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000-634 remonstrate Oriented Analysis and Design - piece 2

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000-634 exam Dumps Source : Object Oriented Analysis and Design - piece 2

Test Code : 000-634
Test designation : Object Oriented Analysis and Design - piece 2
Vendor designation : IBM
real questions : 72 existent Questions

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IBM remonstrate Oriented Analysis and

Object-Oriented analysis and Design | existent Questions and Pass4sure dumps

This chapter is from the ebook 

analysis emphasizes an investigation of the issue and necessities, instead of a solution. for instance, if a novel online buying and selling apparatus is preferred, how will or not it's used? What are its functions?

"evaluation" is a extensive term, most amenable qualified, as in requirements analysis (an investigation of the necessities) or object-oriented evaluation (an investigation of the domain objects).

Design emphasizes a conceptual acknowledge (in utility and hardware) that fulfills the requirements, rather than its implementation. as an instance, an contour of a database schema and software objects. Design concepts frequently exclude low-degree or "obtrusive" particulars—glaring to the meant patrons. in the end, designs can besides live carried out, and the implementation (similar to code) expresses the existent and comprehensive realized design.

As with analysis, the term is optimal certified, as in object-oriented design or database design.

useful evaluation and design had been summarized within the phrase execute the appropriate thing (evaluation), and execute the issue appropriate (design).

Object-Oriented evaluation And Design — Introduction (half 1) | existent Questions and Pass4sure dumps

The thought Of Object-Orientation

Object-orientation is what’s called a programming paradigm. It’s not a language itself but a group of concepts this is supported through many languages.

if you aren’t everyday with the concepts of object-orientation, you may grasp a glance on the legend of Object-Oriented Programming.

If everything they execute in these languages is object-oriented, it skill, they are oriented or concentrated around objects.

Now in an object-oriented language, this one broad software will as a substitute live split aside into self contained objects, practically dote having a few mini-programs, each remonstrate representing a discrete a piece of the software.

and each remonstrate incorporates its personal records and its personal logic, and they talk between themselves.

These objects aren’t random. They represent the style you talk and assume in regards to the hardship you try to resolve on your existent lifestyles.

They symbolize issues dote personnel, photos, pecuniary institution debts, spaceships, asteroids, video phase, audio files, or whatever thing exists to your program.

Object-Oriented analysis And Design (OOAD)

It’s a structured formula for analyzing, designing a system by pass of making exercise of the thing-orientated concepts, and develop a group of graphical apparatus models throughout the building lifestyles cycle of the application.

OOAD in the SDLC

The application life cycle is customarily divided up into degrees going from summary descriptions of the hardship to designs then to code and checking out and eventually to deployment.

The earliest tiers of this technique are evaluation (requirements) and design.

The broad disagreement between analysis and design is often described as “what Vs how”.

In analysis builders toil with users and domain consultants to contour what the system is meant to do. Implementation details are speculated to live more often than not or absolutely unnoticed at this part.

The goal of the evaluation section is to create a model of the device inspite of constraints reminiscent of acceptable expertise. here's typically performed via exercise cases and summary definition of probably the most essential objects the exercise of conceptual model.

The design side refines the analysis mannequin and applies the crucial know-how and different implementation constrains.

It focuses on describing the objects, their attributes, behavior, and interactions. The design mannequin should quiet hold the entire particulars required in order that programmers can set into effect the design in code.

They’re most excellent conducted in an iterative and incremental utility methodologies. So, the actions of OOAD and the developed fashions aren’t done as soon as, they are able to revisit and refine these steps perpetually.

Object-Oriented evaluation

within the object-oriented analysis, we …

  • Elicit necessities: define what does the application deserve to do, and what’s the hardship the utility attempting to resolve.
  • Specify requirements: relate the necessities, usually, the exercise of exercise circumstances (and eventualities) or consumer reports.
  • Conceptual mannequin: identify the faultfinding objects, refine them, and define their relationships and conduct and draw them in a simple diagram.
  • We’re now not going to cover the primary two actions, just the final one. These are already defined in aspect in requirements Engineering.

    Object-Oriented Design

    The evaluation piece identifies the objects, their relationship, and conduct the usage of the conceptual mannequin (an summary definition for the objects).

    while in design phase, they relate these objects (with the aid of growing nature diagram from conceptual diagram — usually mapping conceptual model to classification diagram), their attributes, behavior, and interactions.

    in addition to applying the software design concepts and patterns which could live covered in later tutorials.

    The input for object-oriented design is equipped through the output of object-oriented analysis. however, evaluation and design may additionally ensue in parallel, and the effects of 1 undertaking may besides live used with the aid of the different.

    in the object-oriented design, we …

  • Describe the courses and their relationships the exercise of classification diagram.
  • Describe the interplay between the objects the usage of sequence diagram.
  • follow application design ideas and design patterns.
  • a category diagram gives a visual illustration of the courses you want. And prerogative here is where you rep to live in fact particular about object-oriented principles dote inheritance and polymorphism.

    Describing the interactions between those objects permits you to greater understand the duties of the distinctive objects, the behaviors they should have.

    — other diagrams

    there are many other diagrams they are able to exercise to mannequin the apparatus from diverse perspectives; interactions between objects, constitution of the gadget, or the conduct of the system and how it responds to activities.

    It’s always about deciding on the revise diagram for the revise want. you should realize which diagrams can live constructive when brooding about or discussing a circumstance that isn’t clear.

    device modeling and the distinctive fashions they will exercise will live mentioned subsequent.

    equipment Modeling

    gadget modeling is the procedure of constructing fashions of the system, with each and every model representing a discrete perspectives of that device.

    essentially the most essential factor about a apparatus model is that it leaves out element; It’s an summary illustration of the device.

    The fashions are always based on graphical notation, which is almost always in accordance with the notations within the Unified Modeling Language (UML). different fashions of the gadget dote mathematical mannequin; an in depth system description.

    models are used during the evaluation technique to assist to elicit the requirements, prerogative through the design technique to relate the device to engineers, and after implementation to document the gadget structure and operation.

    different views

    We can besides develop a mannequin to signify the device from different views.

  • exterior, where you mannequin the context or the atmosphere of the device.
  • interplay, where you model the interplay between accessories of a equipment, or between a apparatus and different techniques.
  • Structural, the residence you model the organization of the device, or the structure of the records being processed by pass of the gadget.
  • Behavioral, where you model the dynamic conduct of the device and the pass it reply to pursuits.
  • Unified Modeling Language (UML)

    The unified modeling language become the run-of-the-mill modeling language for object-oriented modeling. It has many diagrams, youngsters, the most diagrams that are widely used are:

  • Use case diagram: It indicates the interplay between a apparatus and it’s ambiance (users or programs) within a selected condition.
  • type diagram: It shows the discrete objects, their relationship, their behaviors, and attributes.
  • Sequence diagram: It indicates the interactions between the diverse objects in the device, and between actors and the objects in a equipment.
  • State desktop diagram: It indicates how the apparatus respond to external and interior activities.
  • exercise diagram: It shows the stream of the information between the techniques within the system.
  • which you can execute diagramming toil on paper or on a whiteboard, as a minimum within the initial stages of a venture. however there are some diagramming tools on the pass to aid you to draw these UML diagrams.

    document: IBM Outpaces opponents in utility construction application market for Seventh Straight yr | existent Questions and Pass4sure dumps

    source: IBM

    June 13, 2008 08:00 ET

    ARMONK, novel york--(Marketwire - June 13, 2008) - IBM (NYSE: IBM) nowadays introduced that analyst difficult Gartner, Inc.* and market research company Evans facts Corp. hold ranked IBM as the chief in the software development application market. These rankings reach just as IBM is projecting more than 12,000 people will attend its 2008 IBM Rational utility construction Conferences in 13 nations everyone over the world.

    Gartner named IBM the worldwide market participate chief in utility construction in line with complete software profits in 2007 and Evans statistics Corp. survey respondents who were users of IBM Rational application Developer ranked it the number one built-in Developer atmosphere (IDE) for person delight. here's the seventh consecutive 12 months that Gartner has ranked IBM the chief and 2d consecutive yr that IBM Rational software Developer became selected as the Developer's option desirable IDE via the 1,200 builders global collaborating in the survey.

    in line with the impartial Gartner report, IBM is the main market participate supplier in complete utility earnings, with 37.eight p.c market participate -- improved market participate than its three closest opponents combined. The global utility development software market grew greater than 10% p.c in 2007 to practically $6.9 billion, according to Gartner.

    IBM became additionally illustrious for its typical management in accordance with complete software salary for 2007 across application building market sub-classes, including SCCM distributed, remonstrate Oriented analysis & Design and Java Platform advert device. Telelogic, currently received by using IBM, had a 2007 marketshare of forty.6 percent in the requirements Elicitation and administration class according to total software salary.

    "With the mount of worldwide allotted application development groups, clients are trying to find skilled carriers to back them collaborate in an open and transparent manner," talked about Dr. Daniel Sabbah, run-of-the-mill manager, IBM Rational software. "We believe the robust response from the Evans facts and Gartner reports coincides with the remarks we've received from shoppers about IBM's strategy around advantageous application beginning."

    IBM Kicks off the realm's Most Attended Developer convention collection

    This marketshare advice coincides with IBM's announcement that over 12,000 participants are expected to attend the 15 IBM Rational application building Conferences planned everyone over. Following the event held last week in Orlando, FL, IBM will grasp the demonstrate on the highway to 17 cities including Sharm El Sheikh, Egypt; San Paulo, Brazil; Bangalore, India; Shanghai, China; Rome and Milan, Italy.

    For conference attendees using an iPhone, IBM is releasing a convention scheduler written in enterprise era Language (EGL) to permit iPhone clients to dynamically adventure the IBM Rational software building conference via an interface that they suppose comfy with. the usage of internet 2.0 and social engineering ideas, clients can give comments on and chat about classes, navigate the conference searching for tracks and pursuits, and exercise artistic technology that implies which talks the consumer should quiet attend next in response to preferences.

    on the annual IBM Rational application construction convention in Orlando, Florida, greater than three,500 attendees discovered about novel software and programs that assist clients seriously change how they are birth software on a world scale. The announcement of latest items, functions and commerce confederate initiatives are designed to seriously change how IBM Rational software can aid consumers drive more advantageous cost and efficiency from their globally disbursed software investments.

    purchasers unable to attend the convention in the community can view the keynote shows on IBM tv.

    IBM helps builders tarry competitive in state-of-the-art quickly-paced development ambiance. ingenious programs comparable to IBM developerWorks, the premier technical resource for software developers, and IBM alphaWorks, IBM's emerging applied sciences outlet, supply an internet neighborhood for the developers of nowadays and tomorrow. builders who are unbiased application carriers can grasp skills of income and marketing tools, skill-constructing courses and technical befriend by joining the international IBM PartnerWorld software. IBM's academic Initiative and IBM Rational application development convention are examples of the continuing researching and group-constructing classes crucial by pass of students, educators and developers worldwide.

    For greater assistance, consult with

    *"Market Share: application construction software, worldwide, 2007" by using Laurie Wurster, Teresa Jones and Asheesh Raina, may additionally 2008.

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    Object Oriented Analysis and Design - piece 2

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    Object-Oriented Analysis & Design | existent questions and Pass4sure dumps

    Object-Oriented Analysis & DesignJune 2, 3, 9, 10Worcester state CollegeTaught by Jan Bergandy, Computer Science, UMass, Dartmouth

    Seminar Objectives:

  • To develop an in-depth understanding of object-oriented paradigm
  • To obtain a working information of object-oriented analysis & design techniques
  • To learn object-oriented modeling using Unified Modeling Language (UML)
  • To learn about basic design patterns and the role of patterns is software development
  • To understand the impact of remonstrate paradigm on software development activities
  • To explore synergy between object-oriented design and object-oriented programming
  • To learn about key remonstrate technologies
  • Who should attend:This workshop is addressed to faculty involved in teaching programming, software design, and other courses related to software development. It is addressed to those who kisser a transition to remonstrate technology and want to learn about challenges and benefits of this transition. The workshop does not require any prior information of object-oriented programming or information of remonstrate paradigm. generic computer fluency and generic information of issues associated with software and software development are expected.

    Seminar Organization:The course will live conducted as a project with instructor giving short presentations pertaining to a specific stage of the analysis and design process. During this course the participants will construct an analysis model for a selected problem. This model will live refined in to the circumstantial design even providing an break for discussion about the relationship between object-oriented design and object-oriented programming. Each student will receive a copy of the course materials and the textbook.

    Tools & Platforms:Rational-Rose CASE toolThe CASE implement is used exclusively to expedite the process of model construction. The students expend no more than half an hour of their time during the entire class on learning how to exercise the tool. Not using the CASE toll will invent it almost impossible to experience hands-on everyone the elements of the object-oriented analysis and design process.

    Textbooks:M. Fowler, ÒUML DistilledÓ, Addison-Wesley, ISBN 0-201-32563-2 (additional/optional )

    E. Gamma, R. Helm, R. Johnson, J. Vlissides, ÒDesign PatternÓ, Addison-Wesley, ISBN 0-201-63361-2


    June 2, 2001, 9:00 - 5:00Topics to live addressed:Object paradigm top-down - analysis & design perspectiveObject paradigm bottom-up - programming perspectiveBasic concepts: abstraction, encapsulation, information hiding, modularityResponsibility view of the requirementsClasses and objects emerging from responsibilitiesComparison of procedural and object-oriented paradigmsClasses and relationships as the building blocks of software architectureCriteria of class qualityIntroduction to Unified Modeling Language (UML)Static & dynamic modelActors and exercise casesTransitioning from functional requirements to objects - introduction

    Project:Analysis of the requirements for the selected projectIdentifying actors and exercise casesConstructing exercise case diagrams

    June 3, 2001, 9:00 - 2:00Topics to live addressed:Transitioning from functional requirements to objectsIdentifying the first group of classesClass specificationClass as an encapsulation of a responsibilityClass, Utility Class, Parameterized Class and its instantiationClass diagram - introductionIdentifying relationships between classesAssociation relationshipsAssociation classesRepresenting relationships with cardinalityAggregation versus compositionRepresenting aggregation and composition relationshipsRepresenting generalization/ specialization (inheritance)PolymorphismAbstract classes and interfacesSpecification of relationshipsImplementing classes & relationships (bottom-up view of relationships)Class diagram

    Project:Identifying first group of classes based on responsibilitiesPreliminary class diagramIdentifying relationships between classesDefining cardinalitiesClass diagram

    June 10, 2001, 9:00 - 5:00 (part I)Topics to live addressed:Static versus dynamic modelIdentifying scenarios through refinement of exercise casesModeling scenarios using object-interaction and sequence diagrams

    Project:Refining exercise casesDeveloping and modeling scenariosIdentifying methodsRefining class specifications

    June 10, 2001 (part II)Topics to live addressed:Events, states and actionsState diagramCriteria for using state diagramsConcurrency, active objectsMutual exclusion problemSequential, guarded, and synchronous objectsModeling concurrencyConcurrent state diagramsActivity diagrams

    Project:Evaluating classes for the need of state diagramsConstructing state diagrams for selected classes(Constructing activity diagrams)Refining class specifications

    June 10, 2001, 9:00 - 2:00Topics to live addressed:Introduction to design patterns: Creational patterns, Abstract Factory, Builder, Prototype, Singleton, Virtual Constructor

    Structural Patterns: Adapter, Bridge, Composite, Decorator, Façade, Proxy

    Behavioral Patterns: Chain of Responsibility, Command, Iterator, Mediator, Memento

    Other distinguished topics to live covered in this course:What to hope from an object-oriented languageDynamic nature of object-oriented systems and the issues of garbage collectionEffective exercise of inheritance and polymorphism and their impact on software qualitySingle versus multiple inheritancePolymorphism versus genericsClass design and data normalization (attribute dependence issues)

    Object-oriented design patterns in the kernel, piece 2 | existent questions and Pass4sure dumps needs you!

    Without subscribers, LWN would simply not exist. gladden consider signing up for a subscription and helping to maintain LWN publishing

    June 7, 2011

    This article was contributed by Neil Brown

    In the first piece of this analysis they looked at how the polymorphic side of object-oriented programming was implemented in the Linux kernel using regular C constructs. In particular they examined mode dispatch, looked at the different forms that vtables could take, and the circumstances where divide vtables were eschewed in preference for storing office pointers directly in objects. In this conclusion they will explore a second distinguished aspect of object-oriented programming - inheritance, and in particular data inheritance.

    Data inheritance

    Inheritance is a core concept of object-oriented programming, though it comes in many forms, whether prototype inheritance, mixin inheritance, subtype inheritance, interface inheritance etc., some of which overlap. The shape that is of interest when exploring the Linux kernel is most dote subtype inheritance, where a concrete or "final" nature inherits some data fields from a "virtual" parent type. They will summon this "data inheritance" to emphasize the fact that it is the data rather than the conduct that is being inherited.

    Put another way, a number of different implementations of a particular interface share, and separately extend, a common data structure. They can live said to inherit from that data structure. There are three different approaches to this sharing and extending that can live institute in the Linux kernel, and everyone can live seen by exploring the struct inode structure and its history, though they are widely used elsewhere.

    Extension through unions

    The first approach, which is probably the most obvious but besides the least flexible, is to declare a union as one factor of the common structure and, for each implementation, to declare an entry in that union with extra fields that the particular implementation needs. This approach was introduced to struct inode in Linux-0.97.2 (August 1992) when

    union { struct minix_inode_info minix_i; struct ext_inode_info ext_i; struct msdos_inode_info msdos_i; } u;

    was added to struct inode. Each of these structures remained void until 0.97.5 when i_data was moved from struct inode to struct ext_inode_info. Over the years several more "inode_info" fields were added for different filesystems, peaking at 28 different "inode_info" structures in when ext3 was added.

    This approach to data inheritance is simple and straightforward, but is besides byalongshot clumsy. There are two obvious problems. Firstly, every novel filesystem implementation needs to add an extra realm to the union "u". With 3 fields this may not appear dote a problem, with 28 it was well past "ugly". Requiring every filesystem to update this one structure is a barrier to adding filesystems that is unnecessary. Secondly, every inode allocated will live the selfsame size and will live large enough to store the data for any filesystem. So a filesystem that wants lots of space in its "inode_info" structure will impose that space cost on every other filesystem.

    The first of these issues is not an impenetrable barrier as they will perceive shortly. The second is a existent problem and the generic ugliness of the design encouraged change. Early in the 2.5 development sequence this change began; it was completed by 2.5.7 when there were no "inode_info" structures left in union u (though the union itself remained until 2.6.19).

    Embedded structures

    The change that happened to inodes in early 2.5 was effectively an inversion. The change which removed ext3_i from struct inode.u besides added a struct inode, called vfs_inode, to struct ext3_inode_info. So instead of the private structure being embedded in the common data structure, the common data structure is now embedded in the private one. This neatly avoids the two problems with unions; now each filesystem needs to only earmark recollection to store its own structure without any need to know anything about what other filesystems might need. Of course nothing ever comes for free and this change brought with it other issues that needed to live solved, but the solutions were not costly.

    The first hardship is the fact that when the common filesystem code - the VFS layer - calls into a specific filesystem it passes a pointer to the common data structure, the struct inode. Using this pointer, the filesystem needs to find a pointer to its own private data structure. An obvious approach is to always residence the struct inode at the top of the private inode structure and simply cast a pointer to one into a pointer to the other. While this can work, it lacks any semblance of nature safety and makes it harder to order fields in the inode to rep optimal performance - as some kernel developers are wont to do.

    The solution was to exercise the list_entry() macro to discharge the necessary pointer arithmetic, subtracting from the address of the struct inode its offset in the private data structure and then casting this appropriately. The macro for this was called list_entry() simply because the "list.h lists" implementation was the first to exercise this pattern of data structure embedding. The list_entry() macro did exactly what was needed and so it was used despite the exceptional name. This rehearse lasted until 2.5.28 when a novel container_of() macro was added which implemented the selfsame functionality as list_entry(), though with slightly more nature safety and a more meaningful name. With container_of() it is a simple matter to map from an embedded data structure to the structure in which it is embedded.

    The second hardship was that the filesystem had to live amenable for allocating the inode - it could no longer live allocated by common code as the common code did not hold enough information to earmark the revise amount of space. This simply involved adding alloc_inode() and destroy_inode() methods to the super_operations structure and calling them as appropriate.

    Void pointers

    As illustrious earlier, the union pattern was not an impenetrable barrier to adding novel filesystems independently. This is because the union u had one more realm that was not an "inode_info" structure. A generic pointer realm called generic_ip was added in Linux-1.0.5, but it was not used until 1.3.7. Any file system that does not own a structure in struct inode itself could define and earmark a divide structure and link it to the inode through u.generic_ip. This approach addressed both of the problems with unions as no changes are needed to shared declarations and each filesystem only uses the space that it needs. However it again introduced novel problems of its own.

    Using generic_ip, each filesystem required two allocations for each inode instead of one and this could lead to more wastage depending on how the structure size was rounded up for allocation; it besides required writing more error-handling code. besides there was recollection used for the generic_ip pointer and often for a back pointer from the private structure to the common struct inode. Both of these are wasted space compared with the union approach or the embedding approach.

    Worse than this though, an extra recollection dereference was needed to access the private structure from the common structure; such dereferences are best avoided. Filesystem code will often need to access both the common and the private structures. This either requires lots of extra recollection dereferences, or it requires holding the address of the private structure in a register which increases register pressure. It was largely these concerns that stopped struct inode from ever migrating to broad exercise of the generic_ip pointer. It was certainly used, but not by the major, high-performance filesystems.

    Though this pattern has problems it is quiet in wide use. struct super_block has an s_fs_info pointer which serves the selfsame purpose as u.generic_ip (which has since been renamed to i_private when the u union was finally removed - why it was not completely removed is left as an exercise for the reader). This is the only pass to store filesystem-private data in a super_block. A simple search in the Linux embrace files shows quite a collection of fields which are void pointers named "private" or something similar. Many of these are examples of the pattern of extending a data nature by using a pointer to a private extension, and most of these could live converted to using the embedded-structure pattern.

    Beyond inodes

    While inodes serve as an effective vehicle to interpose these three patterns they execute not display the full scope of any of them so it is useful to peep further afield and perceive what else they can learn.

    A survey of the exercise of unions elsewhere in the kernel shows that they are widely used though in very different circumstances than in struct inode. The particular aspect of inodes that is missing elsewhere is that a wide ambit of different modules (different filesystems) each wanted to extend an inode in different ways. In most places where unions are used there are a miniature fixed number of subtypes of the foundation nature and there is Little expectation of more being added. A simple sample of this is struct nfs_fattr which stores file impute information decoded out of an NFS reply. The details of these attributes are slightly different for NFSv2 and NFSv3 so there are effectively two subtypes of this structure with the disagreement encoded in a union. As NFSv4 uses the selfsame information as NFSv3 this is very unlikely to ever live extended further.

    A very common pattern in other uses of unions in Linux is for encoding messages that are passed around, typically between the kernel and user-space. struct siginfo is used to convey extra information with a signal delivery. Each signal nature has a different nature of ancillary information, so struct siginfo has a union to encode six different subtypes. union inputArgs appears to live the largest current union with 22 different subtypes. It is used by the "coda" network file system to pass requests between the kernel module and a user-space daemon which handles the network communication.

    It is not transparent whether these examples should live considered as the selfsame pattern as the original struct inode. execute they really represent different subtypes of a foundation type, or is it just one nature with internal variants? The Eiffel object-oriented programming language does not back variant types at everyone except through subtype inheritance so there is clearly a school of thought that would want to handle everyone usages of union as a shape of subtyping. Many other languages, such as C++, provide both inheritance and unions allowing the programmer to invent a choice. So the acknowledge is not clear.

    For their purposes it doesn't really matter what they summon it as long as they know where to exercise each pattern. The examples in the kernel fairly clearly prove that when everyone of the variants are understood by a separate module, then a union is a very appropriate mechanism for variants structures, whether you want to advert to them as using data inheritance or not. When different subtypes are managed by different modules, or at least widely divide pieces of code, then one of the other mechanisms is preferred. The exercise of unions for this case has almost completely disappeared with only struct cycx_device remaining as an sample of a deprecated pattern.

    Problems with void pointers

    Void pointers are not quite so effortless to classify. It would probably live honest to mutter that void pointers are the modern equivalent of "goto" statements. They can live very useful but they can besides lead to very convoluted designs. A particular problem is that when you peep at a void pointer, dote looking at a goto, you don't really know what it is pointing at. A void pointer called private is even worse - it is dote a "goto destination" command - almost pointless without reading lots of context.

    Examining everyone the different uses that void pointers can live set to would live well beyond the scope of this article. Instead they will restrict their attention to just one novel usage which relates to data inheritance and illustrates how the untamed nature of void pointers makes it difficult to recognize their exercise in data inheritance. The sample they will exercise to accountfor this usage is struct seq_file used by the seq_file library which makes it effortless to synthesize simple text files dote some of those in /proc. The "seq" piece of seq_file simply indicates that the file contains a sequence of lines corresponding to a sequence of items of information in the kernel, so /proc/mounts is a seq_file which walks through the mount table reporting each mount on a separate line.

    When seq_open() is used to create a novel seq_file it allocates a struct seq_file and assigns it to the private_data realm of the struct file which is being opened. This is a straightforward sample of void pointer based data inheritance where the struct file is the foundation nature and the struct seq_file is a simple extension to that type. It is a structure that never exists by itself but is always the private_data for some file. struct seq_file itself has a private realm which is a void pointer and it can live used by clients of seq_file to add extra state to the file. For sample md_seq_open() allocates a struct mdstat_info structure and attaches it via this private field, using it to meet md's internal needs. Again, this is simple data inheritance following the described pattern.

    However the private realm of struct seq_file is used by svc_pool_stats_open() in a subtly but importantly different way. In this case the extra data needed is just a separate pointer. So rather than allocating a local data structure to advert to from the private field, svc_pool_stats_open simply stores that pointer directly in the private realm itself. This certainly seems dote a sensible optimization - performing an allocation to store a separate pointer would live a dissipate - but it highlights exactly the source of confusion that was suggested earlier: that when you peep at a void pointer you don't really know what is it pointing at, or why.

    To invent it a bit clearer what is happening here, it is helpful to imagine "void *private" as being dote a union of every different workable pointer type. If the value that needs to live stored is a pointer, it can live stored in this union following the "unions for data inheritance" pattern. If the value is not a separate pointer, then it gets stored in allocated space following the "void pointers for data inheritance" pattern. Thus when they perceive a void pointer being used it may not live obvious whether it is being used to point to an extension structure for data inheritance, or being used as an extension for data inheritance (or being used as something else altogether).

    To highlight this issue from a slightly different perspective it is instructive to examine struct v4l2_subdev which represents a sub-device in a video4linux device, such as a sensor or camera controller within a webcam. According to the (rather helpful) documentation it is expected that this structure will normally live embedded in a larger structure which contains extra state. However this structure quiet has not just one but two void pointers, both with names suggesting that they are for private exercise by subtypes:

    /* pointer to private data */ void *dev_priv; void *host_priv;

    It is common that a v4l sub-device (a sensor, usually) will live realized by, for example, an I2C device (much as a screen device which stores your filesystem might live realized by an ATA or SCSI device). To allow for this common occurrence, struct v4l2_subdev provides a void pointer (dev_priv), so that the driver itself doesn't need to define a more specific pointer in the larger structure which struct v4l2_subdev would live embedded in. host_priv is intended to point back to a "parent" device such as a controller which acquires video data from the sensor. Of the three drivers which exercise this field, one appears to succeed that goal while the other two exercise it to point to an allocated extension structure. So both of these pointers are intended to live used following the "unions for data inheritance" pattern, where a void pointer is playing the role of a union of many other pointer types, but they are not always used that way.

    It is not immediately transparent that defining this void pointer in case it is useful is actually a valuable service to provide given that the device driver could easily enough define its own (type safe) pointer in its extension structure. What is transparent is that an apparently "private" void pointer can live intended for various qualitatively different uses and, as they hold seen in two different circumstances, they may not live used exactly as expected.

    In short, recognizing the "data inheritance through void pointers" pattern is not easy. A fairly deep examination of the code is needed to determine the exact purpose and usage of void pointers.

    A diversion into struct page

    Before they leave unions and void pointers behind a peep at struct page may live interesting. This structure uses both of these patterns, though they are hidden byalongshot due to historical baggage. This sample is particularly instructive because it is one case where struct embedding simply is not an option.

    In Linux recollection is divided into pages, and these pages are set to a variety of different uses. Some are in the "page cache" used to store the contents of files. Some are "anonymous pages" holding data used by applications. Some are used as "slabs" and divided into pieces to acknowledge kmalloc() requests. Others are simply piece of a multi-page allocation or maybe are on a free list waiting to live used. Each of these different exercise cases could live seen as a subtype of the generic class of "page", and in most cases need some dedicated fields in struct page, such as a struct address_space pointer and index when used in the page cache, or struct kmem_cache and freelist pointers when used as a slab.

    Each page always has the selfsame struct page describing it, so if the effective nature of the page is to change - as it must as the demands for different uses of recollection change over time - the nature of the struct page must change within the lifetime of that structure. While many nature systems are designed assuming that the nature of an remonstrate is immutable, they find here that the kernel has a very existent need for nature mutability. Both unions and void pointers allow types to change and as noted, struct page uses both.

    At the first even of subtyping there are only a miniature number of different subtypes as listed above; these are everyone known to the core recollection management code, so a union would live pattern here. Unfortunately struct page has three unions with fields for some subtypes spread over everyone three, thus hiding the existent structure somewhat.

    When the primary subtype in exercise has the page being used in the page cache, the particular address_space that it belongs to may want to extend the data structure further. For this purpose there is a private realm that can live used. However it is not a void pointer but is an unsigned long. Many places in the kernel assume an unsigned long and a void * are the selfsame size and this is one of them. Most users of this realm actually store a pointer here and hold to cast it back and forth. The "buffer_head" library provides macros attach_page_buffers and page_buffers to set and rep this field.

    So while struct page is not the most elegant example, it is an informative sample of a case where unions and void pointers are the only option for providing data inheritance.

    The details of structure embedding

    Where structure embedding can live used, and where the list of workable subtypes is not known in advance, it seems to live increasingly the preferred choice. To gain a full understanding of it they will again need to explore a Little bit further than inodes and contrast data inheritance with other uses of structure embedding.

    There are essentially three uses for structure embedding - three reasons for including a structure within another structure. Sometimes there is nothing particularly spellbinding going on. Data items are collected together into structures and structures within structures simply to highlight the closeness of the relationships between the different items. In this case the address of the embedded structure is rarely taken, and it is never mapped back to the containing structure using container_of().

    The second exercise is the data inheritance embedding that they hold already discussed. The third is dote it but importantly different. This third exercise is typified by struct list_head and other structs used as an embedded anchor when creating abstract data types.

    The exercise of an embedded anchor dote struct list_head can live seen as a style of inheritance as the structure containing it "is-a" member of a list by virtue of inheriting from struct list_head. However it is not a strict subtype as a separate remonstrate can hold several struct list_heads embedded - struct inode has six (if they embrace the similar hlist_node). So it is probably best to assume of this sort of embedding more dote a "mixin" style of inheritance. The struct list_head provides a service - that of being included in a list - that can live mixed-in to other objects, an whimsical number of times.

    A key aspect of data inheritance structure embedding that differentiates it from each of the other two is the actuality of a reference counter in the inner-most structure. This is an observation that is tied directly to the fact that the Linux kernel uses reference counting as the primary means of lifetime management and so would not live shared by systems that used, for example, garbage collection to manage lifetimes.

    In Linux, every remonstrate with an independent actuality will hold a reference counter, sometimes a simple atomic_t or even an int, though often a more categorical struct kref. When an remonstrate is created using several levels of inheritance the reference counter could live buried quite deeply. For sample a struct usb_device embeds a struct device which embeds struct kobject which has a struct kref. So usb_device (which might in gyrate live embedded in a structure for some specific device) does hold a reference counter, but it is contained several levels down in the nest of structure embedding. This contrasts quite nicely with a list_head and similar structures. These hold no reference counter, hold no independent actuality and simply provide a service to other data structures.

    Though it seems obvious when set this way, it is useful to recollect that a separate remonstrate cannot hold two reference counters - at least not two lifetime reference counters (It is fine to hold two counters dote s_active and s_count in struct super_block which import different things). This means that multiple inheritance in the "data inheritance" style is not possible. The only shape of multiple inheritance that can toil is the mixin style used by list_head as mentioned above.

    It besides means that, when designing a data structure, it is distinguished to assume about lifetime issues and whether this data structure should hold its own reference counter or whether it should depend on something else for its lifetime management. That is, whether it is an remonstrate in its own right, or simply a service provided to other objects. These issues are not really novel and apply equally to void pointer inheritance. However an distinguished disagreement with void pointers is that it is relatively effortless to change your mind later and switch an extension structure to live a fully independent object. Structure embedding requires the discipline of thinking clearly about the problem up front and making the prerogative decision early - a discipline that is worth encouraging.

    The other key telltale for data inheritance structure embedding is the set of rules for allocating and initializing novel instances of a structure, as has already been hinted at. When union or void pointer inheritance is used the main structure is usually allocated and initialized by common code (the mid-layer) and then a device specific open() or create() office is called which can optionally earmark and initialize any extension object. By contrast when structure embedding is used the structure needs to live allocated by the lowest even device driver which then initializes its own fields and calls in to common code to initialize the common fields.

    Continuing the struct inode sample from above which has an alloc_inode() mode in the super_block to request allocation, they find that initialization is provided for with inode_init_once() and inode_init_always() back functions. The first of these is used when the previous exercise of a piece of recollection is unknown, the second is adequate by itself when they know that the recollection was previously used for some other inode. They perceive this selfsame pattern of an initializer office divide from allocation in kobject_init(), kref_init(), and device_initialize().

    So apart from the obvious embedding of structures, the pattern of "data inheritance through structure embedding" can live recognized by the presence of a reference counter in the innermost structure, by the delegation of structure allocation to the final user of the structure, and by the provision of initializing functions which initialize a previously allocated structure.


    In exploring the exercise of mode dispatch (last week) and data inheritance (this week) in the Linux kernel they find that while some patterns appear to dominate they are by no means universal. While almost everyone data inheritance could live implemented using structure embedding, unions provide existent value in a few specific cases. Similarly while simple vtables are common, mixin vtables are very distinguished and the aptitude to delegate methods to a related remonstrate can live valuable.

    We besides find that there are patterns in exercise with Little to recommend them. Using void pointers for inheritance may hold an initial simplicity, but causes longer term wastage, can antecedent confusion, and could nearly always live replaced by embedded inheritance. Using NULL pointers to indicate default conduct is similarly a penniless option - when the default is distinguished there are better ways to provide for it.

    But maybe the most valuable lesson is that the Linux kernel is not only a useful program to run, it is besides a useful document to study. Such study can find elegant practical solutions to existent problems, and some less elegant solutions. The willing student can pursue the former to befriend help their mind, and pursue the latter to befriend help the kernel itself. With that in mind, the following exercises might live of interest to some.

  • As inodes now exercise structure embedding for inheritance, void pointers should not live necessary. Examine the consequences and wisdom of removing "i_private" from "struct inode".

  • Rearrange the three unions in struct page to just one union so that the enumeration of different subtypes is more explicit.

  • As was illustrious in the text, struct seq_file can live extended both through "void pointer" and a limited shape of "union" data inheritance. accountfor how seq_open_private() allows this structure to besides live extended through "embedded structure" data inheritance and give an sample by converting one usage in the kernel from "void pointer" to "embedded structure". consider submitting a patch if this appears to live an improvement. Contrast this implementation of embedded structure inheritance with the mechanism used for inodes.

  • Though subtyping is widely used in the kernel, it is not uncommon for a remonstrate to hold fields that not everyone users are interested in. This can indicate that more fine grained subtyping is possible. As very many completely different things can live represented by a "file descriptor", it is likely that struct file could live a candidate for further subtyping.

    Identify the smallest set of fields that could serve as a generic struct file and explore the implications of embedding that in different structures to implement regular files, socket files, event files, and other file types. Exploring more generic exercise of the proposed open() mode for inodes might befriend here.

  • Identify an "object-oriented" language which has an remonstrate model that would meet everyone the needs of the Linux kernel as identified in these two articles.

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    Java and Object-Oriented Programming | existent questions and Pass4sure dumps

    This chapter is from the book 

    Many seasoned Java developers will scoff at the fact that this section even exists in this book. It is here for two very distinguished reasons. The first is that I continually quicken across Java applications built with a procedural mind-set. The fact that you know Java doesn't signify that you hold the aptitude to transform that information into well-designed object-oriented systems. As both an instructor and consultant, I perceive many data-processing shops send COBOL and/or Visual Basic developers to a three-day class on UML and a five-day class on Java and hope miracles. Case in point: I was recently asked to review a Java application to assess its design architecture and institute that it had only two classes—SystemController and ScreenController—which contained over 70,000 lines of Java code.

    The second reason for the stress on how the language maps to object-oriented principles is that people dote language comparisons and how they stack up to their counterparts. To appease those that live and die by language comparisons, let's set Java under the scrutiny of what constitutes an object-oriented language.

    No definitive definition of what makes a language object-oriented is globally accepted. However, a common set of criteria I personally find useful is that the language must back the following:

  • Classes
  • Complex types (Java reference types)
  • Message passing
  • Encapsulation
  • Inheritance
  • Polymorphism
  • These are discussed in the next subsections.

    Java and Classes

    Java allows classes to live defined. There are no stray functions floating around in Java. A class is a static template that contains the defined structure (attributes) and conduct (operations) of a real-world entity in the application domain. At runtime, the class is instantiated, or brought to life, as an remonstrate born in the image of that class. In my seminars, when several folks novel to the remonstrate world are in attendance, I often exercise the analogy of a cookie cutter. The cookie cutter is merely the template used to stamp out what will become individually decorated and unique cookies. The cookie cutter is the class; the unique blue, green, and yellow gingerbread man is the remonstrate (which I confidence supports a gnaw operation).

    Java exposes the class to potential outside users through its public interface. A public interface consists of the signatures of the public operations supported by the class. A signature is the operation designation and its input parameter types (the recrudesce type, if any, is not piece of the operation's signature).

    Good programming rehearse encourages developers to declare everyone attributes as private and allow access to them only via operations. As with most other languages, however, this is not enforced in Java. figure 2-1 outlines the concept of a class and its interface.

    FIGURE 2-1 Public interface of a class

    The figure uses a common eggshell metaphor to relate the concept of the class's interface, as well as encapsulation. The internal details of the class are hidden from the outside via a well-defined interface. In this case, only four operations are exposed in the classes interface (Operation_A, B, C, and D). The other attributes and operations are protected from the outside world. Actually, to the outside world, it's as if they don't even exist.

    Suppose you want to create an Order class in Java that has three attributes—orderNumber, orderDate, and orderTotal—and two operations—calcTotalValue() and getInfo(). The class definition could peep dote this:

    /** * Listing 1 * This is the Order class for the Java/UML book */ package com.jacksonreed; import java.util.*; public class Order { private Date orderDate; private long orderNumber; private long orderTotal; public Order() { } public boolean getInfo() { recrudesce true; } public long calcTotalValue() { recrudesce 0; } public Date getOrderDate() { recrudesce orderDate; } public void setOrderDate(Date aOrderDate) { orderDate = aOrderDate; } public long getOrderNumber() { recrudesce orderNumber; } public void setOrderNumber(long aOrderNumber) { orderNumber = aOrderNumber; } public long getOrderTotal() { recrudesce orderTotal; } public void setOrderTotal(long aOrderTotal) { orderTotal = aOrderTotal; } public static void main(String[] args) { Order order = novel Order(); System.out.println("instantiated Order"); System.out.println(order.getClass().getName()); System.out.println(order.calcTotalValue()); try { Thread.currentThread().sleep(5*1000); } catch(InterruptedException e) {} } }

    A few things are notable about the first bit of Java code presented in this book. Notice that each of the three attributes has a rep and a set operation to allow for the retrieval and setting of the Order object's properties. Although doing so is not required, it is common rehearse to provide these accessor-type operations for everyone attributes defined in a class. In addition, if the Order class ever wanted to live a JavaBean, it would hold to hold "getters and setters" defined in this way.

    Some of the mode code in the main() operation does a few things of note. Of interest is that a try screen exists at the near of the operation that puts the current thread to sleep for a bit. This is to allow the console display to freeze so that you can perceive the results.

    If you nature in this class and then compile it and execute it in your favorite development implement or from the command prompt with

    javac //* to compile it java order //* to quicken it

    you should rep results that peep dote this:

    instantiated Order com.jacksonreed.Order 0


    Going forward, I engage you will perceive no code samples with class, operation, or impute names of foo, bar, or foobar.

    More on Java and Classes

    A class can besides hold what are called class-level operations and attributes. Java supports these with the static keyword. This keyword would travel prerogative after the visibility (public, private, protected) component of the operation or attribute. Static operations and attributes are needed to invoke either a service of the class before any existent instances of that class are instantiated or a service that doesn't directly apply to any of the instances. The classic sample of a static operation is the Java constructor. The constructor is what is called when an remonstrate is created with the novel keyword. Perhaps a more business-focused sample is an operation that retrieves a list of Customer instances based on particular search criteria.

    A class-level impute can live used to store information that everyone instances of that class may access. This impute might be, for example, a import of the number of objects currently instantiated or a property about Customer that everyone instances might need to reference.

    Java and involved Types (Java Reference Types)

    A involved type, which in Java is called a reference type, allows variables typed as something other than primitive types (e.g., int and boolean) to live declared. In Java, these are called reference types. In object-oriented systems, variables that are "of" a particular class, such as Order, Customer, or Invoice, must live defined. Taken a step further, Order could consist of other class instances, such as OrderHeader and OrderLine.

    In Java, you can define different variables that are references to runtime objects of a particular class type:

    Public Order myOrder; Public Customer myCustomer; Public Invoice myInvoice;

    Such variables can then live used to store actual remonstrate instances and subsequently to serve as recipients of messages sent by other objects. In the previous code fragment, the variable myOrder is an instance of Order. After the myOrder remonstrate is created, a message can live sent to it and myOrder will respond, provided that the operation is supported by myOrder's interface.

    Java and Message Passing

    Central to any object-oriented language is the aptitude to pass messages between objects. In later chapters you will perceive that toil is done in a system only by objects that collaborate (by sending messages) to accomplish a goal (which is specified in a use-case) of the system.

    Java doesn't allow stray functions floating around that are not attached to a class. In fact, Java demands this. Unfortunately, as my previous legend suggested, just saw that a language requires everything to live packaged in classes doesn't signify that the class design will live robust, let lonely correct.

    Java supports message passing, which is central to the exercise of Java's object-oriented features. The format closely resembles the syntax of other languages, such as C++ and Visual Basic. In the following code fragment, assume that a variable called myCustomer, of nature Customer, is defined and that an operation called calcTotalValue() is defined for Customer. Then the calcTotalValue() message being sent to the myCustomer remonstrate in Java would peep dote this:


    Many developers feel that, in any other structured language, this is just a fancy pass of calling a procedure. Calling a procedure and sending a message are similar in that, once invoked, both a procedure and a message implement a set of well-defined steps. However, a message differs in two ways:

  • There is a designated receiver, the object. Procedures hold no designated receiver.

  • The interpretation of the message—that is, the how-to code (called the method) used to respond to the message—can vary with different receivers. This point will become more distinguished later in the chapter, when polymorphism is reviewed.

  • The concepts presented in this book depend heavily on classes and the messaging that takes residence between their instances, or objects.

    Java and Encapsulation

    Recall that a class exposes itself to the outside world via its public interface and that this should live done through exposure to operations only, and not attributes. Java supports encapsulation via its aptitude to declare both attributes and operations as public, private, or protected. In UML this is called visibility.

    Using the code from the previous Order example, suppose you want to set the value of the orderDate attribute. In this case, you should execute so with an operation. An operation that gets or sets values is usually called a getter or a setter, respectively, and collectively such operations are called accessors. The local copy of the order date, orderDate, is declared private. (Actually, everyone attributes of a class should live declared private or protected, so that they are accessible only via operations exposed as public to the outside world.)

    Encapsulation provides some powerful capabilities. To the outside world, the design can cloak how it derives its impute values. If the orderTotal impute is stored in the Order object, the corresponding rep operation defined previously looks dote this:

    public long getOrderTotal() { recrudesce orderTotal; }

    This snippet of code would live invoked if the following code were executed by an interested client:

    private long localTotal; private Order localOrder; localOrder = novel Order(); localTotal = localOrder.getOrderTotal()

    However, suppose the impute orderTotal isn't kept as a local value of the Order class, but rather is derived via another mechanism (perhaps messaging to its OrderLine objects). If Order contains OrderLine objects (declared as a Vector or ArrayList of OrderLine objects called myOrderLines) and OrderLine knows how to obtain its line totals via the message getOrderLineTotal(), then the corresponding rep operation for orderTotal within Order will peep dote this:

    public long getOrderTotal() { long totalAmount=0; for (int i=0; i < myOrderLines.length; i++) { totalAmount = totalAmount + myOrderLines[i].getOrderLineTotal(); } recrudesce totalAmount; }

    This code cycles through the myOrderLines collection, which contains everyone the Orderline objects related to the Order object, sending the getOrderLineTotal() message to each of Order's OrderLine objects. The getOrderTotal() operation will live invoked if the following code is executed by an interested client:

    long localTotal; Order myOrder; myOrder = novel Order(); localTotal = localOrder.getOrderTotal()

    Notice that the "client" code didn't change. To the outside world, the class quiet has an orderTotal attribute. However, you hold hidden, or encapsulated, just how the value was obtained. This encapsulation allows the class's interface to remain the selfsame (hey, I hold an orderTotal that you can inquireof me about), while the class retains the flexibility to change its implementation in the future (sorry, how they execute commerce has changed and now they must derive orderTotal dote this). This kindhearted of resiliency is one of the compelling commerce reasons to exercise an object-oriented programming language in general.

    Java and Inheritance

    The inclusion of inheritance is often the most cited reason for granting a language object-oriented status. There are two kinds of inheritance: interface and implementation. As they shall see, Java is one of the few languages that makes a transparent distinction between the two.

    Interface inheritance (Figure 2-2) declares that a class that is inheriting an interface will live amenable for implementing everyone of the mode code of each operation defined in that interface. Only the signatures of the interface are inherited; there is no mode or how-to code.

    FIGURE 2-2 Interface inheritance

    Implementation inheritance (Figure 2-3) declares that a class that is inheriting an interface may, at its option, exercise the mode code implementation already established for the interface. Alternatively, it may pick to implement its own version of the interface. In addition, the class inheriting the interface may extend that interface by adding its own operations and attributes.

    FIGURE 2-3 Implementation inheritance

    Each nature of inheritance should live scrutinized and used in the appropriate setting. Interface inheritance is best used under the following conditions:

  • The foundation class presents a generic facility, such as a table lookup, or a derivation of system-specific information, such as operating-system semantics or unique algorithms.

  • The number of operations is small.

  • The foundation class has few, if any, attributes.

  • Classes realizing or implementing the interface are diverse, with Little or no common code.

  • Implementation inheritance is best used under the following conditions:

  • The class in question is a domain class that is of primary interest to the application (i.e., not a utility or controller class).

  • The implementation is complex, with a large number of operations.

  • Many attributes and operations are common across specialized implementations of the foundation class.

  • Some practitioners contend that implementation inheritance leads to a symptom called the breakable foundation class problem. Chiefly, this term refers to the fact that over time, what were once common code and attributes in the superclass may not tarry common as the commerce evolves. The result is that many, if not all, of the subclasses, override the conduct of the superclass. Worse yet, the subclasses may find themselves overriding the superclass, doing their own work, and then invoking the selfsame operation again on the superclass. These practitioners espouse the idea of using only interface inheritance. Particularly with the advent of Java and its raising of the interface to a first-class type, the concept and usage of interface-based programming hold gained tremendous momentum.

    As this book evolves, keeping in mind the pointers mentioned here when deciding between the two types of inheritance will live helpful. Examples of both constructs will live presented in the theme project that extends throughout this book.

    Implementation Inheritance

    Java supports implementation inheritance with the extends keyword. A class wanting to grasp edge of implementation inheritance simply adds an extendsClassName statement to its class definition. To continue the previous example, suppose you hold two different types of orders, both warranting their own subclasses: Commercial and Retail. You would quiet hold an Order class (which isn't instantiated directly and which is called abstract). The previous fragment showed the code for the Order class. Following is the code for the Commercial class.

    package com.jacksonreed; public class Commercial extends Order { public Commercial() { } /* Unique Commercial code goes here */ }

    Implementation inheritance allows the Commercial class to utilize everyone attributes and operations defined in Order. This will live done automatically by the Java Virtual Machine (JVM) in conjunction with the language environment. In addition, implementation inheritance has the aptitude to override and/or extend any of Order's behavior. Commercial may besides add completely novel conduct if it so chooses.

    Interface Inheritance

    Java supports interface inheritance with the implements keyword. A class wanting to realize a given interface (actually being amenable for the mode code) simply adds an implements InterfaceName statement. However, unlike extension of one class by another class, implementation of an interface by a class requires that the interface live specifically defined as an interface beforehand.

    Looking again at the previous sample with Order, let's assume that this system will hold many classes—some built in this release, and some built in future releases—that need the aptitude to cost themselves. recollect from earlier in this chapter that one of the indicators of using interface inheritance is the situation in which there is Little or no common code but the functional intent of the classes is the same. This pricing functionality includes three services: the abilities to figure tax, to figure an extended price, and to figure a total price. Let's summon the operations for these services calcExtendedPrice(), calcTax(), and calcTotalPrice(), respectively, and assign them to a Java interface called IPrice. Sometimes interface names are prefixed with the epistle I to distinguish them from other classes:

    package com.jacksonreed; interface IPrice { long calcExtendedPrice(); long calcTax(); long calcTotalPrice(); }

    Notice that the interface contains only operation signatures; it has no implementation code. It is up to other classes to implement the actual conduct of the operations. For the Order class to implement, or realize, the IPrice interface, it must embrace the implements keyword followed by the interface name:

    public class Order implements IPrice { }

    If you try to implement an interface without providing implementations for everyone of its operations, your class will not compile. Even if you don't want to implement any mode code for some of the operations, you quiet must hold the operations defined in your class.

    One very powerful aspect of interface inheritance is that a class can implement many interfaces at the selfsame time. For example, Order could implement the IPrice interface and perhaps a search interface called ISearch. However, a Java class may extend from only one other class.

    Java and Polymorphism

    Polymorphism is one of those $50 words that dazzles the uninformed and sounds really impressive. In fact, polymorphism is one of the most powerful features of any object-oriented language.

    Roget's II: The novel Thesaurus cross-references the term polymorphism to the main entry of variety. That will execute for starters. Variety is the key to polymorphism. The Latin root for polymorphism means simply "many forms." Polymorphism applies to operations in the object-oriented context. So by combining these two thoughts, you could mutter that operations are polymorphic if they are identical (not just in designation but besides in signatures) but tender variety in their implementations.

    Polymorphism is the aptitude of two different classes each to hold an operation that has the selfsame signature, while having two very different forms of mode code for the operation. Note that to grasp edge of polymorphism, either an interface inheritance or an implementation inheritance relationship must live involved.

    In languages such as COBOL and FORTRAN, defining a routine to have the selfsame designation as another routine will antecedent a compile error. In object-oriented languages such as Java and C++, several classes might hold an operation with the selfsame signature. Such duplication is in fact encouraged because of the power and flexibility it brings to the design.

    As mentioned previously, the implements and extends keywords let the application grasp edge of polymorphism. As they shall see, the sample project presented later in this book is an order system for a company called Remulak Productions. Remulak sells musical equipment, as well as other types of products. There will live a Product class, as well as Guitar, SheetMusic, and Supplies classes.

    Suppose, then, that differences exist in the fundamental algorithms used to determine the best time to reorder each nature of product (called the economic order quantity, or EOQ). I don't want to let too much out of the bag at this point, but there will live an implementation inheritance relationship created with Product as the forefather class (or superclass) and the other three classes as its descendants (or subclasses). The scenario that follows uses implementation inheritance with a polymorphic example. Note that interface inheritance would succumb the selfsame benefits and live implemented in the selfsame fashion.

    To facilitate extensibility and live able to add novel products in the future in a sort of plug-and-play fashion, they can invent calcEOQ() polymorphic. To execute this in Java, Product would define calcEOQ() as abstract, thereby informing any inheriting subclass that it must provide the implementation. A key concept behind polymorphism is this: A class implementing an interface or inheriting from an forefather class can live treated as an instance of that forefather class. In the case of a Java interface, the interface itself is a cogent type.

    For example, assume that a collection of Product objects is defined as a property of the Inventory class. Inventory will back an operation, getAverageEOQ(), that needs to figure the tolerable economic order quantity for everyone products the company sells. To execute this requires that they iterate over the collection of Product objects called myProducts to rep each object's unique economic order quantity individually, with the goal of getting an average:

    public long getAverageEOQ() { long totalAmount=0; for (int i=0; i < myProducts.length; i++) { totalAmount = totalAmount + myProducts[i].calcEOQ(); } recrudesce totalAmount / myProducts.length; }

    But wait! First of all, how can Inventory hold a collection of Product objects when the Product class is abstract (no instances were ever created on their own)? recollect the maxim from earlier: Any class implementing an interface or extending from an forefather class can live treated as an instance of that interface or extended class. A Guitar "is a" Product, SheetMusic "is a" Product, and Supplies "is a" Product. So anywhere you reference Guitar, SheetMusic, or Supplies, you can substitute Product.

    Resident in the array myProducts within the Inventory class are individual concrete Guitar, SheetMusic, and Supplies objects. Java figures out dynamically which remonstrate should rep its own unique calcEOQ() message. The beauty of this construct is that later, if you add a novel nature of Product—say, Organ—it will live totally transparent to the Inventory class. That class will quiet hold a collection of Product types, but it will hold four different ones instead of three, each of which will hold its own unique implementation of the calcEOQ() operation.

    This is polymorphism at its best. At runtime, the class related to the remonstrate in question will live identified and the revise "variety" of the operation will live invoked. Polymorphism provides powerful extensibility features to the application by letting future unknown classes implement a predictable and well-conceived interface without affecting how other classes deal with that interface.

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