Effective Java: Programming Language Guide in Java

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Effective Java: Programming Language Guide
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4 Classes and Interfaces
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Classes and interfaces lie at the heart of the Java programming language They are its basic units of abstraction The language provides many powerful elements that you can use to design classes and interfaces This chapter contains guidelines to help you make the best use of these elements so that your classes and interfaces are usable, robust, and flexible
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Item 12: Minimize the accessibility of classes and members
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The single most important factor that distinguishes a well-designed module from a poorly designed one is the degree to which the module hides its internal data and other implementation details from other modules A well-designed module hides all of its implementation details, cleanly separating its API from its implementation Modules then communicate with one another only through their APIs and are oblivious to each others' inner workings This concept, known as information hiding or encapsulation, is one of the fundamental tenets of software design [Parnas72] Information hiding is important for many reasons, most of which stem from the fact that it effectively decouples the modules that comprise a system, allowing them to be developed, tested, optimized, used, understood, and modified individually This speeds up system development because modules can be developed in parallel It eases the burden of maintenance because modules can be understood quickly and debugged with little fear of harming other modules While information hiding does not, in and of itself, cause good performance, it enables effective performance tuning Once a system is complete and profiling has determined which modules are causing performance problems (Item 37), those modules can be optimized without affecting the correctness of other modules Information hiding increases software reuse because individual modules do not depend on one another and frequently prove useful in contexts other than the one for which they were developed Finally, information hiding decreases the risk in building large systems; individual modules may prove successful even if the system does not The Java programming language has many facilities to aid information hiding One such facility is the access control mechanism [JLS, 66], which determines the accessibility of classes, interfaces, and members The accessibility of an entity is determined by the location where it is declared and by which, if any, of the access modifiers (private, protected, and public) is present in the entity's declaration Proper use of these modifiers is essential to information hiding The rule of thumb is that you should make each class or member as inaccessible as possible In other words, you should use the lowest possible access level consistent with the proper functioning of the software that you are writing For top-level (non-nested) classes and interfaces, there are only two possible access levels: package-private and public If you declare a top-level class or interface with the public modifier, it will be public; otherwise, it will be package-private If a top-level class or interface can be made package-private, it should be By making it package-private, you make it part of the package's implementation rather than its exported API, and you can modify it, replace it, or eliminate it in a subsequent release without fear of harming existing clients If you make it public, you are obligated to support it forever to maintain compatibility
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Effective Java: Programming Language Guide
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If a package-private top-level class or interface is used only from within a single class, you should consider making it a private nested class (or interface) of the class in which it is used (Item 18) This further reduces its accessibility It is, however, not as important to do this as it is to make an unnecessarily public class package-private because a package-private class is already part of the package's implementation rather than its API For members (fields, methods, nested classes, and nested interfaces) there are four possible access levels, listed here in order of increasing accessibility:
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private The member is accessible only inside the top-level class where it is declared package-private The member is accessible from any class in the package where it is declared Technically known as default access, this is the access level you get if no access modifier is specified protected The member is accessible from subclasses of the class where it is declared (subject to a few restrictions [JLS, 662]) and from any class in the package where it is declared public The member is accessible from anywhere
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After carefully designing your class's public API, your reflex should be to make all other members private Only if another class in the same package really needs to access a member should you remove the private modifier, making the member package-private If you find yourself doing this often, you should reexamine the design of your system to see if another decomposition might yield classes that are better decoupled from one another That said, both private and package-private members are part of a class's implementation and do not normally impact its exported API These fields can, however, leak into the exported API if the class implements Serializable (Item 54, Item 55) For members of public classes, a huge increase in accessibility occurs when the access level goes from package-private to protected A protected member is part of the class's exported API and must be supported forever Furthermore, a protected member of an exported class represents a public commitment to an implementation detail (Item 15) The need for protected members should be relatively rare There is one rule that restricts your ability to reduce the accessibility of methods If a method overrides a superclass method, it is not permitted to have a lower access level in the subclass than it does in the superclass [JLS, 8463] This is necessary to ensure that an instance of the subclass is usable anywhere that an instance of the superclass is usable If you violate this rule, the compiler will generate an error message when you try to compile the subclass A special case of this rule is that if a class implements an interface, all of the class methods that are also present in the interface must be declared public This is so because all methods in an interface are implicitly public Public classes should rarely, if ever, have public fields (as opposed to public methods) If a field is nonfinal or is a final reference to a mutable object, you give up the ability to limit the values that may be stored in the field by making it public You also give up the ability to take any action when the field is modified A simple consequence is that classes with public mutable fields are not thread-safe Even if a field is final and does not refer to a mutable object, by making the field public, you give up the flexibility to switch to a new internal data representation in which the field does not exist
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