Effective Java: Programming Language Guide in Java

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Effective Java: Programming Language Guide
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implementation can substantially improve the performance of the clear operation on this list and its subLists This implementation gets a list iterator positioned before fromIndex and repeatedly calls ListIteratornext followed by ListIteratorremove, until the entire range has been removed Note: If ListIteratorremove requires linear time, this implementation requires quadratic time Parameters:
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fromIndex toIndex index of first element to be removed index after last element to be removed
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This method is of no interest to end users of a List implementation It is provided solely to make it easy for subclasses to provide a fast clear method on sublists In the absence of the removeRange method, subclasses would have to make do with quadratic performance when the clear method was invoked on sublists or rewrite the entire subList mechanism from scratch not an easy task! So how do you decide what protected methods or fields to expose when designing a class for inheritance Unfortunately, there is no magic bullet The best you can do is to think hard, take your best guess, and then test it by writing some subclasses You should provide as few protected methods and fields as possible because each one represents a commitment to an implementation detail On the other hand, you must not provide too few, as a missing protected method can render a class practically unusable for inheritance When you design for inheritance a class that is likely to achieve wide use, realize that you are committing forever to the self-use patterns that you document and to the implementation decisions implicit in its protected methods and fields These commitments can make it difficult or impossible to improve the performance or functionality of the class in a subsequent release Also, note that the special documentation required for inheritance clutters up the normal documentation, which is designed for programmers who create instances of your class and invoke methods on them As of this writing, there is little in the way of tools or commenting conventions to separate ordinary API documentation from information of interest only to programmers implementing subclasses There are a few more restrictions that a class must obey to allow inheritance Constructors must not invoke overridable methods, directly or indirectly If this rule is violated, it is likely that program failure will result The superclass constructor runs before the subclass constructor, so the overriding method in the subclass will get invoked before the subclass constructor has run If the overriding method depends on any initialization performed by the subclass constructor, then the method will not behave as expected To make this concrete, here's a tiny class that violates this rule:
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Effective Java: Programming Language Guide public class Super { // Broken - constructor invokes overridable method public Super() { m(); } public void m() { }
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Here's a subclass that overrides m, which is erroneously invoked by Super's sole constructor:
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final class Sub extends Super { private final Date date; // Blank final, set by constructor Sub() { date = new Date(); } // Overrides Superm, invoked by the constructor Super() public void m() { Systemoutprintln(date); } public static void main(String[] args) { Sub s = new Sub(); sm(); } }
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You might expect this program to print out the date twice, but it prints out null the first time because the method m is invoked by the constructor Super() before the constructor Sub() has a chance to initialize the date field Note that this program observes a final field in two different states The Cloneable and Serializable interfaces present special difficulties when designing for inheritance It is generally not a good idea for a class designed for inheritance to implement either of these interfaces, as they place a substantial burden on programmers who extend the class There are, however, special actions that you can take to allow subclasses to implement these interfaces without mandating that they do so These actions are described in Item 10 and Item 54 If you do decide to implement Cloneable or Serializable in a class designed for inheritance, you should be aware that because the clone and readObject methods behave a lot like constructors, a similar restriction applies: Neither clone nor readObject may invoke an overridable method, directly or indirectly In the case of the readObject method, the overriding method will run before the subclass's state has been deserialized In the case of the clone method, the overriding method will run before the subclass's clone methods has a chance to fix the clone's state In either case, a program failure is likely to follow In the case of the clone method, the failure can do damage to the object being cloned as well as to the clone itself Finally, if you decide to implement Serializable in a class designed for inheritance and the class has a readResolve or writeReplace method, you must make the readResolve or
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will be silently ignored by subclasses This is one more case where an implementation detail becomes part of a class's API to permit inheritance By now, it should be apparent that designing a class for inheritance places substantial limitations on the class This is not a decision to be undertaken lightly There are some situations where it is clearly the right thing to do, such as abstract classes, including skeletal implementations of interfaces (Item 16) There are other situations where it is clearly the wrong thing to do, such as immutable classes (Item 13) But what about ordinary concrete classes Traditionally, they are neither final nor designed and documented for subclassing, but this state of affairs is dangerous Each time a change is made in such a class, there is a chance that client classes that extend the class will break This is not just a theoretical problem It is not uncommon to receive subclassing-related bug reports after modifying the internals of a nonfinal concrete class that was not designed and documented for inheritance The best solution to this problem is to prohibit subclassing in classes that are not designed and documented to be safely subclassed There are two ways to prohibit subclassing The easier of the two is to declare the class final The alternative is to make all the constructors private or package-private and to add public static factories in place of the constructors This alternative, which provides the flexibility to use subclasses internally, is discussed in Item 13 Either approach is acceptable This advice may be somewhat controversial, as many programmers have grown accustomed to subclassing ordinary concrete classes to add facilities such as instrumentation, notification, and synchronization or to limit functionality If a class implements some interface that captures its essence, such as Set, List, or Map, then you should feel no compunction about prohibiting subclassing The wrapper class pattern, described in Item 14, provides a superior alternative to inheritance for altering the functionality If a concrete class does not implement a standard interface, then you may inconvenience some programmers by prohibiting inheritance If you feel that you must allow inheritance from such a class, one reasonable approach is to ensure that the class never invokes any of its overridable methods and to document this fact In other words, eliminate the class's self-use of overridable methods entirely In doing so, you'll create a class that is reasonably safe to subclass Overriding a method will never affect the behavior of any other method You can eliminate a class's self-use of overridable methods mechanically, without changing its behavior Move the body of each overridable method to a private helper method and have each overridable method invoke its private helper method Then replace each self-use of an overridable method with a direct invocation of the overridable method's private helper method
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