Effective Java: Programming Language Guide in Java

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exceptions to this rule For example, a field representing a serial number or other unique ID or a field representing the object's creation time will need to be fixed, even if it is primitive or immutable Is all this complexity really necessary Rarely If you extend a class that implements Cloneable, you have little choice but to implement a well-behaved clone method Otherwise,you are probably better off providing some alternative means of object copying or simply not providing the capability For example, it doesn't make much sense for immutable classes to support object copying, because copies would be virtually indistinguishable from the original A fine approach to object copying is to provide a copy constructor A copy constructor is simply a constructor that takes a single argument whose type is the class containing the constructor, for example,
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public Yum(Yum yum);
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A minor variant is to provide a static factory in place of a constructor:
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public static Yum newInstance(Yum yum);
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The copy constructor approach and its static factory variant have many advantages over Cloneable/clone: They do not rely on a risk-prone extralinguistic object creation mechanism; they do not demand unenforceable adherence to ill-documented conventions; they do not conflict with the proper use of final fields; they do not require the client to catch an unnecessary checked exception; and they provide a statically typed object to the client While it is impossible to put a copy constructor or static factory in an interface, Cloneable fails to function as an interface because it lacks a public clone method Therefore you aren't giving up interface functionality by using a copy constructor instead of a clone method Furthermore, a copy constructor (or static factory) can take an argument whose type is an appropriate interface implemented by the class For example, all general-purpose collection implementations, by convention, provide a copy constructor whose argument is of type Collection or Map Interface-based copy constructors allow the client to choose the implementation of the copy, rather than forcing the client to accept the implementation of the original For example, suppose you have a LinkedList l, and you want to copy it as an ArrayList The clone method does not offer this functionality, but it's easy with a copy constructor: new ArrayList(l) Given all of the problems associated with Cloneable, it is safe to say that other interfaces should not extend it and that classes designed for inheritance (Item 15) should not implement it Because of its many shortcomings, some expert programmers simply choose never to override the clone method and never to invoke it except, perhaps, to copy arrays cheaply Be aware that if you do not at least provide a well-behaved protected clone method on a class designed for inheritance, it will be impossible for subclasses to implement Cloneable
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Item 11: Consider implementing Comparable
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Unlike the other methods discussed in this chapter, the compareTo method is not declared in Object Rather, it is the sole method in the javalangComparable interface It is similar in character to Object's equals method, except that it permits order comparisons in addition to simple equality comparisons By implementing Comparable, a class indicates that its instances have a natural ordering Sorting an array of objects that implement Comparable is as simple as this:
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Arrayssort(a);
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It is similarly easy to search, compute extreme values, and maintain automatically sorted collections of Comparable objects For example, the following program, which relies on the fact that String implements Comparable, prints an alphabetized list of its command-line arguments with duplicates eliminated:
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public class WordList { public static void main(String[] args) { Set s = new TreeSet(); saddAll(ArraysasList(args)); Systemoutprintln(s); } }
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By implementing Comparable, you allow your class to interoperate with all of the many generic algorithms and collection implementations that depend on this interface You gain a tremendous amount of power for a small amount of effort Virtually all of the value classes in the Java platform libraries implement Comparable If you are writing a value class with an obvious natural ordering, such as alphabetical order, numerical order, or chronological order, you should strongly consider implementing this interface This item tells you how to go about it The general contract for the compareTo method is similar in character to that of the equals method Here it is, copied from the specification for Comparable: Compares this object with the specified object for order Returns a negative integer, zero, or a positive integer as this object is less than, equal to, or greater than the specified object Throws ClassCastException if the specified object's type prevents it from being compared to this object In the following description, the notation sgn(expression) designates the mathematical signum function, which is defined to return -1, 0, or 1, according to whether the value of expression is negative, zero, or positive The implementor must ensure sgn(xcompareTo(y)) == -sgn(ycompareTo(x)) for all x and y (This implies that xcompareTo(y) must throw an exception if and only if ycompareTo(x) throws an exception)
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