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Java Cookbook
book

Java Cookbook

by Ian F. Darwin
June 2001
Intermediate to advanced content levelIntermediate to advanced
888 pages
21h 1m
English
O'Reilly Media, Inc.
Content preview from Java Cookbook

The Clone Method

Problem

You want to clone yourself. Or at least your objects.

Solution

Override Object.clone( ) .

Discussion

To clone something is to make a duplicate of it. The clone( ) method in Java makes an exact duplicate of an object. Why do we need cloning? Java’s method calling semantics are call-by-reference, which allows the called method to modify the state of an object that is passed into it. Cloning the input object before calling the method would pass a copy of the object, keeping your original safe.

How can you clone? Cloning is not “enabled” by default in classes that you write.

Object o = new Object(  );
Object o2 = o.clone(  );

If you try calling clone( ) without any special preparation, as in this excerpt from Clone0.java, you will see a message like this (from the Jikes compiler; the javac message may not be as informative):

Clone0.java:4:29:4:37: Error: Method "java.lang.Object clone(  );" in class "java/
lang/Object" has protected or default access. Therefore, it is not accessible in 
class "Clone0" which is in a different package.

You must take two steps to make your class cloneable:

  1. Override Object’s clone( ) method.

  2. Implement the empty Cloneable interface.

Using cloning

The class java.lang.Object declares its clone protected and native . Protected classes can be called by a subclass or those in the same package (i.e., java.lang), but not by unrelated classes. That is, you can call Object.clone( ) -- the native method that does the magic of duplicating the object -- only ...

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Publisher Resources

ISBN: 0596001703Supplemental ContentCatalog PageErrata