The clone() Method in Java

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If a class, or one of its superclasses, implements the Cloneable interface, you can use the clone() method to create a copy from an existing object. To create a clone, you write:

aCloneableObject.clone();

Object's implementation of this method checks to see whether the object on which clone() was invoked implements the Cloneable interface. If the object does not, the method throws a CloneNotSupportedException exception. Exception handling will be covered in a later lesson. For the moment, you need to know that clone() must be declared as

protected Object clone() throws CloneNotSupportedException
    -- or --
public Object clone() throws CloneNotSupportedException

if you are going to write a clone() method to override the one in Object.

If the object on which clone() was invoked does implement the Cloneable interface, Object's implementation of the clone() method creates an object of the same class as the original object and initializes the new object's member variables to have the same values as the original object's corresponding member variables.

The simplest way to make your class cloneable is to add implements Cloneable to your class's declaration. then your objects can invoke the clone() method.

For some classes, the default behavior of Object's clone() method works just fine. If, however, an object contains a reference to an external object, say ObjExternal, you may need to override clone() to get correct behavior. Otherwise, a change in ObjExternal made by one object will be visible in its clone also. This means that the original object and its clone are not independent—to decouple them, you must override clone() so that it clones the object and ObjExternal. Then the original object references ObjExternal and the clone references a clone of ObjExternal, so that the object and its clone are truly independent.



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