General Java Questions I

Q: I try to copy an object of my own using the clone() method from

java.lang.Object, but this is a protected method so I can t use it. Is there some other

way to get my objective of duplicating an arbitrary object?

Answer: If you want to clone your object, you need to make it cloneable. To achieve

this, you need to do two things:

1. implement the interface Cloneable

2. override the method clone(), so that it 

a. becomes public

b. calls super.clone()

c. if necessary, clones any members, or

d. if a member can t be cloned, creates a new instance.

Simple example:

public MyClass implements Cloneable {

   int someNumber;

   String someString;

   public Object clone() {

   // primitives and Strings are no

   // problem

      return super.clone();

  }

}

In this case the method clone() of the class MyClass returns a new instance of

MyClass, where all members have exactly the same value. That means, the object

reference  someString  points to the same object. This is called a shallow

copy. In many cases this is no problem. Strings are immutable and you do not

need a new copy. But if you need new copies of members, you have to do it in

the clone() method. Here is another simple example:

public class SomeMember implements Cloneable {

   long someLong;

   public Object clone() {

      return super.clone();

   }

}

public AnotherClass extends MyClass {

    SomeMember someMember;

    public Object clone() {

        AnotherClass ac = (AnotherClass)(super.clone());

        if (someMember != null) {

            ac.someMember = (SomeMember)(someMember.clone());

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