How to clone a Java object with the clone() method

Question

I don\'t understand the mechanism of cloning custom object. For example:

public class Main{

    public static void main(String [] args) {

        Person pe         


        

Answer 1

The clone method is meant to make a deep copy. Make sure you understand the difference between deep and shallow copies. In your case a copy constructor may be the pattern you want. In some cases you can't use this pattern however, for example because you're subclassing class X and you don't have access to the constructor of X that you need. If X overrides its clone method correctly (if necessary) then you could make a copy in the following way:

class Y extends X implements Cloneable {

    private SomeType field;    // a field that needs copying in order to get a deep copy of a Y object

    ...

    @Override
    public Y clone() {
        final Y clone;
        try {
            clone = (Y) super.clone();
        }
        catch (CloneNotSupportedException ex) {
            throw new RuntimeException("superclass messed up", ex);
        }
        clone.field = this.field.clone();
        return clone;
    }

}

In general when overriding your clone method:

• Make the return type more specific
• Start with calling super.clone()
• Do not include throws clause when you know clone() will also work for any subclass (weakness of the clone-pattern; make class final if possible)
• Leave immutable and primitive fields alone, but clone mutable object fields manually after the call to super.clone() (another weakness of the clone-pattern, since these fields cannot be made final)

The clone() method of Object (which will eventually be called when all superclasses obey the contract) makes a shallow copy and takes care of the correct runtime type of the new object. Note how no constructor is called in the entire process.

If you want to be able to call clone() on instances, then implement the Cloneable interface and make the method public. If you don't want to be able to call it on instances, but you do want to make sure subclasses can call their super.clone() and get what they need, then don't implement Cloneable and keep the method protected if your superclass hasn't declared it public already.

The clone pattern is difficult and has a lot of pitfalls. Be sure it's what you need. Consider copy constructors, or a static factory method.