Java Object Oriented Design - Java Object Clone

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Java does not provide an automatic mechanism to clone (copy) an object.

Cloning an object means copying the content of the object bit by bit.

To support clone operation, implement the clone() method in the class.

The declaration of the clone() method in the Object class is as follows:

protected  Object clone()  throws   CloneNotSupportedException

clone() method is declared protected. Therefore, we cannot call it from the client code. The following code is not valid:

Object obj  = new Object();
Object clone = obj.clone(); // Error. Cannot  access protected clone() method

We need to declare the clone() method public in a class to clone objects of the class.

Its return type is Object. It means you will need to cast the returned value of the clone() method.

Suppose MyClass is cloneable. The cloning code will look as

MyClass mc  = new MyClass();
MyClass clone = (MyClass)mc.clone(); // Need to use  a  cast

The clone() method in the Object class throws a CloneNotSupportedException.

To call the clone() method, we need to place the call in a try-catch block, or rethrow the exception.

Example

The following code shows how to implement a clone method.

class MyClass implements Cloneable {
  private double value;
// w  ww . j a v a 2s  .  c om
  public MyClass(double value) {
    this.value = value;
  }

  public void setValue(double value) {
    this.value = value;
  }

  public double getValue() {
    return this.value;
  }

  public Object clone() {
    MyClass copy = null;
    try {
      copy = (MyClass) super.clone();
    } catch (CloneNotSupportedException e) {
      e.printStackTrace();
    }
    return copy;
  }
}

public class Main {
  public static void main(String[] args) {
    MyClass dh = new MyClass(100.00);

    MyClass dhClone = (MyClass) dh.clone();

    System.out.println("Original:" + dh.getValue());
    System.out.println("Clone :" + dhClone.getValue());

    dh.setValue(200.00);
    dhClone.setValue(400.00);

    System.out.println("Original:" + dh.getValue());
    System.out.println("Clone :" + dhClone.getValue());
  }
}

The code above generates the following result.

Example 2

The following code is not returning Object type from clone method, which is compiled only in Java 5 or later.

class MyClass  implements Cloneable  {
    public MyClass clone()  { 
       Object copy  = null;
       return  (MyClass)copy;
    }
}

The following code shows how to do shallow clone.

class MyClass implements Cloneable {
  private double value;
//from   w  w w  . j av a2 s . c  o  m
  public MyClass(double value) {
    this.value = value;
  }

  public void setValue(double value) {
    this.value = value;
  }

  public double getValue() {
    return this.value;
  }

  public Object clone() {
    MyClass copy = null;
    try {
      copy = (MyClass) super.clone();
    } catch (CloneNotSupportedException e) {
      e.printStackTrace();
    }
    return copy;
  }
}

class ShallowClone implements Cloneable {
  private MyClass holder = new MyClass(0.0);

  public ShallowClone(double value) {
    this.holder.setValue(value);
  }

  public void setValue(double value) {
    this.holder.setValue(value);
  }

  public double getValue() {
    return this.holder.getValue();
  }

  public Object clone() {
    ShallowClone copy = null;
    try {
      copy = (ShallowClone) super.clone();
    } catch (CloneNotSupportedException e) {
      e.printStackTrace();
    }
    return copy;
  }
}

public class Main {
  public static void main(String[] args) {
    ShallowClone sc = new ShallowClone(100.00);
    ShallowClone scClone = (ShallowClone) sc.clone();

    System.out.println("Original:" + sc.getValue());
    System.out.println("Clone :" + scClone.getValue());

    sc.setValue(200.00);

    System.out.println("Original:" + sc.getValue());
    System.out.println("Clone :" + scClone.getValue());
  }
}

The code above generates the following result.

Example 3

The code in the clone() method of the ShallowClone class is the same as for the clone() method of the MyClass class.

When the ShallowClone class calls the clone() method of the Object class using super.clone(), it receives a shallow copy of itself. That is, it shares the DoubleHolder object used in its instance variable with its clone.

In a deep cloning, you need to clone all objects referenced by all reference instance variables of an object.

class MyClass implements Cloneable {
  private double value;
//from   w ww  .ja  v  a  2  s  .c o  m
  public MyClass(double value) {
    this.value = value;
  }

  public void setValue(double value) {
    this.value = value;
  }

  public double getValue() {
    return this.value;
  }

  public Object clone() {
    MyClass copy = null;
    try {
      copy = (MyClass) super.clone();
    } catch (CloneNotSupportedException e) {
      e.printStackTrace();
    }
    return copy;
  }
}

class DeepClone implements Cloneable {
  private MyClass holder = new MyClass(0.0);

  public DeepClone(double value) {
    this.holder.setValue(value);
  }

  public void setValue(double value) {
    this.holder.setValue(value);
  }

  public double getValue() {
    return this.holder.getValue();
  }
  public Object clone() {
    DeepClone copy = null;
    try {
      copy = (DeepClone) super.clone();
      copy.holder = (MyClass) this.holder.clone();
    } catch (CloneNotSupportedException e) {
      e.printStackTrace();
    }
    return copy;
  }
}

public class Main {
  public static void main(String[] args) {
    DeepClone sc = new DeepClone(100.00);
    DeepClone scClone = (DeepClone) sc.clone();

    System.out.println("Original:" + sc.getValue());
    System.out.println("Clone :" + scClone.getValue());

    sc.setValue(200.00);

    System.out.println("Original:" + sc.getValue());
    System.out.println("Clone :" + scClone.getValue());
  }
}

The code above generates the following result.

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