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When Cloning a subclass a nonfinal class that defines a clone() method that fails to call super.clone(), cloning a subtype will produce an object of the wrong typeclass.

The Java API [API 2013] for the clone() method [API 2011] says:

By convention, the returned object should be obtained by calling super.clone. If a class and all of its superclasses (except Object) obey this convention, it will be the case that x.clone().getClass() == x.getClass().

Noncompliant Code Example

In this noncompliant code example, the clone() method in the class Base does not fails to call super.clone(). Hence, the object devClone ends up being of type Base instead of Derived, with resulting incorrect application of the doLogic() method.:

Code Block

bgColor	#FFcccc

class Base implements Cloneable {
  public Object clone() throws CloneNotSupportedException {
    return new Base();	 
  }
  protected void doLogic() {
    System.out.println("Superclass doLogic");
  }
}

class Derived extends Base {
  public Object clone() throws CloneNotSupportedException {
    return super.clone();
  }
  protected void doLogic() {
    System.out.println("Subclass doLogic");
  }
  public static void main(String[] args) {
    Derived dev = new Derived();
    try {
      Base devClone = (Base)dev.clone(); // Has type Base instead of Derived
      devClone.doLogic();  // Prints "Superclass doLogic" instead of "Subclass doLogic"
    } catch (CloneNotSupportedException e) { /* ... */ }
  }
}

Consequently, the object devClone ends up being of type Base instead of Derived, and the doLogic() method is incorrectly applied.

Compliant Solution

This compliant solution correctly calls super.clone() in the Base class's clone() method:

Code Block

bgColor	#ccccff

class Base implements Cloneable {
  public Object clone() throws CloneNotSupportedException {
    return super.clone();	 
  }
  protected void doLogic() {
    System.out.println("Superclass doLogic");
  }
}

class Derived extends Base {
  public Object clone() throws CloneNotSupportedException {
    return super.clone();
  }
  protected void doLogic() {
    System.out.println("Subclass doLogic");
  }
  public static void main(String[] args) {
    Derived dev = new Derived();
    try {
      // Has type Derived, as expected
      Base devClone = (Base)dev.clone(); // Has type Derived, as expected
      devClone.doLogic();  // Prints "Subclass doLogic", as expected
    } catch (CloneNotSupportedException e) { /* ... */ }
  }
}

Applicability

Failing to call super.clone() may result in cause a cloned object having the wrong type, with resulting unexpected or incorrect results when it is used.to have the wrong type.