MET55-J. Ensure that the clone method calls super.clone

This is a stub. It needs an example of an error caused by using the base class's object where the subclass's object was expected. It should not produce a RuntimeException (ClassCastException) to qualify.

When a nonfinal class defines a clone() method that does not call super.clone(), cloning a subtype will produce an object of the wrong type.

The Java API for the clone() method [API 2006] 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 call super.clone(). Hence, the object devClone ends up being of type Base instead of Derived, with resulting incorrect application of the doLogic() method.

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) { /* ... */ }
  }
}

Compliant Solution

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

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 {
      Base devClone = (Base)dev.clone(); // has type Derived, as expected
      devClone.doLogic();  // prints "Subclass doLogic", as expected
    } catch (CloneNotSupportedException e) { /* ... */ }
  }
}