MET53-J. Ensure that the clone() method calls super.clone()

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

The Java API [ API 2013 ] for the clone() method 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 fails to call super.clone() :

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:

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();
      devClone.doLogic();  // Prints "Subclass doLogic" as expected
    } catch (CloneNotSupportedException e) { /* ... */ }
  }
}

Applicability

Failing to call super.clone() may cause a cloned object to have the wrong type.

Automated Detection

Bibliography