``++Cloneable++`` is the marker ``++Interface++`` that indicates that ``++clone()++`` may be called on an object. Overriding ``++clone()++`` without implementing ``++Cloneable++`` can be useful if you want to control how subclasses clone themselves, but otherwise, it's probably a mistake.


The usual convention for ``++Object.clone()++`` according to Oracle's Javadoc is:

. ``++x.clone() != x++``
. ``++x.clone().getClass() == x.getClass()++``
. ``++x.clone().equals(x)++``

Obtaining the object that will be returned by calling ``++super.clone()++`` helps to satisfy those invariants:

. ``++super.clone()++`` returns a new object instance
. ``++super.clone()++`` returns an object of the same type as the one ``++clone()++`` was called on
. ``++Object.clone()++`` performs a shallow copy of the object's state


== Noncompliant Code Example

[source,java]
----
class BaseClass {  // Noncompliant; should implement Cloneable
  @Override
  public Object clone() throws CloneNotSupportedException {    // Noncompliant; should return the super.clone() instance
    return new BaseClass();
  }
}

class DerivedClass extends BaseClass implements Cloneable {
  /* Does not override clone() */

  public void sayHello() {
    System.out.println("Hello, world!");
  }
}

class Application {
  public static void main(String[] args) throws Exception {
    DerivedClass instance = new DerivedClass();
    ((DerivedClass) instance.clone()).sayHello();              // Throws a ClassCastException because invariant #2 is violated
  }
}
----


== Compliant Solution

[source,java]
----
class BaseClass implements Cloneable {
  @Override
  public Object clone() throws CloneNotSupportedException {    // Compliant
    return super.clone();
  }
}

class DerivedClass extends BaseClass implements Cloneable {
  /* Does not override clone() */

  public void sayHello() {
    System.out.println("Hello, world!");
  }
}

class Application {
  public static void main(String[] args) throws Exception {
    DerivedClass instance = new DerivedClass();
    ((DerivedClass) instance.clone()).sayHello();              // Displays "Hello, world!" as expected. Invariant #2 is satisfied
  }
}
----


== See

* https://cwe.mitre.org/data/definitions/580[MITRE, CWE-580] - clone() Method Without super.clone()
* https://wiki.sei.cmu.edu/confluence/x/FjZGBQ[CERT, MET53-J.] - Ensure that the clone() method calls super.clone()


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