A class that implements Cloneable should override Object.clone. For non-trivial objects, the Cloneable contract requires a deep copy of the object's internal state. A class that does not have a clone method indicates that the class is breaking the contract and will have undesired behavior.

The necessity of creating a deep copy of an object's internal state means that, for most objects, clone must be overridden to satisfy the Cloneable contract. Implement a clone method that properly creates the internal state of the cloned object.

Notable exceptions to this recommendation are:

  • Classes that contain only primitive types (which will be properly cloned by Object.clone as long as its Cloneable supertypes all call super.clone).
  • Subclasses of Cloneable classes that do not introduce new state.

In the following example, WrongStack does not implement clone. This means that when ws1clone is cloned from ws1, the default clone implementation is used. This results in operations on the ws1clone stack affecting the ws1 stack.

In the following modified example, RightStack does implement clone. This means that when rs1clone is cloned from rs1, operations on the rs1clone stack do not affect the rs1 stack.
  • J. Bloch, Effective Java (second edition), Item 11. Addison-Wesley, 2008.
  • Java API Specification: Object.clone().