Cloneable

Cloneable is a marker interface in the Java programming language. It declares no methods and is implemented by a class to indicate that its instances can be cloned using the protected Object.clone() method. If an object's class does not implement Cloneable and its clone() method is invoked, the runtime throws CloneNotSupportedException.

Object.clone() performs a field-by-field copy of the object, producing a shallow copy. To enable public access to cloning, most classes override clone(), declare it public, and call super.clone() to obtain the base copy before adjusting fields. Because the default clone is shallow, mutable fields that are references are shared between the original and the clone unless explicitly cloned within clone().

Implementing a robust clone requires careful handling of nested objects, final fields, and potential state inconsistency. Some classes implement deep copy by cloning their fields or by creating new instances in the clone() method. Others provide copy constructors or static factory methods to achieve copying without relying on Object.clone().

Criticism: Cloneable is considered flawed by many Java experts, as it relies on a specific inheritance structure and the Object.clone() semantics. As a result, many projects avoid Cloneable in favor of copy constructors, factory methods, or serialization-based approaches. Still, it remains part of the standard library and is used in legacy code and some APIs.

See also: java.lang.Object, CloneNotSupportedException, Copy constructor, Serialization.