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clone (Java method) 54
clone (Java method)
clone() is a method in the Java programming language for object duplication. In Java, objects are manipulated
through reference variables, and there is no operator for copying an object -- the assignment operator duplicates the
reference, not the object. The clone() method provides this functionality.
Overview
Classes that want copying functionality must implement some method to do so. To a certain extent that function is
provided by "clone()".
clone() acts like a constructor. Typically it calls the clone() method of its superclass to obtain the copy, etc. until it
eventually reaches Object's clone() method. The special clone() method in the base class Object provides a standard
mechanism for duplicating objects.
The class Object's clone() method creates and returns a copy of the object, with the same class and with all the fields
having the same values. However, clone() throws a CloneNotSupportedException unless the class you are trying to
use it on implements the marker interface Cloneable.
The default implementation of Object.clone() performs a shallow copy. When a class desires a deep copy or some
other custom behavior, they must perform that in their own clone() method after they obtain the copy from the
superclass.
The syntax for calling clone in Java is:
Object copy = obj.clone();
or commonly
MyClass copy = (MyClass) obj.clone();
which provides the typecasting needed to assign the generic Object reference returned from clone to a reference to a
MyClass object.
One disadvantage with the design of the clone() method is that the return type of clone() is Object, and needs to be
explicitly cast back into the appropriate type. However, overriding clone() to return the appropriate type is preferable
and eliminates the need for casting in the client (using covariant return types, since J2SE 5.0).
Another disadvantage is that one often cannot access the clone() method on an abstract type. Most interfaces and
abstract classes in Java do not specify a public clone() method. As a result, often the only way to use the clone()
method is if you know the actual class of an object; which is contrary to the abstraction principle of using the most
generic type possible. For example, if one has a List reference in Java, one cannot invoke clone() on that reference
because List specifies no public clone() method. Actual implementations of List like ArrayList and LinkedList all
generally have clone() methods themselves, but it is inconvenient and bad abstraction to carry around the actual class
type of an object.