Mutable, Immutable, and Cloneable Objects - Data Structures and Abstractions with Java - page 800

Note: A collection can clone the objects that a client adds to it, but you will have duplicates
of each entry in the collection. For complex objects, the time and memory needed to make
each copy can be substantial.

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Realize that some devious programming could defeat the purpose of our sorted list of clones. You
could write a clone method that retains a reference to the clone. Then, knowing when a sorted list
calls clone , you could get and retain the reference to the clone. Restricting the contents of a collection
to immutable objects is a safer way to maintain its integrity.

C HAPTER S UMMARY

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An object that belongs to a class having public mutator (set) methods is mutable because the client can use
the methods to change the values of the object's data fields. If a client cannot change the values of an
object's data fields, the object's class is read only, and the object is immutable.

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A read-only class is final, has private data fields that, if mutable, are final, and has no public set methods.

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Companion classes represent the same data in both immutable and mutable forms.

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The class Object includes a protected method clone that makes an identical copy of an object. A class can
override clone and declare it public, thus making it available to a client of the class. Such a class must imple-
ment the Java interface Cloneable . If a class does not override clone , it has no clone method, since clone is
protected in Object .

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A clone method should invoke super.clone() to ensure that all aspects of an object are copied.

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Every array has a clone method that copies the array but not the objects in it. A separate step is necessary to
clone these objects.

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To create a deep clone of a chain of linked nodes, you must clone the nodes, including their data objects.

P ROGRAMMING T IPS

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Use an immutable object if it will be shared or added to a collection that can be corrupted by changes to the
object. Use a mutable object if its data will change frequently.

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Not all classes should have a public clone method. In fact, most classes, including read-only classes, do not
have one.

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If your program produces the exception CloneNotSupportedException even though you implemented a
method clone in your class, you probably forgot to write implements Cloneable in your class definition.

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Every public clone method must invoke the method clone of the base class by executing super.clone .
Ultimately, Object 's protected clone method will be invoked. That invocation must appear in a try block,
even though a CloneNotSupportedException will never occur.

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Shallow copies of data fields that reference immutable objects are typically sufficient for a clone. Sharing an
immutable object is usually safe.

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When bounding generic types, use an interface that declares a public method clone instead of using
Cloneable . The new interface must extend Cloneable , however.