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Concurrency
Immutable Objects
An object is considered
immutable if its state cannot change
after it is constructed. Maximum reliance on immutable objects is
widely accepted as a sound strategy for creating simple, reliable
code.
Immutable objects are particularly useful in concurrent
applications. Since they cannot change state, they cannot be corrupted
by thread interference or observed in an inconsistent state.
Programmers are often reluctant to employ immutable objects, because
they worry about the cost of creating a new object as opposed to
updating an object in place. The impact of object creation is often
overestimated, and can be offset by some of the efficiencies
associated with immutable objects. These include decreased overhead
due to garbage collection, and the elimination of code needed to
protect mutable objects from corruption.
The following subsections take a class whose instances are mutable and
derives a class with immutable instances from it. In so doing, they
give general rules for this kind of conversion and demonstrate some
of the advantages of immutable objects.
