Immutable or Not

There are good reasons to make things immutable, but there are also good reasons to make them mutable. Although you should make each decision on a case-by-case basis, the following factors should help:

• If the object is supposed to be a constant, it should always be immutable.
• If the object will be changed frequently, it should be a mutable object. For example, a class such as StringBuffer is changed frequently, so it wouldn't make much sense as an immutable object.
• If the object is very large, be careful if you opt for immutability. Large immutable objects need to be copied in order to be changed; this copying can slow down a program significantly.
• Sets and other collections returned from a method should be immutable to preserve encapsulation.