The following rules define a simple strategy for creating immutable objects. Not all classes documented as "immutable" follow these rules. This does not necessarily mean the creators of these classes were sloppy — they may have good reason for believing that instances of their classes never change after construction. However, such strategies require sophisticated analysis and are not for beginners.
- Don't provide "setter" methods — methods that modify fields or objects referred to by fields.
- Make all fields
finalandprivate. - Don't allow subclasses to override methods. The simplest way to do this is to declare the class as
final. A more sophisticated approach is to make the constructorprivateand construct instances in factory methods. - If the instance fields include references to mutable objects, don't allow those objects to be changed:
- Don't provide methods that modify the mutable objects.
- Don't share references to the mutable objects. Never store references to external, mutable objects passed to the constructor; if necessary, create copies, and store references to the copies. Similarly, create copies of your internal mutable objects when necessary to avoid returning the originals in your methods.
