Question

I am looking for some efficient way for building a immutable class, just like Java’s String class.

Answer 1

1. All the fields must be
   private and preferably final
2. Ensure the class cannot be
   overridden – make the class final,
   or use static factories and keep
   constructors private
3. Fields must be populated from the
   Constructor/Factory
4. Don’t provide any setters for the
   fields
5. Watch out for collections. Use
   Collections.unmodifiable*. Also, collections should contain only immutable Objects
6. All the getters must provide
   immutable objects or use defensive copying
7. Don’t provide any methods that
   change the internal state of the
   Object.

Tom Hawtin pointed out that final can be optional. String class has a cache hash var that is only assigned when the hash function is called.