Please explain the difference between Immutable and Singleton in Java?

Question

Also it would be helpful if can tell when to use immutable class and when singleton?

Answer 1

An immutable object is initialized by its constructor only, while a singleton is instantiated by a static method.

1.  A set of functions (or static methods) which manipulate some shared mutable state constitute a singleton.

2.  If a singleton A provides a reference to mutable object B, then B is a singleton as well.

3.  This implies that every mutable member of a singleton collection is itself a singleton.

4.  A transitively immutable object is NOT a singleton even if globally accessible. It is a constant.

5.  A stand-alone function that does not access any singletons is NOT itself a singleton, assuming that code is immutable.