Can somebody explain immutable classes in JAVA

Question

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

Answer 1

Immutable objects are simply objects whose state (the object's data) cannot change after construction. Examples of immutable objects from the JDK include String and Integer.

Guidelines to make a class immutable:
1) Don’t provide “setter” methods — methods that modify fields or objects referred to by fields.
2) Make all fields final and private
3) Don’t allow subclasses to override methods
4) Special attention when having mutable instance variables