Immutable object in Java

Immutable objects whose state (i.e. the object’s data) does not change once it is instantiated (i.e. it becomes a read-only object after instantiation).
Immutable classes are ideal for representing numbers (e.g. java.lang.Integer, java.lang.Float, java.lang.BigDecimal etc are immutable objects), enumerated types, colors (e.g. java.awt.Color is an immutable object), short lived objects like events, messages etc.
The immutable class are written in following guidelines:

1. A class is declared as final. (The final class cannot be extended)
2. All its fields are final (final fields cannot be mutated once assigned).
3. Do not provide any methods that can change the state of the immutable object in any way – not just setXXX methods, but any methods which can change the state.
4. The “this” reference is not allowed to escape during construction from the immutable class and the immutable class should have exclusive access to fields that contain references to mutable objects like arrays, collections and mutable classes like Date etc by:
   • Declaring the mutable references as private.
   • Not returning or exposing the mutable references to the caller

Benefits:

1. Immutable classes can greatly simplify programming by freely allowing you to cache and share the references to the immutable objects without having to

defensively copy them or without having to worry about their values becoming stale or corrupted.

1. Immutable classes are inherently thread-safe and you do not have to

synchronize access to them to be used in a multi-threaded environment. So there is no chance of negative performance consequences.

1. Eliminates the possibility of data becoming inaccessible when used as keys in HashMaps or as elements in Sets