What is Immutable objects ? Why String is immutable in Java ?

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.

Immutable objects greatly simplify your program since they:

1) Are simple to construct, test, and use.

2) Are automatically thread-safe and have no synchronization issues.

3) Do not need a copy constructor.

4)Allow hashCode to use lazy initialization, and to cache its return value

5)Do not need to be copied defensively when used as a field

 You can make a class immutable by following these guidelines:

1) Ensure the class cannot be overridden - make the class final, or use static factories and keep constructors private

2)Make fields private and final

3)Do not provide any methods which can change the state of the object in any way - not just setXXX methods, but any method which can change state