How do you create an immutable class in Java?

An object is considered immutable if its state cannot change after it is constructed. Immutable objects are particularly useful in concurrent applications. Since they cannot change state, they cannot be corrupted by thread interference or observed in an inconsistent state.

The below are the rules for creating an immutable class.

• Don't provide "setter" methods.
• Make all fields final and private.
• Don't allow subclasses to override methods. The simplest way to do this is to declare the class as final. A more sophisticated approach is to make the constructor private and construct instances in factory methods.
• If the instance fields include references to mutable objects, don't allow those objects to be changed:
• Don't provide methods that modify the mutable objects.
• Don't share references to the mutable objects. Never store references to external, mutable objects passed to the constructor; if necessary, create copies, and store references to the copies. Similarly, create copies of your internal mutable objects when necessary to avoid returning the originals in your methods.