What happens when a class is immutable?

An immutable class is a class whose state cannot be modified after it is created. Once an object of an immutable class is created, it cannot be modified in any way.

Here are some popular characteristics of immutable classes:

• All fields of the class are final and therefore cannot be modified after the object is created.
• The class has no setter methods, so the values of the fields cannot be changed once they are set in the constructor.
• The class should not provide any methods that can modify its state.
• If a class has mutable fields (like collections) it should return a new instance of the collection with the new state, instead of modifying the existing one.
• The class should be thread-safe since it can be shared among multiple threads without the need for explicit synchronization.