Is it necessary to make immutable class as final

Question

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why to declare immutable class as final in java

Why declare an immutable class as final in Java ? Please explain with some example if it is really necessary to declare it as final (consider all fields to be private and that the class has no setter methods).

Answer 1

First this What is immutable

Immutable means that once an object of that Class has been created
it can't be altered.
Ex: String class

2nd thing how to make a class immutable .

1. Make the class final
2. make all members final, set them explicitly, in a static block, or in the constructor
3. Make all members private
4. No Methods that modify state