Java : private method vs final class

Question

I came across this question when I was studying immutability in java. It is said that in order to make a class immutable , the class should be declared final so that it's methods can not be extended. I have a question such that, I could achieve the same if I mark the methods private or methods final in java. If I do so I don't have to mark the class as final. I am getting this correct. Your thoughts on this highly appreciated.

Answer 1

The thing with a non-final class with no public mutating methods or fields is that you can still subclass it.

Say you have this immutable class:

class Point {
    private int x, y;
    // getters

    // constructors
}

This class has no public mutating methods or fields, but it is not final, so other people can still subclass it to make it mutable:

class MyPoint extends Point {
    public int foo;
}

Now suddenly you can change foo's value however you want. MyPoint now becomes mutable!

This is why you should mark immutable classes as final.