Why would one declare an immutable class final in Java?

If you don't mark the class final, it might be possible for me to suddenly make your seemingly immutable class actually mutable. For example, consider this code:

public class Immutable {
     private final int value;

     public Immutable(int value) {
         this.value = value;
     }

     public int getValue() {
         return value;
     }
}

Now, suppose I do the following:

public class Mutable extends Immutable {
     private int realValue;

     public Mutable(int value) {
         super(value);

         realValue = value;
     }

     public int getValue() {
         return realValue;
     }
     public void setValue(int newValue) {
         realValue = newValue;
     }

    public static void main(String[] arg){
        Mutable obj = new Mutable(4);
        Immutable immObj = (Immutable)obj;              
        System.out.println(immObj.getValue());
        obj.setValue(8);
        System.out.println(immObj.getValue());
    }
}

Notice that in my Mutable subclass, I've overridden the behavior of getValue to read a new, mutable field declared in my subclass. As a result, your class, which initially looks immutable, really isn't immutable. I can pass this Mutable object wherever an Immutable object is expected, which could do Very Bad Things to code assuming the object is truly immutable. Marking the base class final prevents this from happening.

Hope this helps!

Contrary to what many people believe, making an immutable class final is not required.

The standard argument for making immutable classes final is that if you don't do this, then subclasses can add mutability, thereby violating the contract of the superclass. Clients of the class will assume immutability, but will be surprised when something mutates out from under them.

If you take this argument to its logical extreme, then all methods should be made final, as otherwise a subclass could override a method in a way that doesn't conform to the contract of its superclass. It's interesting that most Java programmers see this as ridiculous, but are somehow okay with the idea that immutable classes should be final. I suspect that it has something to do with Java programmers in general not being entirely comfortable with the notion of immutability, and perhaps some sort of fuzzy thinking relating to the multiple meanings of the final keyword in Java.

Conforming to the contract of your superclass is not something that can or should always be enforced by the compiler. The compiler can enforce certain aspects of your contract (eg: a minimum set of methods and their type signatures) but there are many parts of typical contracts that cannot be enforced by the compiler.

Immutability is part of the contract of a class. It's a bit different from some of the things people are more used to, because it says what the class (and all subclasses) can't do, while I think most Java (and generally OOP) programmers tend to think about contracts as relating to what a class can do, not what it can't do.

Immutability also affects more than just a single method — it affects the entire instance — but this isn't really much different than the way equals and hashCode in Java work. Those two methods have a specific contract laid out in Object. This contract very carefully lays out things that these methods cannot do. This contract is made more specific in subclasses. It is very easy to override equals or hashCode in a way that violates the contract. In fact, if you override only one of these two methods without the other, chances are that you're violating the contract. So should equals and hashCode have been declared final in Object to avoid this? I think most would argue that they should not. Likewise, it is not necessary to make immutable classes final.

That said, most of your classes, immutable or not, probably should be final. See Effective Java Second Edition Item 17: "Design and document for inheritance or else prohibit it".

So a correct version of your step 3 would be: "Make the class final or, when designing for subclassing, clearly document that all subclasses must continue to be immutable."