Question

What exactly is the difference between a final class and having a class constructor as private.

I know both can’t be subclassed(correct me if i am wrong). Is their any difference?

Answer 1

A final class cannot be extended. It prevents this

final class FinalClass {

}

// and later

class ExtendedClass extends FinalClass { // ERROR

}

This is useful for things like String – you wouldn’t want someone to be able to overwrite the logic of String, one of the most commonly used Objects, and be able to, oh I don’t know, add networking and send all the strings back you use. It’s possible to do if you can extend String.

A private constructor cannot be called outside the class.

class PrivateCons {

    private PrivateCons() {

    }
}

// later
PrivateCons pc = new PrivateCons(); // ERROR

Often this ends up working like this: (java.lang.Math is a good example)

class FuncLib {
    private FuncLib() { } // prevent instantiation
    public static void someFunc(...) { }
    public static int anotherFunc(...) { }
}

Or it ends up working like this // Integer does this actually

class VerySpecial {

    private static Map<String,VerySpecial> cache;

    public static VerySpecial generate(String data) {
        VerySpecial result = cache.get(data);
        if(result == null) {
            result = new VerySpecial(data);
            cache.put(data,result);
        }
        return result;
    }

    private String data;

    private VerySpecial() { }

    private VerySpecial(String data) { this.data = data}

}

When you extend a class, your constructor by default attempts to call the default (no argument) constructor. If that is private, then you must explicitly call a non-private constructor when you extend it. If you have no non-private constructors to call you won’t be able to extend it. Thanks for comments for pointing this out. 🙂