Question

How would one go about defining a method in a super class but leave the implementation to the sub classes?

Answer 1

Leave the method abstract:

abstract class A {
    abstract void doStuff();
}

class B extends A {
    void doStuff() {
        // stuff
    }
}

Note that…

If a class includes abstract methods, the class itself must be declared abstract

Abstract classes can’t be constructed (so new A() would be an error, but new B() would be fine), and subclasses need to implement all abstract methods unless they’re also abstract.

abstract class A {
    abstract void doStuff();
}

abstract class B extends A {
    // This is fine since B is abstract
}

class C extends A {
    // This is an error since doStuff() isn't implemented
}

With an abstract class, you can mix abstract and normal methods:

abstract class A {
    abstract void doStuff();

    void doOtherStuff() {
        // Subclasses don't need to implement this
    }
}

If all of the methods are abstract, it may make more sense to use an interface, since classes an implement multiple interfaces:

interface DoesStuff {
    void doStuff();
}

interface DoesOtherStuff {
    void doOtherStuff();
}

class B implements DoesStuff, DoesOtherStuff {
    void doStuff() {
        // stuff
    }

    void doOtherStuff() {
        // other stuff
    }
}

Note: Methods in an interface are automatically abstract and public, unlike methods in classes, which are by default package private and not abstract.