Question

In Java…

I am creating a class Foo which contains a method doAction(). My requirements:

1. doAction() must have a default implementation (i.e. function body) in Foo.
2. All subclasses of Foo must override doAction(), meaning that subclasses will get a compiler error if they do not provide a new implementation.
3. I need to be able to instantiate Foo.

abstract would work, except that it does not allow me specify a function body for doAction().

Answer 1

Edit

It is impossible to simultaneously satisfy all of the requirements, end of story. You must give up at least one condition, and probably consider an entirely different approach to the problem you’re trying to solve.

---

Use two separate methods. Either:

abstract class Foo {
    
    // Override this method
    abstract void doActionInSubclass();
    
    // You can't override a final method
    // And you don't want subclases to override this one
    final void doAction () {
        // do whatever default-y things you want here
        doActionInSubclass();
    }
}

Or just make the "required" method completely separate from the one you want to force subclasses to override:

abstract class Foo {
    abstract void mustOverrideThisInConcreteSubclasses();
    
    final void doAction() {
        // default-y things here
    }
}