How to force a method to be overridden in java?

I have to create a lot of very similar classes which have just one method different between them. So I figured creating abstract class would be a good way to achieve this. But the method I want to override (say, method foo()) has no default behavior. I don't want to keep any default implementation, forcing all extending classes to implement this method. How do I do this?

Solution 1:

You need an abstract method on your base class:

public abstract class BaseClass {
    public abstract void foo();
}

This way, you don't specify a default behavior and you force non-abstract classes inheriting from BaseClass to specify an implementation for foo.

Solution 2:

Just define foo() as an abstract method in the base class:

public abstract class Bar {
   abstract void foo();
}

See The Java™ Tutorials (Interfaces and Inheritance) for more information.

Solution 3:

Just make the method abstract.

This will force all subclasses to implement it, even if it is implemented in a super class of the abstract class.

public abstract void foo();