Implement an interface and override methods in Java?

Question

Why do you have to override all methods of an interface?

For instance if I have

public class Foo extend JFrame implements ActionListener, KeyListener {
          


        

Answer 1

If you implement an interface, that means that the implemented object can be used in any scenario where a implementation is expected. From that point of view is quite obvious that you must implement all methods of an interface, otherwise you cannot really say that you have implemented it. e.g. if I have a ICalculator interface with a Add and Subtract method, and you want only to implement Add, could the resulting class really be used by another project that needs a ICalculator?

However, I quite understand your ire about implementing some of the interfaces in the frameworks today, as they clearly break the Interface Segragation Principle. What that means is that some interfaces are clumps of several functionalities that can be used together to do something useful. E.g. if the ICalculator inteface, instead of just basic Add, Divide etc.. methods, I started adding methods like StandardDeviation that are not essential to the nature of the Calculator object.

Bottom line, yes, you have to implement each and every one of the interface's members, however, If you are sure and know that some of them will never be used, feel free to leave them empty, or even better, make them throw exceptions, so you'll be sure that never really means never.

Answer 2

You really should grab a good book about object orientated programming and revisit the first chapters. An interface is a completely abstract definition of methods, so you cannot implement an interface partially.

Answer 3

Concrete classes must always implement all of the methods of an interface. If you weren't already extending JFrame you could extend KeyAdapter. It implements empty methods for KeyListener to avoid writing them out. You could use an anonymous class with that inside of your Foo class like this:

addKeyListener(new KeyAdapter() {
    public void keyTyped(KeyEvent e) {
        // handle typed key here
    }
});

Answer 4

This is exactly what the KeyAdapter class is for: it implements all methods of the interface (but none of the implementations do anything). Since you cannot extend two classes, you should use an inner class for your listener (which is a cleaner design anyway):

public class Foo extend JFrame {
      foo(){
          ...
          component.addKeyListener(new MyKeyListener());
          ...
      }

    private class MyKeyListener extends KeyAdapter{
        @Override
        public void keyPressed(KeyEvent arg) {          
        }
    }
}

Answer 5

You can make a parent class of Foo which has these empty methods and have Foo only implement the methods you want to see.

When you implement an interface, you must implement all its methods somewhere for a concrete object to be created.

Answer 6

Interfaces have no default implementation. If you weren't forced to implement each method in the interface, then what would happen when some other code tries to call these methods through the interface?