Overloading inside interface in Java

Question

I need to implements various update methods for Observers in my project, to overload the Object argument with a specific type. So instead of using

public void update (Observable o, Object arg0){
   if(arg0 instanceof Class1) ....
   else if (arg0 instanceof Class2) ....

or a switch (which is a very 'C' way of working), I thought of using an interface that extends Observer and overloads the update method, like this

public interface ExtendedObserver extends Observer{
   public void update (Observable obj, Class1 cls);
   public void update (Observable obj, Class2 cls);
}

and then implement it into my Observers. Every different Observer will use a specific update method.

I wanted to know if it's a correct way of working, because

I don't think overloading a method inside an interface is correct
I still have to add the unused update methods inside every class that implements the interface.

If you are using Java 8, you can supply empty default implementations for all the methods, so you will not have to redefine all of them in implementors. Btw it would be nice to know what exactly you want to do in these if-instanceof branches. Probable it's better to move this functionality to Class1, Class2? — Tagir Valeev
– Tagir Valeev, Commented May 12, 2015 at 10:03

Prasad Kharkar · Accepted Answer · 2015-05-12 10:02:41Z

1

There is no problem in overloading a method inside interface
An interface is a contract that is mandatory for implementing classes to implement. If you do not want classes to implement that method. There is no need to include it in interface as well.
So you can write that overloaded method in those classes only where it is needed. Or Make an abstract class implement the interface and write overloaded method in it.
This way you will not end of exposing unnecessary method contract to other classes

answered May 12, 2015 at 10:02

Prasad Kharkar

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Zhongxia Yan · Accepted Answer · 2015-05-12 10:04:15Z

1

You could try changing the interface to an abstract class and implementing the two methods there. Also, consider using generics.

answered May 12, 2015 at 10:04

Zhongxia Yan

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Tagir Valeev · Accepted Answer · 2015-05-12 10:06:55Z

1

If you control Class1 and Class2, it would be better to change them like this:

interface Updatable {
    void doUpdate(Observable obj);
}

class Class1 implements Updatable {
    ...
    void doUpdate(Observable obj) { /*class1-specific code*/ }
}

class Class2 implements Updatable {
    ...
    void doUpdate(Observable obj) { /*class2-specific code*/ }
}

This way your main update method will be like this:

public void update (Observable o, Updatable arg0){
    arg0.doUpdate(o);
}

answered May 12, 2015 at 10:06

Tagir Valeev

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Comments

B. Kemmer · Accepted Answer · 2015-05-12 10:12:13Z

1

i don't think this is the right way of implementing an Interface. An Interface is a specific Trait (Scala) of a Class. So if it is only one Parameter Type which is changing i would recommend using bounded Type Parameters.

public interface ExtendedObserver<T> extends Observer{
    public void update (Observable obj, T cls);
}

If that doesn't help you, i would recommend you to outsource Class-Specific methods in their own Interface.

edited May 12, 2015 at 10:12

answered May 12, 2015 at 10:07

B. Kemmer

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