Java: Implement Setter Method With Subclass of Parameter Type?

Question

I have two Interface here:

public interface Configuration {

}

public abstract interface Configurable {

    public Configurable setConfig(Configuration config);

}

And when one of my implementation tries to override the setConfig()method, the compiler will complain that is not overriding a super class method.

// This is not correct.
@Override
public SubConfigurable setConfig(SubConfiguration config) {
    this.config = config;
    return this;
}

I understand that I could always pass in a Configuration object and cast down to its real type but I am wondering if there is better way to target this. Or maybe I am just in a wrong design approach?

Thanks in advance.

I don't think that's the actual code. Did you mean SubConfiguration by chance? — user2864740
– user2864740, Commented Apr 3, 2014 at 0:12

rgettman · Accepted Answer · 2014-04-03 00:04:30Z

4

It is possible to make the interface generic, specifying the type of configuration with a generic type parameter, with an upper bound.

public interface Configurable<C extends Configuration> {
    public Configurable setConfig(C config);
}

Then the subclass can specify which Configuration is required.

public class SubConfigurable implements Configurable<SubConfigurable> {

    @Override
    public SubConfigurable setConfig(SubConfiguration config) {
        this.config = config;
        return this;
    }
}

This is not necessary for the return type, because Java return types are covariant. A subclass can return a subtype of the original method and still override that original method. Parameter types in overriding methods are not covariant, but using generics in this way is a valid solution.

answered Apr 3, 2014 at 0:04

rgettman

179k30 gold badges282 silver badges365 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

Radiodef Over a year ago

And I think Configurable#setConfig should then be declared to return a Configurable<C> to work completely generically.

Neal Xiong Over a year ago

That is exactly what I was looking for! Thx. Actually I tried the other way around (Configuration<T extends Configurable>) and it did not work. I think I still need to understand generic more.

Tomasz Bekas · Accepted Answer · 2014-04-03 00:04:31Z

0

Take a look at Java Generics. Also java interface does not have to implicitly be declared as abstract, it is already abstract.

answered Apr 3, 2014 at 0:04

Tomasz Bekas

6961 gold badge6 silver badges16 bronze badges

Collectives™ on Stack Overflow

Java: Implement Setter Method With Subclass of Parameter Type?

2 Answers 2

2 Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

2 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related