Generic parameters in method - Java

Question

I have an interface:

public interface InterfaceA {
  public ArrayList<? extends Object> b();
  public Object b(String string);
  public void c(Object object);
}

The first two methods work fine when I implement them:

public class ClassA implements InterfaceA {
   public ArrayList<SomeObject> b() {
      ...
   }

   public SomeObject b(String string) {
      ...
   }
}

But I can't implement the third one like this:

public void c(SomeObject object) {
    ...
}

Is there any way of declare a method with a "generic" parameter to implement it with different objects?

FYI: public ArrayList<?> b(); will be enough. All Java classes are descendants of Object. — PM 77-1
– PM 77-1, Commented May 23, 2015 at 0:46
You're right, I just saw it like that and didn't change it, thanks. — Andrés Marotta
– Andrés Marotta, Commented May 23, 2015 at 0:57

Elliott Frisch · Accepted Answer · 2015-05-23 00:58:35Z

2

Yes. It's possible to make a generic method. Something like,

public <T> void c(T object);

It is also possible to make a generic interface. Something like,

public interface InterfaceA<T> {
    public ArrayList<T> b();
    public T b(String string);
    public void c(T object);
}

and then an implementation might look like

public class ClassA<T> implements InterfaceA<T> {
    @Override
    public ArrayList<T> b() {
        // ...
    }

    @Override
    public T b(String string) {
        // ...
    }

    @Override
    public void c(T object) {
        // ...  
    }
}

edited May 23, 2015 at 0:58

answered May 23, 2015 at 0:43

Elliott Frisch

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4 Comments

Andrés Marotta Over a year ago

Well, that didn't work, :/ Thanks anyway, I'm going to try with the other answer.

Andrés Marotta Over a year ago

I declare the method in the interface like you said: public <T> void c(T object) { ... } And try to implement it in the class like this: public c(SomeObject object) { ... } And I got an error.

Elliott Frisch Over a year ago

@AndrésMarotta Ahhh... no you implement it with the generic signature too.. but then you can call it with a SomeObject. Or you make the entire class / interface generic. Your question isn't entirely clear.

Andrés Marotta Over a year ago

I will try to explain me better (I'm not a native english speaker). I have some classes that has the same methods, but with different objects. For example, every class makes an insert of an object in the DB, but every class does it with "his" object. Now, I want to declare an interface to "group" those methods and force the classes to implement theme. For that, the interface most be "generic" about the type of objects that are going to past through the classes that implement it. I hope that's more clear.

Radiodef · Accepted Answer · 2015-05-23 01:10:17Z

1

We can't, because if a method parameter is different, it's an overload.

The closest we can do is declare a type parameter on the class:

public interface InterfaceA<T> {
    public void c(T object);
}

public class ClassA implements InterfaceA<SomeObject> {
    @Override
    public void c(SomeObject object) {
        object.foo();
    }
}

class SomeObject {
    void foo() {
        System.out.println(1);
    }
}

But that may or may not be what you're after.

The problem with covariant method parameters (as I understood what you're asking about) is like this:

InterfaceA a = new ClassA();
// passing Object to method which actually
// expects a SomeObject
a.c( new Object() );

Wikipedia has some interesting things to say about this: Covariant method argument type. Apparently Eiffel has it and it's a bit controversial.

edited May 23, 2015 at 1:10

answered May 23, 2015 at 0:45

Radiodef

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3 Comments

Andrés Marotta Over a year ago

I tried that and the only thing that fix is the error. I can do this: public void c(SomeObject object) { ... } And I'm not getting a compile error. But I can't use the "object" like a SomeObject type. I mean, it doesn't get his methods.

Radiodef Over a year ago

You should be able to inside the body of ClassA.c. See my edit.

Andrés Marotta Over a year ago

Oh, it was my mistake. I didn't change the name of the parameter. That works! Thank you!

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