CODEWITHSUNDEEP
javalistgenericsextend
Ekz
Ekz

Reputation: 71

extending List and its generic

I'm trying to do such thing:

public class MyClass1 implements IMyInterface{
    public boolean myMethod(){
        ...
    }
}
public class MyClass2 implements IMyInterface{
    public boolean myMethod(){
        ...
    }
}
public class MyList<T extends IMyInterface> extends ArrayList<T>{
    public T getSomething(){
        for (int i = 0; i < size(); i++) {
            if (get(i).myMethod())
                return get(i);
        }
        return null;
    }
}

So I can use it like this

public class MyClass3{
    public void doSomething(MyList<IMyInterface> list){
        ...
    }
}
public class MyClass4{
    public MyList<MyClass1> list1;
    public MyList<MyClass2> list2;

    public MyClass4(){
        MyClass3.doSomething(list1);
        MyClass3.doSomething(list2);
    }
}

But I getting a error with this message: The method doSomething(MyList) in the type MyClass3 is not applicable for the arguments (MyList)

Upvotes: 0

Views: 170

Answers (3)

ben75
ben75

Reputation: 28706

This signature : 

public void doSomething(MyList<IMyInterface> list)

will match as soon as the arg is a MyList of IMyInterface. But in your code you try to call it with a MyList of MyClass1 and this is not the same.

Modify the signature so that it will match all MyList of IMyInterface and also all MyList of subtype of IMyInterface. To do that you must use an upper bound widcard like this :

public void doSomething(MyList<? extends IMyInterface> list)

Upvotes: 0

Debojit Saikia
Debojit Saikia

Reputation: 10632

The method definition

public void doSomething(MyList<IMyInterface> list){
        ...
}

says that it will only accept MyList of type IMyInterface. You cannot pass it a MyList of type MyClass1 or MyClass2. MyList<MyClass1> is a subtype of the raw type MyList, but not of the parameterized type MyList<IMyInterface>.

You need to use bounded wildcard type to deal with situations like this. You need to make the parameter to accept a MyList of type IMyInterface or some subtype of IMyInterface. So in order to make it work, change the method parameter of doSomething to this;

public void doSomething(MyList<? extends IMyInterface> list){
...
}

Upvotes: 1

msam
msam

Reputation: 4287

you need to use wilcards

change doSomething(MyList<IMyInterface> list) to doSomething(MyList<? extends IMyInterface> list)

Upvotes: 1

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