why does the genericity of class effect the genericity of method

Asked Jun 06 '19 at 07:20

Active Jun 06 '19 at 07:28

Viewed 31 times

public class Test {

public void test(){
    Demo demo = new Demo();
    TextView textView = demo.getView(1);
}


class Demo {

    private View ContentView = null;

    public <T extends View> T getView(int id){
        return ContentView.findViewById(id);
    }
}

}

The above is no problem.The following compilation fails, requiring a strong wall conversion type

public class Test {

public void test(){
    Demo demo = new Demo();
    TextView textView = demo.getView(1);
}


class Demo<B> {

    private View ContentView = null;

    public <T extends View> T getView(int id){
        return ContentView.findViewById(id);
    }
}

}

I just added a generic type of the class and had to force it when calling getView ().why?thanks

edited Jun 06 '19 at 07:28

asked Jun 06 '19 at 07:20

super_mark

What? You obviously added a generic type to a method and not to a class. Besides, your two code examples are identical. – Smutje Jun 06 '19 at 07:21
I can't spot the difference between source 1 and source 2. – minus Jun 06 '19 at 07:23
@minus `class Demo` the generic `B` is the difference – Vladyslav Matviienko Jun 06 '19 at 07:29
@VladyslavMatviienko now I see it ... but it has been edited since I wrote my comment. – minus Jun 06 '19 at 07:33
@minus before edit it was `Test` – Vladyslav Matviienko Jun 06 '19 at 07:36
Yes, I just edited it. Why do we need to overtype when we add a generic class? – super_mark Jun 06 '19 at 07:37
1

In the second example, `demo` has a raw type, and raw types erase all generics, even those unrelated to the omitted type parameter. – Andy Turner Jun 06 '19 at 07:45
@AndyTurner you should post it as an answer, it is truly an interesting and general question, and interesting answer. – minus Jun 06 '19 at 07:49
@minus it is quite adequately covered in the dupe. It's not necessarily easy to find that question, if you were looking for it by yourself, but that's why dupes are shown in a big yellow box. – Andy Turner Jun 06 '19 at 08:08
@minus but yes, it's an interesting thing that catches a lot of people out. At one point, I remember dupe-hammering this question roughly once a week. – Andy Turner Jun 06 '19 at 08:14

why does the genericity of class effect the genericity of method

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