Call method using type variable generics

Question

I have a scenario that i want to create one support class called D which contains a generic method. I have set the upper bound for type variable.

class A{
    void show(){
        System.out.println("Hello A");
    }
}

class B extends A{
    void msg(){
        System.out.println("Hello B");
    }
}

class C extends A{
    void msg(){
        System.out.println("Hello C");
    }
}

class D{
    <T extends A> void display(T ob){
        ob.msg(); //here i want to do some tricks
    }
}

First i want to share my objective. Here msg() function of B and C class has different implementations. I want to create one support class called D that has one display method, using display method i want to call either msg() function of B or C class dependent on instantiation. Can you tell me how can i achieve it?

Can;t you check to see if ob instanceof B || ob instanceof C? Or maybe thats not what you are asking?? — ShayHaned
– ShayHaned, Commented Jun 30, 2017 at 6:55
class A don't have a msg method, it is only declare in the subclass. So if A can be abstract, add an abstract method, if not, add an empty method (not good in the first place...). Or rename show in A to match the methods name in B and C (if they should be the same) — AxelH
– AxelH, Commented Jun 30, 2017 at 6:55
One of options: introduce an interface M{void msg();} which will be implemeted by B and C, so no generics required. — kan
– kan, Commented Jun 30, 2017 at 6:56
@kan but using T extends A, T will only "see" the methods of A, if the interface is only implemented in B and C, this won't work. It need to be implemented in A. — AxelH
– AxelH, Commented Jun 30, 2017 at 6:57

Jesper · Accepted Answer · 2017-06-30 06:57:44Z

2

You need to have the method msg() in class A, otherwise the display() method in class D does not know if this method exist or not in the object that you're passing to it. (What if someone makes a class E that extends A but does not have a msg() method, and you pass an E to D.display()?).

If you don't want to implement the msg() method in class A, then you can make it abstract (and you'll also have to make the class abstract).

abstract class A {
    public abstract void msg();

    // ...
}

answered Jun 30, 2017 at 6:57

Jesper

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Comments

ΦXocę 웃 Пepeúpa ツ · Accepted Answer · 2017-06-30 06:59:44Z

0

more like an architecture style, I would use an interface for that, so your generic method constrains to <T extends If> void display(T ob) where If is the interface with the abstract method msg

interface If {
    void msg();
}

class A {
    void show() {
        System.out.println("Hello A");
    }
}

class B extends A implements If {
    @Override
    public void msg() {
        System.out.println("Hello B");
    }
}

class C extends A implements If {
    @Override
    public void msg() {
        System.out.println("Hello C");
    }
}

class D {
    <T extends If> void display(T ob) {
        ob.msg(); // here i want to do some tricks
    }
}

edited Jun 30, 2017 at 6:59

answered Jun 30, 2017 at 6:58

ΦXocę 웃 Пepeúpa ツ

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3 Comments

AxelH Over a year ago

Isn't it changing the method definition ? You can't pass an A instance anymore. But indeed, this is a clean solution

ΦXocę 웃 Пepeúpa ツ Over a year ago

OP doesnt need an A object, OP needs something that can implement the msg method

AxelH Over a year ago

What I mean is you can't use it like A a = new B(); new D().display(a);.What the original definition was accepting. And of course, you lose the possibilities to use A.show(). Not a problem here, but this need to be point out ;)

vsbehere · Accepted Answer · 2017-06-30 07:03:01Z

0

You don't need generics for this, there is basic concept called dynamic binding in Java.

abstract class A{
    void show(){
        System.out.println("Hello A");
    }       
     abstract void msg();    
}

class B extends A{
    @Override
    void msg(){
        System.out.println("Hello B");
    }
}

class C extends A{

       @Override
       void msg(){
             System.out.println("Hello C");
       }
}

class D{
    void display(A ob){
        ob.msg(); 
    }
}

Here an appropriate instance provided to method will determine which class method should in called at runtime.

edited Jun 30, 2017 at 7:03

answered Jun 30, 2017 at 6:59

vsbehere

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