使用类型变量泛型的调用方法

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?

Answer 1

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();

    // ...
}

Answer 2

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
    }
}

Answer 3

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.