Implementing generic method in java

Question

I have below code which is giving error when i try to access the object's method. What is that i am doing wrong here.

public class Main{
public static void main(String[] args) {
    MyFirstObject myObj1 = new MyFirstObject();
    MySecondObject myObj2 = new MySecondObject();

MyGenerics mg = new MyGenerics();
System.out.println( mg.partTwo(myObj1, myObj2) );

}
}

class MyFirstObject{
    int value(){
        return 1;
    }
}
class MySecondObject{
    int value(){
        return 2;
    }
}

class MyGenerics {


   static <T,U> int partTwo (T o1, U o2) 
    { 
        System.out.println(o1.value()); 
                           return 1;
    } 
}

Error is : Main.java:31: error: cannot find symbol System.out.println(o1.value()); ^ symbol: method value() location: variable o1 of type T where T,U are type-variables: T extends Object declared in method partTwo(T,U) U extends Object declared in method partTwo(T,U) 1 error

What is that method suppose to do and what do the Generic paramaters T and U represent in the code you wrote? — user6073886
– user6073886, Commented Jun 16, 2020 at 9:52
Your MyGenerics class is lacking type parameters <T,U>, which also need to be given on creation. — Adder
– Adder, Commented Jun 16, 2020 at 9:53
Why does class need generic parameters? I only need to create a generic method. — nilesh panchal
– nilesh panchal, Commented Jun 16, 2020 at 10:05
You need to tell the class somehow which classes T and U stand for. — Adder
– Adder, Commented Jun 16, 2020 at 10:55

Daimon Cool · Accepted Answer · 2020-06-16 11:01:28Z

2

When you pass T or U, java doesn't know what the object it is and what methods it contains, so you need some specification. For example, you can create an interface and implement it in classes.

Example:

public class Main {
    public static void main(String[] args) {
        MyFirstObject myObj1 = new MyFirstObject();
        MySecondObject myObj2 = new MySecondObject();

        MyGenerics mg = new MyGenerics();
        System.out.println(mg.partTwo(myObj1, myObj2));

    }
}

interface Action {
    int value();
}

class MyFirstObject implements Action {
    public int value() {
        return 1;
    }
}

class MySecondObject implements Action {
    public int value() {
        return 2;
    }
}

class MyGenerics {

    static <T extends Action, U extends Action> int partTwo(T o1, U o2) {
        System.out.println(o1.value());
        return 1;
    }
}

answered Jun 16, 2020 at 11:01

Daimon Cool

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1 Comment

nilesh panchal Over a year ago

Will this also ensure that it can accept only objects which implement interface containing value function.

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