Java difference between two methods (generics)

Question

Consider the class hierarchy

public class A {
    public void say() {
        System.out.println("I am A");
    }
}

and

public class B extends A {
    public void say() {
        System.out.println("I am B");
    }
}

In a third class I have two different methods

private static void print(A input) {
}

private static <T extends A> void print2(T input) {
}

What is the "difference" between them?

I can both call them with an instance of A and all subclasses of A:

public class Test {

    private static void print(A input) {
        input.say();
    }

    private static <T extends A> void print2(T input) {
    }

    public static void main(String[] args) {
        B b = new B();
        print(b);
        print2(b);
    }
}

Thank you for your help!

P.S.: The difference between

private static void print(java.util.List<A> input) {
}
private static <T extends A> void print2(java.util.List<T> input) {
}

is clear!

Depends what the method does.

khelwood
– khelwood

2015-12-11 14:33:18 +00:00
Commented Dec 11, 2015 at 14:33 — khelwood
– khelwood, Commented Dec 11, 2015 at 14:33

Denis Lukenich · Accepted Answer · 2015-12-11 14:37:03Z

4

Well from a practical reason there is not so much difference there. You could call the second method by

<B>test2(new B());

It would fail then when you try to use is with an A

<B>test2(new A()); //Compile error

Although this does not make a big use for it.

However: The generic declaration does make sence when you e.g. add the return-types.

public void <T extends A> T test2(T var) {
    //do something
    return var;
}

that you could call with:

B b = new B();
B b2 = test2(b);

while calling a similar method without generics would require a cast.

edited Dec 11, 2015 at 14:37

answered Dec 11, 2015 at 14:34

Denis Lukenich

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2 Comments

mrbela Over a year ago

Thank you! Are there other differences?

Denis Lukenich Over a year ago

Generics start getting interesting on class-level. On Method-level using polymorphism is often equal.

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