how type variable allowing wrong type?

Question

package org.my.java;

public class TestTypeVariable {

    static <T,A extends T> void typeVarType(T t, A a){
        System.out.println(a.getClass());
        System.out.println(t.getClass());
    }

    public static void main(String[] s){
        int i= 1;
        typeVarType("string", i);
    }
}

when run, following is the output :

class java.lang.Integer
class java.lang.String

How can A be of type Integer when it has been already upper-bounded to String?

Please explain me on it.

Trillium

I cannot compile your example: Bound mismatch: The generic method typeVarType(T, A) of type TestTypeVariable is not applicable for the arguments (String, Integer). The inferred type Integer is not a valid substitute for the bounded parameter <A extends T> Which is what I would expect. Did you compile the source successfully? – Scheider

With the compiler resolving T to Object and A to Integer, there is no issue. – Zinnia

@Scheider It passes compilation in Java 8, but not in Java 7. – Ultann

This is already being answered in this post #12475550 – Dorsman

Answer 1

Two things here:

• there is a simple solution to the "bad" typing: T isn't String but Object. And Integer extends Object. But please note: this only works with the "enhanced" type inference capabilities of Java8. With Java7, your input will not compile!
• misconception on your end: getClass() happens at runtime, and therefore returns the specific class of the objects passed - independent on what the compiler thinks about generics at compile time.

Blunk

It compiles fine. typeVarType("string", i); has T=Object, A=Integer . – Unplaced

it is compilable and runable. getClass() i just used to print run-time type of an object.but thats not an issue. main point is how java allowing integer type for an string upper bounded type – Trillium

@Unplaced That happens when one believes in other comments without further checking. I thought "T should be Object" myself ... thanks, and updated. – Blunk

It compiles fine in Java 8, but doesn't pass compilation in Java 7. In this case I think it makes more sense for it not to pass compilation, since <T,A extends T> makes no sense (due to type erasure). – Ultann

Of course, the inferred type for T could also be Object & Serializable & Comparable<?>… – Hemorrhoidectomy

I wish you could quote the JLS here, i swear given this on an interview I would say it fails – Functionary