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.

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 Which is what I would expect. Did you compile the source successfully? — SilverNak, Sep 25 '17 at 08:17
With the compiler resolving `T` to `Object` and `A` to `Integer`, there is no issue. — daniu, Sep 25 '17 at 08:19
@SilverNak It passes compilation in Java 8, but not in Java 7. — Eran, Sep 25 '17 at 08:29
This is already being answered in this post https://stackoverflow.com/questions/12475050/why-t-extends-string-is-allowed-but-gives-warning — thedevd, Sep 25 '17 at 08:45

GhostCat · Answer 1 · 2017-09-25T08:38:06.040

15

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.

edited Sep 25 '17 at 08:38

answered Sep 25 '17 at 08:18

GhostCat

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It compiles fine. `typeVarType("string", i);` has `T=Object, A=Integer` . – khelwood Sep 25 '17 at 08:20
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 – Atul Sep 25 '17 at 08:22
@khelwood That happens when one believes in other comments without further checking. I thought "T should be Object" myself ... thanks, and updated. – GhostCat Sep 25 '17 at 08:22
2

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 `` makes no sense (due to type erasure). – Eran Sep 25 '17 at 08:26
Of course, the inferred type for `T` could also be `Object & Serializable & Comparable>`… – Holger Sep 25 '17 at 13:46
I wish you could quote the JLS here, i swear given this on an interview I would say it fails – Eugene Sep 25 '17 at 19:58

how type variable allowing wrong type?

1 Answers1