Accessing type arguments from generic types used in typebounds

Question

I have discovered the following problems with generics. Consider the generic interface

public interface A<X> {
    X get();
    void doStuff(X x);
}

Now, let's assume the following method definition:

public <T extends A<?>> void foo(T t) {
    bar(t);
}

Due to the wildcard, the type information for the return type of get() is insufficient. Therefore, I have to delegate to another method that "binds" this wildcard to a fresh type variable:

private <X> void bar(A<X> t) {
    X x = t.get();
    t.doStuff(x);
}

The invocation of bar() in foo is not allowed, the compiler outputs the following error message:

The method bar(A) in the type test is not applicable for the arguments (T)

However, if I change the method foo() to

public <T extends A<?>> void foo(T t) {
    A<?> u = t; // No explicit cast required, no "unchecked" warning!
    bar(u);
}

it works. Why? Is this a compiler error? Any comments on this would be very much appreciated.

Notes:

The reason why I don't simply declare method foo as void foo(A) is that I'm actually using the upper type bound interesection (&).
The reason why I don't declare X as a type variable in foo() is that I actually have the problem at class level and don't want to unnecessarily increase the number of type parameters of this class.

are you using the -Xlint compiler flag? If not, try compiling with that — Hunter McMillen
I agree with @kan. The java compiler does, in fact compile this. I used 1.6.0_27. Eclipse does give the exact error you gave, so maybe you're using it? — Joseph Gordon
Created the eclipse bug: bugs.eclipse.org/bugs/show_bug.cgi?id=364200 using @kan's example. — Joseph Gordon

kan kan · Accepted Answer · 2011-11-18T17:51:25

I've checked the code:

public class Test
{
    public interface A<X> {
        X get();
        void doStuff(X x);
    }
    public <T extends A<?>> void foo(T t) {
        bar(t);
    }

    private <X> void bar(A<X> t) {
        X x = t.get();
        t.doStuff(x);
    }
}

it works. javac 1.6.0_22. Where do you have the error? Or I using another code?

Accessing type arguments from generic types used in typebounds

2 Answers