Inner class type parameter bound by enclosing class type variable

Question

Is there any way to use a type variable declared by an enclosing class as a bound on a type variable declared in an inner class?

class Test<E> {
   class Inner<T extends E> {}
   <T extends E> void doStuff(T arg) {}
   public static void main(String[] args) {
      new Test<Number>().doStuff(new Integer(0)); // works fine, as expected
      new Test<Number>().new Inner<Integer>(); // won't compile
   }
}

javac gives this error:

Test.java:6: type parameter java.lang.Integer is not within its bound
             new Test<Number>().new Inner<Integer>();
                                             ^

I can't find any combination of types that will satisfy the compiler. What's the difference between the type parameter T as declared by Inner versus doStuff? Why does one work and the other doesn't?

I'm not looking for an alternative, I just want to gain a better understanding of how the language works.

Answer 1

https://bugs.java.com/bugdatabase/view_bug?bug_id=6557954

Bug ID: 6557954
Votes   2
Synopsis    Inner class type parameters doesn't get substituted when checking type well-formedness
Category    java:compiler
Release Fixed    7(b40)
State   10-Fix Delivered, bug
Priority:   5-Very Low
Submit Date 16-MAY-2007
Posted Date : 2008-07-02 16:22:46.0

Description

The compiler fails to accept this program:

class Foo<T> {
  class Bar<U extends T> {}
  Foo<Number>.Bar<Integer> f;
}

Evaluation

This is a problem in Check.java as when checking for bound-conformance actual type parameters are subsituted only in topmost type-variable's bound. In this case we have that Foo.Bar is to be checked against the actual type-parameters T=Number, U=Integer

So it should be the case that:

Number <: Object
Integer <: [Number/T]T = Number

unfortunately, javac misses the second substitution so that the check becomes:

Integer <: T 

which is wrong and cause the error.

Edit:

On my system, the code in the question compiles without error with Java 7 javac:

C:\workspace\Sandbox\src>"%JAVA_HOME%\bin\javac.exe" -version
javac 1.7.0-ea

But it fails with the error indicated in the question for Java 6 javac:

C:\workspace\Sandbox\src>"%JAVA_HOME%\bin\javac.exe" -version
javac 1.6.0_17