inconsistent compiler behavior with generics example

From:

Roger Levy <sinoslav@gmail.com>

Newsgroups:

comp.lang.java.programmer

Date:

Mon, 03 Sep 2007 18:33:11 -0000

Message-ID:

<1188844391.820927.287400@g4g2000hsf.googlegroups.com>

I have this new function that I wrote inside Eclipse (OS X, Java
1.5.0) and it both compiles and runs in Eclipse, and compiles using
javac within OS X. But it fails to compile through either (1) javac
on Linux (version 1.6.0), or (2) Apache ant on either OS X or Linux.
Here's the program and I'm appending the compile-time error message.

Thanks

Roger

***

import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collection;
import java.util.List;

public class Blah {

  public static <E,L1 extends Collection<E>, L2 extends
Collection<L1>> L2 asListOfLists(E[][] array) {
    List<List<E>> result = new ArrayList<List<E>>();
    for(E[] a : array ) {
      result.add(Arrays.asList(a));
    }
    return (L2) result;
  }

  public static void main(String[] args) {
    System.out.println(asListOfLists(new Integer[][] { { 1,2 },
{ 3,4 }} ));
  }

}

Error:

$ javac -d . Blah.java
lBlah.java:19: incompatible types; inferred type argument(s)
java.util.Collection<java.lang.Integer>,java.lang.Object do not
conform to bounds of type variable(s) L1,L2
found : <L1,L2>L2
required: java.lang.Object
    System.out.println(asListOfLists(new Integer[][] { { 1,2 },
{ 3,4 }} ));
                                    ^
Note: Blah.java uses unchecked or unsafe operations.
Note: Recompile with -Xlint:unchecked for details.
1 error