Name: vsR10008 Date: 07/27/2000



JVM rejects a class which implements the same interface twice.
Howevere we can't find in JVMS (Second Edition) that this is prohibited.
We are not sure if this is a bug in implementation or an omission in the spec.
Something should be fixed.

Let's consider the following example:

interface A {...}
interface A1 extends A {}
interface A2 extends A {}
class B implements A1,A2 {...}
class C implements A,A {...}

Class B and C are semantically equal; so
JVM should accept both B and C or reject them both.
Meanwhile JVM accepts "B" but rejects "C".

Note, that JLS explicitly prohibits such usage of interfaces
(JLS, 2nd edition, "8.1.4 Superinterfaces"):

  A compile-time error occurs if the same interface is mentioned two or
  more times in a single "implements" clause.

We cannot compile class C from the example above with usual Java compiler,
so we use jcod utility to make appropriate classfile.

The following sources demonstrate the bug:

--------------------------------------------------------------- D.java ---
interface A {
    void m() {}
}
interface A1 extends A {}
interface A2 extends A {}
class B implements A1,A2 {
    public void m() {}
}
// class C implements A,A {} -- see C.jcod

public class D {
    public void m() {}
    public static void main(String[] argv) {
        String[] cn={"B","C"};
        for(int i=0;i<cn.length;i++) {
           try {
              System.out.print("Loading class "+cn[i]+"...");
              Class.forName(cn[i]);
              System.out.println("Loaded!");
           } catch (Throwable t) {
              System.out.println("ERROR:"+t);
           }
        }
    }
}
--------------------------------------------------------------- C.jcod ---
class C {
  0xCAFEBABE;
  3; // minor version
  45; // version
  [] { // Constant Pool
    ; // first element is empty
    Method #3 #11; // #1
    class #12; // #2
    class #13; // #3
    class #14; // #4
    Utf8 "<init>"; // #5
    Utf8 "()V"; // #6
    Utf8 "Code"; // #7
    Utf8 "LineNumberTable"; // #8
    Utf8 "SourceFile"; // #9
    Utf8 "D.java"; // #10
    NameAndType #5 #6; // #11
    Utf8 "C"; // #12
    Utf8 "java/lang/Object"; // #13
    Utf8 "A"; // #14
  } // Constant Pool

  0x0020; // access
  #2;// this_cpx
  #3;// super_cpx

  [] { // Interfaces
  // ... implements A,A
    #4;
    #4;
  } // Interfaces

  [] { // fields
  } // fields

  [] { // methods
    { // Member
      0x0000; // access
      #5; // name_cpx
      #6; // sig_cpx
      [] { // Attributes
        Attr(#7) { // Code
          1; // max_stack
          1; // max_locals
          Bytes[] {
            0x2AB70001B1;
          };
          [] { // Traps
          } // end Traps
          [] { // Attributes
            Attr(#8) { // LineNumberTable
              [] { // LineNumberTable
                0 5;
              }
            } // end LineNumberTable
          } // Attributes
        } // end Code
      } // Attributes
    } // Member
  } // methods

  [] { // Attributes
    Attr(#9) { // SourceFile
      #10;
    } // end SourceFile
  } // Attributes
} // end class C
--------------------------------------------------------------------------

Execution log:

> uname -a

SunOS novo12 5.7 Generic_Patch sun4u sparc SUNW,Ultra-2

> java -version

java version "1.4.0beta"
Java(TM) 2 Runtime Environment, Standard Edition (build 1.4.0beta-b25)
Java HotSpot(TM) Core VM (build 1.3-internal, interpreted mode)

> java -classic -version

java version "1.4.0beta"
Java(TM) 2 Runtime Environment, Standard Edition (build 1.4.0beta-b25)
Classic VM (build 1.4.0beta-b25, green threads, nojit)

> jcod C.jcod

> javac D.java

> java D

Loading class B...Loaded!
Loading class C...ERROR:java.lang.ClassFormatError: C (Repeative interface name)

> java -classic D

Loading class B...Loaded!
Loading class C...ERROR:java.lang.ClassFormatError: C (Repeative interface name)

This bug doesn't occur in JDK1.0, JDK1.0.1, JDK1.0.2

In JDK 1.1-1.1.2 JVM rejects class "C" throwing
"java.lang.IllegalAccessError: Unimplemented interface method".

In JDK 1.1.3-1.1.5 JVM rejects class "C" throwing
"java.lang.VerifyError: Bad interface index"

In JDK 1.1.6-1.1.8 JVM rejects class "C" throwing
java.lang.ClassFormatError: Bad interface index

In JDK 1.2-1.4 JVMs (both classic and HotSpot) reject class "C" throwing
java.lang.ClassFormatError: C (Repeative interface name)

======================================================================

In the above example, class B is legal and class C is not. The VM is doing the right thing.

###@###.### 2002-04-11