Question about compiler method binding regarding generic methods

[laeubi]

This is not really a question about JDT directly, but ti feels JDT devlopers are the right audience to ask!

I'm currently working on a tool that checks OSGi version ranges and came about an interesting problem:

• We have org.osgi.framework.Version that implements comparable
• One point in time, there was no generic, so the class provided the method signature compareTo(Object)
• Today we override it and it has become compareTo(Version other)
• Now I have a classfile that has a method reference (compiled against the one with generics) that looks like this org/osgi/framework/Version#compareTo(Lorg/osgi/framework/Version;)I

Given I can really run the old (non generic) version against that compiled classfile, will it still work or result in a NoSuchMethod or similar?!?

[stephan-herrmann]

Here's a DIY experiment to answer the question:

public class Test implements Comparable<Test> {
	public int compareTo(Test other) { return -1; }
}

$ ecj -23 Test.java
$ javap -v Test
[...]
  public int compareTo(Test);
    descriptor: (LTest;)I
    flags: (0x0001) ACC_PUBLIC
    Code:
      stack=1, locals=2, args_size=2
         0: iconst_m1
         1: ireturn
      LineNumberTable:
        line 3: 0

  public int compareTo(java.lang.Object);
    descriptor: (Ljava/lang/Object;)I
    flags: (0x1041) ACC_PUBLIC, ACC_BRIDGE, ACC_SYNTHETIC
    Code:
      stack=2, locals=2, args_size=2
         0: aload_0
         1: aload_1
         2: checkcast     #1                  // class Test
         5: invokevirtual #16                 // Method compareTo:(LTest;)I
         8: ireturn
      LineNumberTable:
        line 1: 0

Generating the method int compareTo(Object) is needed for overriding a method of this exact signature from Comparable. Only with this "bridge method" it is possible to do this:

Comparable<Test> c = new Test();
c.compareTo(new Test()); // expecting Test.compareTo() to be invoked!