We recently thought about an interesting approach of finding a difference between APIs (public methods) of two versions of a Java software. I was doing it for JDK1.5 (Tiger) and JDK1.6 (Mustang b76).

Sometime you might want to take a quick look and find out what exactly has been changed in your (or somebody's else code) between two versions. Why? Well, if you want to focus your test development on the new API only it might be a very sound idea. I'd spoke about some of other methods to help you to achieve better software quality
Well, there are some methods to do so. E.g. you can use "Since" tags from a Javadoc Well, good if you have any...

Or you can store your product API's snapshot like RefactorIt does. Boy, what if you didn't create this snapshot in a first place and now you badly need one? "Sorry partner - no can do for ya..."

Anyways, the idea was on the surface as usual:
In one way or another build a list of all public (protected/private/static/etc.) methods in your software. You can use grep or javap along with grep for these purposes. Don't forget to do this for both versions of your application - it's about the difference, right? :-)

Now, you have these API lists, you just need to compare them. And again it's up to you: you can use Unix diff command, Emacs ediff, or vimdiff. I'd wrote that self-explanatory Perl script (feel free to modify it, criticize it, or merely throw it away). Fix &open_file()'s loop if your API lists weren't created in the form of "filename:method signature" per line...

#!/usr/bin/perl

if ($#ARGV != 1) {
    print "Please supply two lists of APIs you want to compare\n";
    exit 1;
}

my %diff = ();
my $left = &open_file ($ARGV[0]);  #Presumably, the earlier version
my $right = &open_file ($ARGV[1]); #Presumably, the latest version

foreach $file (keys %$right) {
    if (!exists $left->{$file}) {
	$diff{$file} =  \@{ $right->{$file} } ;
    } else {
	@l_stuff = @{ $left->{$file} };
	@r_stuff = @{ $right->{$file} };
	foreach $r ( @r_stuff )  {
	    if (&is_there($r, @l_stuff) != 1) {
		push @{ $diff{$file}}, $r;
	    }
	}
    }
}

foreach $d (keys %diff) {
    @extract = @{ $diff{$d} };
    print "$d\n   @extract\n";
}

#=========== Some subs
sub is_there () {
    my ($search, @list) = @_;

    for (@list) {
	return 1 if $_ eq $search;
    }
    return 0;
} 

sub open_file () {
    local ($name) = @_;
    open FILE, $name or
        die "Can't open specified file!$name\n";
    my %ret = ();
    
    while () {
        ($key, $value) = split(':', $_);
        push @{ $ret{$key} }, $value;
    }
    return \%ret;
}

CU,
Cos