Java .class files low-level (hex) editing?

Question

Hello, all!
I have a problem:
There is an application (namely a J2ME instant messanger for Nokia mobiles). It is a non-commercial application. But unfortunately, there is a bad bug in the application. It is not even a bug, it is more a feature - but it does not support Russian (cyrillic) characters encoding. Russian characters in all incoming messages are displayed with a kind of hierogliphs.
I wrote to developers (and not only I did - I know at least 3 russian people who also wrote a developer about this) - but unsuccessful. Developer keeps silent.
So what I decided is to fix the bug by myself :) By decompilation, e t.c.
Unfortunally, the midlet is hard obfuscated :( So it took me some hours of reading obfuscated code, and now I see some ways which I must try to solve the problem. I think I must write a code which processes the desired String variables (Despite obfuscation, I can see which Strings to process to obtain effect), and inject this code somewhere to be executed, and from where it could reach the desired Strings.
But the great problem is that obfuscated classes produce a lot of erroneus code, which can not be compiled back. So I decide to reject the decompilation at all, or at least of the big classes (where it is a lot of errors, which I can not correct manually).
In some moments, the application stores the desired Strings in the java.lang.Hashmap. So if I could place my processing code into the Hashmap methods, then I could easily process what I need.
Evidently, I can not edit the real basic java classes. But why not to make the applicaton using my class with identical functionality, instead of java.jang.Hashmap?
My idea now is to edit the .class file of the midlet (yeah, edit with notepad), replacing all the references to "java/lang/Hashmap" or "java.lang.Hasmap" with "java/lang/Fuckmap" and "java.lang.Fuckmap".
So did I (replaced it in norepad).
I read the Sun's documentation about .class file structure, and it seems that nothing is against it. There is no encryption, no checksums, e t.c.
Then I produced "Fuckmap" class this way:
import java.lang.Hashmap;
class Fuckmap extends Hashmap {}
(compiled and preverified it of course)
And placed it into java/lang folder inside the application jar.
I instead of working properly, I get the "Application error - invalid constant pool entry"). What do I do wrong?

Answer 1

2 lexzeus, mlk:
I understood you.
I jave just tried it, and used "jaga/util/Fuckmap" (jaga, not java). But still the same ("Invalid constant pool entry). I tried two times (maybe I could make something wrong?), both times took "clean" jar package, e t.c.
Do you really think jav.alang.Hashmap should help now? I have ony little hope :(