See How to Edit .class file In JavaBy Sir Handler | hats Off Sir

Some broad level requirements that
this tool is based on are:
Intuitive Graphical User
Interface: The user interface
should be clean and intuitive.
Screens should not be crowded.Components like tab panels should
be used wherever appropriate to
reduce screen clutter.
Clear separation for the Novice
and Power User: The features
should be so arranged that the
novice user can reach the required
option easily, while all features are
available for the power user deeper
down the screen hierarchy.
Cascading changes: The core
engine should be able to detect
cascading changes if any and
confirm all changes before
proceeding.
Consistency check: Consistency
check should be provided.
Providing this check at all stages
might not be feasible. To handle
such cases the user should be given
control to do this check optionally.
Facility to switch off consistency
check: Why on earth would
somebody want this? Well, to test
the Java Virtual Machine itself, to
see how well it stands up to errors
in the class file. The Java compiler
would never produce an invalid
class file. This is probably the only
way to do it.
Creating fresh classes: It should
be possible to create classes out of
scratch using this tool. This will
serve as a useful tool for learning
the Java Virtual Machine
instructions.
Facility to examine compiler
specific attributes: Some compilers
put attributes specific to their
product to facilitate debugging
and other product specific
features. For example, the compiler
might embed an entry to the source
file browser file or may be a line
number attribute in the class
method descriptions. This utility
should be able to detect, display
and manipulate such features
wherever possible.