To my experience, jclasslib is one of the best bytecode viewers.

As for editors, there are two types: bytecode manipulation libraries, and editors with GUIs. This question has been asked few times on SO, you could check the answers and the links that were provided.

Just be careful that editing bytecode in not as straightforward as you think. The JVMS imposes many restrictions on how class files should be, and there is a great chance that one of your edit will violate one of them.

Check these other questions:

Editing a .class file directly, playing around with opcodes

Is it possible to view bytecode of Class file?

Programming in Java bytecode

To my experience, jclasslib is one of the best bytecode viewers.

As for editors, there are two types: bytecode manipulation libraries, and editors with GUIs. This question has been asked few times on SO, you could check the answers and the links that were provided.

Just be careful that editing bytecode in not as straightforward as you think. The JVMS imposes many restrictions on how class files should be, and there is a great chance that one of your edit will violate one of them.

Check these other questions:

Editing a .class file directly, playing around with opcodes

Is it possible to view bytecode of Class file?

Programming in Java bytecode