How to generate bytecode and save to .class file?

I have the following weird requirement.

I am given:

1. A list of some method names.
2. Names and types of parameters of the above methods.
3. The functionality of the above methods. This is as follows: For each parameter, the method converts it to string using toString and obtains an array of strings. To this array, the method applies a function foo. The function foo takes as input a String [] type and outputs String. The methods return what foo returns. foo's code is given inside a Java object and is to be accessed as a black-box.

The info in 1. and 2. can be in a text or XML file. For this purpose, we can consider it to be available inside a Java object in whatever way we choose.

The task is to create a .class file (i.e., bytecode) that implements those methods and can be run on the JVM.

I think this assembler library would be one way to do it. Can anyone suggest an easier way?

[EDIT:] I can think of one other way: first generate the .java file and then compile it to get the .class file.

[The context:] I have to do this for several hundreds of methods. I want the shortcut so I can automate my job rather than manually write the code.