Convert a Standard Java code to an API - repost

I have a standard piece of Java Code. Now I want to convert this code to an API such that developers could plugin the API but shouldn't be allowed to read the Source Code. Similar to .dll Extension in .NET Environment I need a step by step instruction. a) How to write it. a simple string which comes from API. b) How do you package that API c) How do you integrate that API to the Java Project (Eclipse) d) How do you declare that API and Access it. Print screen will work. Remember API source code shouldn't be readable to the person who has attach the API file in his project. Like how Drop box has an API and so on. Thank You