How do I manipulate classes in java

The thing is that both lua and JavaScript are prototype based, Java is not. You can use reflection to accomplish something similar, but not at the level of JavaScript. Reflection

Inheritance in JavaScript is accomplished by the prototype chain. Basically, when bark is not found in the snoopy object, it is looked up in its prototype snoopy.prototype. If it is found there, that version is used. If not (for example when calling bark.toString()), the prototype chain is traversed until a prototype is found which has that member. The prototype itself is shared between all 'instances' and since it's just a normal object, you can add or remove members to it later on.

Inheritance in Java is class-based. You cannot add or remove members from a class definition at run time unless you recompile and reload the whole class. It's a different programming paradigm which means that you'll have to program (slightly) differently for it using other techniques and patterns.