This interface contains shareable instruction objects. In order to save memory you can use some instructions multiply, since they have an immutable state and are directly derived from Instruction. I.e. they have no instance fields that could be changed. Since some of these instructions like ICONST_0 occur very frequently this can save a lot of time and space. This feature is an adaptation of the FlyWeight design pattern, we just use an array instead of a factory. The Instructions can also accessed directly under their names, so it's possible to write il.append(Instruction.ICONST_0);