You can use the User Defined Java Class step to enter your own Java class to drive the functionality of a complete step. You can program your own plugin into a step, yet the goal of this step is not to do full-scale Java development inside of a step. A whole plugin system is available to help with that part (see the Administer Pentaho Data Integration and Analytics document). The goal is for you to just define Java methods and logic. For this step, the Janino project libraries are used to compile Java code in the form of classes at runtime.