In some cases you may have a subsystem which you want to always be running a command no matter what. So what do you do when the command you are currently running ends? That's where default commands come in.

By now you've learned how to code and deploy your first C++ program. This article highlights additional resources as you look to add features and move beyond the basics presented so far.

This article is about command based programming in Java, if you are programming in C++ click here to skip ahead to learning about subsytems.

WPILib supports a method of writing programs called "Command based programming". Command based programming is a design pattern to help you organize your robot programs. Some of the characteristics of robot programs that might be different from other desktop programs are:

• Activities happen over time, for example a sequence of steps to shoot a Frisbee or raise an elevator and place a tube on a goal.
• These activities occur concurrently, that is it might be desirable for an elevator, wrist and gripper to all be moving into a pickup position at the same time to increase robot performance.
• It is desirable to test the robot mechanisms and activities each individually to help debug your robot.
• Often the program needs to be augmented with additional autonomous programs at the last minute, perhaps at competitions, so easily extendable code is important.

Command based programming supports all these goals easily to make the robot program much simpler than using some less structured technique.

This document describes how to rewrite a simple autonomous into a command based autonomous. Hopefully, going through this process will help those more familiar with the older simple autonomous method understand the command based method better. By re-writing it as a command based program, there are several benefits in terms of testing and reuse. For this example, all of the logic is abstracted out into functions primarily so that the focus of this example can be on the structure.