Follow the tutorials to learn to program. Use the Guides to see example programs is many languages.
Drawing the French tricolour with Java
You can create images using Java. The first tutorial Flags looks at drawing flags of the world, from simple flags such as Libya to more complicated flags such as the Stars and Stripes of the USA. In Java you can use methods such as
fillRect to create simple images.
Consult the User's Guide for information on changing material in progzoo.
You can mess about in the Playpen.