This is a forum for questions about Processing, not Java. Processing is very much like Java, but is a lot easier to use. One of the example sketches that comes with Processing shows you how to texture a cube. If you don't have Processing installed on your system yet, you can get it from the download page, here:  http://processing.org/download/ . Once it's installed, start up Processing, and navigate to "File > Examples > 3D > Textures > TextureCube".