To write Java you need a text editor, a Java compiler, and a Java runtime environment. The easiest way to get a Java compiler and runtime environment is to download Sun's Java Software Development Kit (SDK). The SDK also includes a variety of tools—it's a "must-have" item for any Java developer. (You'll have to supply your own text editor; Window's WordPad and NotePad are both acceptable.)
After you've learned Java, you might want to look at some of the integrated development environments (IDEs) being offered by Borland, WebGain, and others. These environments typically enable you to write Java faster, although not all of them give you access to the latest Java features.
Sun continues to improve Java and periodically ...