The actual Java code we'll write is stored in plain-text files holding Java statements and declarations. To store Java code in a file, you can use a simple text editor or as fancy a word processor as you like, as long as the result is a plain-text file without any fancy formatting that the Java compiler can't handle. You can use whatever text editor you prefer, such as vi in UNIX or WordPad in Windows.
You must give such files the extension .java because the Java compiler expects that extension. As you saw, I saved the application named ch10_01 in a file named ch10_01.java. This Java file is the one you'll pass to the Java compiler to create a bytecode file.
The tools we'll use are ready; it's time to start writing code.