# What is Java?¶

Java is a programming language, which means that we can use Java to tell a computer what to do. Computers don’t actually speak Java so we have to compile (translate) Java source files (they end in .java) into class files (they end in .class). The source file is something humans can read and edit and the class file is code that a computer can understand and can run.

In this book the Java code is actually being sent to a server to compile and run (as long as you have an internet connection) and the output will be shown in the browser, so you won’t actually create a source file or class file.

If you have the Java development environment loaded on your computer you can use the Java command javac to compile your Java source code at a command line. You can use java ClassName to execute the main method in a Java class. In Java all source code must be part of a class (defined inside a class).

Figure 1: Compiling Java source (Person.java) into a class file (Person.class)

You can copy the Java source code shown in this book into a file and save it if you want to run it locally on your computer. Be sure to name the file the same name as the class name with ”.java” as the extension. All code (programs) in Java must be part of a class (defined inside a class) in a source file and the name of the class must match the file name.

You can also use an integrated development environment on your local computer. We recommend Dr Java at http://www.drjava.org (see the appendix for more information), but there are many good ones such as JGrasp, BlueJ, Greenfoot, and Eclipse.

Next Section - What is a Class and an Object?