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Java and the Mac OS X Terminal

This document instructs you on how to use the Mac OS X Terminal with Java.

Java

You will use the Java compiler javac to compile your Java programs and the Java interpreter java to run them. To check if Apple's implementation of Java 2 Standard Edition (J2SE 5.0) is already installed:

Run Software Update.
Run Applications/Utilities/Java/J2SE 5.0/Java Preferences (or Applications/Utilities/Java/Java Preferences if using Mac OS X v. 10.5) and verify that the default version is 1.5.

If the J2SE 5.0 folder does not exist:

Download and install J2SE 5.0 Release 6 from Apple.

Command line interface

You will type commands in an application called the Terminal.

Open a terminal window. You can find this under Go -> Applications -> Utilities. Drag the Terminal to your dock since you will be using it frequently.
You should now have a Terminal window somewhere on the screen. It will have a prompt that looks something like:
```
machine:~ username$
```

To check that you are running the right version of Java, type the commands in boldface below. You should see something similar to the information printed below. The important part is that is says 1.5 (and not 1.4).

machine:~ username$ java -version
java version "1.5.0_13"
Java(TM) 2 Runtime Environment, Standard Edition (build 1.5.0_13-b05-237)
Java HotSpot(TM) Client VM (build 1.5.0_13-119, mixed mode, sharing)

Then type

machine:~ username$ javac -version
javac 1.5.0_13
javac: no source files
...

Since you will be using the Terminal frequently, you may want to customize the default window settings (e.g., Monaco 13pt font with antialiasing).

Create the program

Now you are ready to write your first Java program.

Using the ASCII editor of your choice, type the Java program HelloWorld.java exactly as it appears below. If you omit even a semicolon, the program won't work..
```
public class HelloWorld { 
    public static void main(String[] args) { 
        System.out.println("Hello, World");
    }
}
```
Finally, save the file to the subdirectory /introcs/hello and name it HelloWorld.java. The file name is case sensitive and must exactly match the name of the class in the Java program.

Compile the program

You will use the javac command to convert your Java program into a form more amenable for execution on a computer.

From the Terminal, navigate to the directory containing your .java files, say ~wayne/introcs/hello, by typing the cd command below.
```
machine:~ username$ cd introcs/hello
machine:~/introcs/hello username$
```
Assuming the file, say HelloWorld.java is in the current working directory, type the javac command below to compile it.
```
machine:~/introcs/hello username$ javac HelloWorld.java
machine:~/introcs/hello username$
```
If everything went well, you should see no error messages.

Execute the program

You will use the java command to execute your program.

From the Terminal, type the java command below.
```
machine:~/introcs/hello username$ java HelloWorld
Hello, World!
```
If all goes well, you should see the output of the program - Hello, World.

Input and Output

If your program gets stuck in an infinite loop, type Ctrl-c to break out.

Troubleshooting

When I try to run java I get: Exception in thread "main" java.lang.NoClassDefFoundError. First, be sure that HelloWorld.class is in the current directory. Be sure to type java HelloWorld without a trailing .class or .java. If this was not your problem, it's possible that your CLASSPATH was set by some other program so that it no longer includes the current working directory. Try running your program with the command line

machine:~/introcs/hello wayne$ java -cp ./ HelloWorld

If this works, your classpath is set incorrectly. Ask the teaching staff for help.

How do I get the menu to display at the top of the screen instead of at the top of the frame? Execute with java -Dapple.laf.useScreenMenuBar=true

Where can I learn more about the command line? Here is a short tutorial on the command-line.