CS 117 Lab

Getting Started with Linux

Linux tutorial

Getting started with Java

Follow the instructions below. If you have questions along the way, ask. If you want to try something, go ahead. Whenever there are questions in the lab, answer them to your own satisfaction--you do not need to hand in your answers.

The only thing you need to hand in is in section IV below.

I. Hello, World

Goal: To get a simple program compiled and running.

1. Once you have logged on to the computer, launch nedit by clicking on its icon on the desktop or typing nedit in a terminal window. Enter the following java code in the edit window and save it in a file called HelloWorld.java.

   
   
   /*
   	The traditional first program.  
   */
   
   class HelloWorld
   {
   	public static void main( String[] args )
   	{
   		System.out.println( "Hello, world." );
   	}
   }
   

2. In a terminal window:
   • Compile the program by entering: javac HelloWorld.java
   • Run the program by entering: java HelloWorld

3. What do "/*" and "*/" do?

4. Change the first appearance of the word "println" to "print". What happens?

II. Hello, World --- Version 2

1. Edit, compile and run the following modified version of the HelloWorld program. /* The traditional first program. */

   
   import javax.swing.*;
   
   class HelloWorldJ
   {
   	public static void main( String[] args )
   	{
   		String myMessage;
   		
   		myMessage = new String("Hello, world.");
   		JOptionPane.showMessageDialog(null,myMessage);
   		System.exit(0);
   	}
   }
   

2. Here is what each new line of code means:
   • import javax.swing.*; : This is a library of pre-written Java classes that our program can now use (We "imported" the library into our program).
   • String myMessage; : myMessage is declared to be a variable that will hold a string of characters.
   • myMessage = new String("Hello, world."); : Allocate the computer memory to hold this string and put the string "Hello World." into it.
   • JOptionPane.showMessageDialog(null,myMessage); : JOptionPane is a class from the swing library we imported, and showMessageDialog is a class method. "null" and "myMessage" are parameters the method needs to execute.
   • System.exit(0) : System is a prewritten Java class and exit is a method of this class.

III. Arithmetic

Goal: To see how basic arithmetic operations behave, and, as an added bonus, to see how to accept input from the person using the program.

1. Create a new class called "Arithmetic". Copy and paste the code from Arithmetic.java into an nedit window.

2. Compile and run the program. Enter integers when the program asks you to. What does the operation "%" do? How about "/"? (Try running the program several times with different input.)

3. Now change the type of a and b from "int" to "double", and change the two "Integer.parseInt" methods to "Double.parseDouble". Recompile. How does the behavior of "%" change? Get rid of the "%" line and recompile.

4. Your a and b variables now have type double. When you run the program, how is the behavior of "/" different from when a and b had type int? The type "double" is badly named--what does it mean in this context?

5. Notice the section at the top bracketed by /*...*/. This is a comment, as is anything following //. Suppose you want to "comment out" a block of code that's bugging you. That is, you want to make the compiler ignore some code without actually removing the code from your file. The /*...*/ style comments give you a convenient tool for doing this. Try commenting out everything in main. Recompile. What happens, and why? How can you fix the problem? Which style of comments is appropriate for documentation in your code, and which style of comments is appropriate for commenting out several lines of code at once?

IV. Putting it all together

1. Create a class called "Temperature" with a main method that asks for a temperature in Celsius and prints out the temperature in Fahrenheit. Use JOptionPane.showInputDialog for input and JOptionPane.showMessageDialog for output. The formula to convert Celsius to the equivalent Fahrenheit is:

   fahrenheit = 1.8 x celsius + 32

2. Put the names of all students who worked on your Temperature class in the comment at the top of your source file. Then, submit the program electronically. To find out how to do so, go to submitting programs.