If you already know how to log into cdf and use the pico text editor, feel free to jump to the exercises below.

If at any point you would like some clarifications on this page, feel free to ask the TA for help.

and see if you can follow the instructions on how to use CodeWarrior. CodeWarrior has a text editor, and it can also compile and run programs. Try this for now. Once you are used to it, go to the Examples and Exercises sections.

If you use CodeWarrior, you can ignore the sections on "Loggin In", "Text Editor", and "Does it work?".

class HelloWorld
{
   public static void main( String argv[] )
   {
      System.out.println( "Hello World!!!" );
   }
}

Steps 1 and 2 you saw in the previous section. Now we discuss what steps 3 and 4 do.

Compiling the code.

Remember what this means? It means that the English-like instructions you learned in class, i.e., the Java code, is going to be translated into something that the computer understands, i.e., zeros and ones).

In general, to compile a program, type:

Nothing shows up. After you type the javac command and nothing shows up on the screen, except for another eddie% prompt, then that means the translation was successful! Congratulate yourself! Now you can run the code.

What are all these messages on the screen? After you type the javac command and if some or lots of messages show up on the screen, then that means your program had errors in it and it could not be translated into zeros and ones. Sorry.

The messages that showed up are called "error messages". They indicate where the messages are (the line numbers at the beginning of the message) and what kind of error it is. Of course, the computer is not omnipotent. It cannot always be correct about these errors. So, these error messages are just a guess at what might be wrong. Use them as a guide to help you fix the program. Sometimes errors are due to simple mistakes like typos and missing ". Begin with those.

Running the code.

After compiling, the computer creates a file that contains the zeros and ones instructions. This file is called an "executable". If your program is called HelloWorld.java, then the corresponding executable is named HelloWorld.class. In general, the executable is named FILENAME.class. To see what it does, you have to "run" it. So, type:

Notice the command we use is java (without the "c" at the end) and that we do not type in .class. In general, the command you use is:

I think it works. If the output on the screen shows what you expected, then your program works!

It doesn't do what I expected it to. Then something went wrong. Go back and see what mistakes you might have made by checking each line in the file. Ask your friends or TA for help if necessary.

Each example has a set of "Things to try". These suggestions will help you understand how Java works. Try them. Try making one change in your file, save it, compile it, and if possible, run it. See what happens, and see if you understand why. If there is an error, do you know why it happened? If the output looks different, did you expect it? If the output looks the same, is this correct?

If you understand these examples and all the suggestions in "Things to try", you are ready to write your own Java programs.

1. A program that prints out the sums of numbers.

class Sum
{
   public static void main( String[] args )
   {
       // variable declarations
       int a = 2;
       int b = 3;
       int firstsum;
       double x = 4.5;
       double y, secondsum;

       // arithmetic and assignment
       firstsum = a + b;

       y = firstsum;

       secondsum = x + y;
            
       // print to screen, uses concatenation
       System.out.print( "\n The first sum is " );
       System.out.println( firstsum );
       System.out.println( "\n" + "The second sum is " + secondsum + "\n" );
   }
}

Things to try:

• remove the 3 lines of comments (the ones beginning with //)
• add a comment before the main procedure
• add a comment before the class that explains what this program does
• change the values 2 to 2.5 and 3 to 3.9
• change the int variables to double
• change the double variables to int
• change the System.out.print to System.out.println
• change the System.out.println to System.out.print

2. A program that calculates the area of a donut (i.e., there is a big circle with a small circle inside it, calculate the area that is taken up by the space between these two circles).

class Donut
{
   public static void main( String[] args )
   {
      // variable declarations
      double pi = 3.14;
      double outRad = 5.2;         // radius of outter circle
      double inRad = 2.79;         // radius of inner circle
      double areaO, areaI, areaDonut;

      // make calculations
      areaO = pi * outRad * outRad;
      areaI = pi * inRad * inRad;
      areaDonut = areaO - areaI;
            
      // print output to screen
      System.out.print( "\n The area of the donut is: " );
      System.out.print( areaDonut + "\n" );
   }
}

Things to try:

• add a comment before the class that explains what this program does
• change the value 3.14 to 3
• change the values 5.2 to 5.0 and 2.79 to 3
• change the System.out.println to System.out.print

3. A program that

class Donut
{
   public static void main( String[] args )
   {
      // variable declarations
      double outRad = 5.2;         // radius of outter circle
      double inRad = 2.79;         // radius of inner circle
      double areaO, areaI, areaDonut;

      // make calculations
      areaO = areaCircle( outRad );
      areaI = areaCircle( inRad );
      areaDonut = areaO - areaI;
            
      // print output to screen
      System.out.print( "\n The area of the donut is: " );
      System.out.print( areaDonut + "\n" );
   }
    
   static private double areaCircle( double r )
   {
      // variable declarations
      double pi = 3.14;
      double area;

      // compute
      area = pi * r * r;

      // pass the result back to caller
      return area;
   }
}

Things to try:

• change areaO = areaCircle( outRad ); to areaO = areaCircle( inRad ); and change areaI = areaCircle( inRad ); to areaI = areaCircle( outRad );
• change area = pi * r * r; to two lines: area = r * r; and followed by area = area * pi;

4. A program that

import java.lang.Math;

class distance
{
   public static void main( String[] args )
   {
      // variable declarations
      double x1, x2, y1, y2, dist;
      double tempX, tempY;

      // initialize the (x,y) values
      x1 = 2;  y1 = 3;
      x2 = 5;  y2 = 7;

      // compute the squares
      tempX = square( x2 - x1 );
      tempY = square( y2 - y1 );

      // compute the distance
      dist  = Math.sqrt( tempX + tempY );

      System.out.print( "The distance between " +
      "(" + x1 + "," + y1 + ")" + " and " +
      "(" + x2 + "," + y2 + ")" + " is " + dist );
   }

   static private double square( double n )
   {
      return n * n;
   }
}

Things to try:

• split x1 = 2; y1 = 3; into two lines
• split x2 = 5; y2 = 7; into two lines
• if you do not use the square procedure, how would you calculate the values for tempX and tempY?
• add a square procedure to the previous donut example and change the calculation of area to area = pi * square( r );

5. A program that prints out the face of a die in random (i.e., a random number between 1 and 6). Note: the material in this example is harder than the prevoius, so don't worry if you don't understand it completely. This example is just for fun.

import java.lang.Math;

class RollDice
{
   public static void main( String[] args )
   {
      int face;

      // use random to generate real random numbers between 0 and 1
      // then scale them to the faces of a die between 1 and 6
      face = 1 + ( int )( Math.random() * 6 );
      System.out.println( "Face   " + face );
   }
}

Things to try:

• run the program multiple times (i.e., type java RollDice several times at the eddie% prompt)
• take out the "1 +": so change face = 1 + ( int )( Math.random() * 6 ); to face = ( int )( Math.random() * 6 );
• change * 6 to * 8
• change * 6 to * 3
• change * 6 to * 100
• change System.out.println( "Face " + face ); to System.out.println( "Face " );
• change System.out.println( "Face " + face ); to System.out.println( face );

Once you have typed in the exercise and saved it, you can exit pico to go back to the eddie% prompt. Then, you can compile and run the program to see if it works. Going through each of these exercises will give you more practise and a better understand of how Java works.

If you understand how these work, you are in good shape for the course. Good luck!

1. A program that does nothing.

 
class Nothing
{
   public static void main( String argv[] )
   {
   }
}

2. A program that prints "Hellow World!" to the screen.

class ...
{
   public static void main( String argv[] )
   {
       ...
   }
}

3. A program that adds two integers and prints out their sum.

class ...
{
   public static void main( String argv[] )
   {
      // declare variables
      ...

      // initialize the variables
      ...

      // add them
      ... 
      
      // print output to screen 
      ... 
   }
}
\end{verbatim}

4. A program that two numbers (integers or decimals) and prints out their sum.

class ...
{ 
   public static void main( String argv[] )
   {
      // declare and initialize variables
      ...

      // add them
      ...

      // print output to screen
      ...
   }
}

5. A program that prints out what the user types in.

class ...
{
   public static void main( String argv[] )
   {
      // prepare for input from screen
      BufferedReader input = new BufferedReader(new InputStreamReader(System.in));
      String response;

      // get some words from user
      System.out.println( ... );

      // read in the input
      response = input.readLine();

      // print output to screen
      ...
   }
}

6. A program that lets the user guess a letter in the alphabet. (We didn't go over this one in class, so don't worry if you don't know how to complete it. Do have a try anyway.)

class ...
{
   public static void main( String argv[] )
   {
      // prepare for input from screen
      ...

      // make up a default answer and store it somewhere
      letter = "b";

      // ask the user for a letter
      ...

      // read in the input 
      ...
      
      // print the result to the screen 
      if( letter.equals( ... ) ) 
         ...
      else 
         ...
   }
}

Copyright © 2001 by bowen hui