Computing Assignment 4: Introduction to Java

Purpose: In this assignment, you will attempt to enter and run your first Java programs. Hopefully, this well begin to get you accustomed to Java's syntax, compiler, and interpreter.

Warning: At times, Java is incredibly verbose. At other times, it is confusingly terse. Why do we do things the way we do? Because the designers decided that we whould. (I.e., "it's just the way things are").

Your First Program: Hello World

Many people begin working in a new programming language by writing a short program that prints the words "Hello World". Here's one such program, written in Java.

import rebelsky.io.SimpleOutput;

/**
 * A simple Java program that illustrates textual output.
 *
 * @author Samuel A. Rebelsky
 * @version 1.0 of January 1998
 */
public class HelloWorld 
{
  /**
   * Print the string "Hello world." to standard output.
   */
  public static void main(String[] args) 
  {
    // The object we're using to print output
    SimpleOutput out;
    // Set up output
    out = new SimpleOutput();
    // And print
    out.println("Hello world");
    // That's it
    System.exit(0);
  } // main
} // HelloWorld


 

Let's deconstruct this program.

import Statement

import rebelsky.io.SimpleOutput;

This tells the Java compiler that we'll be using a SimpleOutput object which is in the library rebelsky.io. As you might guess, this is a library that I've created.

Introduction

/**
 * A simple Java program that illustrates textual output.
 *
 * @author Samuel A. Rebelsky
 * @version 1.0 of January 1998
 */

This is a comment about the program. A comment is a note for the reader of the program (rather than for the computer). Java supports a variety of commenting styles. This is a javadoc comment. It begins with a slash and two stars and ends with a star and a slash. Every class should have a javadoc comment which consists of a few-sentence summary of the intent of the class, a blank line, and the author and version (as above).

Class Header

public class HelloWorld

This is the beginning of the real code. We're defining a class (more or less, a combination of functions and templates for objects) named HelloWorld that can be used anywhere (public).

Begin Class Definition

{

This brace begins the definition (code) for the class. You'll see a corresponding closing brace later.

Method Summary

  /**
   * Print the string "Hello world." to standard output.
   */

Another javadoc comment. Each method (function, algorithm) you create should have a javadoc comment.

Method Header

  public static void main(String[] args)

This is the start of a method (function, algorithm) definition. In particular, we're defining a method called main. Every Java program must have one main function that begins just like this. For now, we won't worry about why it looks like this, it just does. When you execute a Java program, execution begins at the start of your main method.

Begin Method Definition

  {

This open brace begins the definition of main. You'll see another one later.

Comment

    // The object we're using to print output

This is another comment. You can begin a comment with two slashes. The comment then ends at the end of the line. I'd recommend making it a point to comment add a note as to the purpose of every variable.

Variable Declaration

    SimpleOutput out;

This is a variable declaration. It says that out refers to an object of type SimpleOutput.

Object Creation and Assignment

    // Set up output
    out = new SimpleOutput();

Initially, variables have no values. This line (and associated comment) tells Java to create a new SimpleOutput object and associate it with the variable we've just declared. The default SimpleOutput object writes to the screen. Eventually, you'll learn how to write to files.

Output

    // And print
    out.println("Hello world");

This says to print the string "Hello World". More precisely, this says to call the println method of the out object we've just created, and to pass it the string "Hello World". Note that the ordering here is similar to that we'd use if we were talking to someone, as in "Sam, write 'Hello World' on the board."

Cleanup

    // That's it
    System.exit(0);

This tells the program to end, and notes that the program finished without errors (0 means "without errors"; other numbers mean "with some error"). This isn't strictly necessary. However, while you don't need to include a System.exit statement, I strongly recommend that you do.

End Method Definition

  } // main

This matches the brace that opened the body of main. The comment isn't strictly necessary, but it makes it easier to read the proram. I'd recommend making it a point to comment every end brace to indicate what you are ending.

End Class Definition

} // HelloWorld

This matches the brace that opened the definition of HelloWorld. Again, the comment isn't strictly necessary, but it makes the program more readable.

Running The Program

Now, you're ready to create and run your first Java program. Copy the code from above and save the program as HelloWorld.java. Capitalization is important! You can cut and paste to copy the program, or you can retype it by hand. You may even want to do both (first copy and paste; then retype) as you'll learn different things from the different procedures. To copy and paste, select the text with the left mouse button, then move to your emacs window and click with them middle mouse button. Isn't that intuitive?

Next, you need to compile it. Use
% /home/rebelsky/bin/jc HelloWorld.java
(don't type the percent sign). If you've mistyped something, you may see an error. Let me know, and we'll fix the error together.

Finally, you can run the program. Type
% /home/rebelsky/bin/ji HelloWorld

Once you've gotten the program to run, try changing the message that it prints and recompiling. To do this, you'll need to open the program from within Xemacs (which you should remember how to do from a previous assignment), change the appropriate line, and save.

Your Second Program: Greetings

We've seen how to print output and a sketch of the overall structure of a simple Java program. Now, let's move on to one that also reads input.

import rebelsky.io.SimpleInput;
import rebelsky.io.SimpleOutput;

/**
 * A simple Java program that illustrates textual input and output.
 *
 * @author Samuel A. Rebelsky
 * @version 1.0 of January 1998
 */
public class Welcome
{
  /**
   * Prompt for the user's name and print a greeting.
   */
  public static void main(String[] args) 
  {
    // The object we're using to read input.
    SimpleInput in;
    // The object we're using to print output.
    SimpleOutput out;
    // The user's name.
    String name;
    // Set up input and output.
    in = new SimpleInput();
    out = new SimpleOutput();
    // Prompt for the name.
    out.print("What is your name? ");
    // Get the name.
    name = in.readLine();
    // Print a friendly message.
    out.println("Hello " + name + ", welcome to the joy of Java.");
    // That's it
    System.exit(0);
  } // main
} // Welcome

Again, we'll look at it, line by line (or at least chunk by chunk).

import Statements

import rebelsky.io.SimpleInput;
import rebelsky.io.SimpleOutput;

/**
 * A simple Java program that illustrates textual input and output.
 *
 * @author Samuel A. Rebelsky
 * @version 1.0 of January 1998
 */
public class Welcome
{

  /**
   * Prompt for the user's name and print a greeting.
   */
  public static void main(String[] args)
  {

    // The object we're using to read input.
    SimpleInput in;
    // The object we're using to print output.
    SimpleOutput out;
    // The user's name.
    String name;

    // Set up input and output.
    in = new SimpleInput();
    out = new SimpleOutput();

    // Prompt for the name.
    out.print("What is your name? ");

    // Get the name.
    name = in.readLine();

    // That's it
    System.exit(0);
  } // main
} // Welcome

import rebelsky.io.SimpleInput;
import rebelsky.io.SimpleOutput;

/**
 * A simple Java program that illustrates numeric input and output.
 *
 * @author Samuel A. Rebelsky
 * @version 1.0 of January 1998
 */
public class Square
{
  /**
   * Prompt for a number and print the square of the number.
   */
  public static void main(String[] args) 
  {
    // The object we're using to read input.
    SimpleInput in;
    // The object we're using to print output.
    SimpleOutput out;
    // The value entered.
    double val;
    // Set up input and output.
    in = new SimpleInput();
    out = new SimpleOutput();
    // Prompt for the value.
    out.print("Please enter a value and I will square it: ");
    // Get the name.
    val = in.readDouble();
    // Print a friendly message.
    out.println(val + " squared is " + val*val);
    // That's it
    System.exit(0);
  } // main
} // Square

    double val;

    val = in.readDouble();

    out.println(val + " squared is " + val*val);