Class 03: Lab: Getting Started with Java

Held Wednesday, January 26, 2000

Overview

Today, we ground our discussion of Java in practice, writing, compiling, and executing Java programs.

Notes

• Some more responses to the introductory survey are now available.
• Final reminder: I'm holding a meeting on summer opportunities in computer science at noon on Thursday in Science 2424 (down the hall). There is a good chance that I will take at least one first year student from this class (assuming, of course, that some of you apply and indicate an interest in majoring in computer science).
• Assigned:
  • Read Laboratory J2 in preparation for Friday's class.

Contents

• The Basics of Java Programming, Revisited
  • Your Second Java Program
• Preparation
• Lab J1
• Reflection

Summary

• The structure of Java programs, revisited
• Using Java in the MathLAN, revisited
• Laboratory J1: An Introduction to Java
• Due:
  • Preparatory reading of Lab J1
  • Preparatory reading of Using Java in the MathLan

The Basics of Java Programming, Revisited

• This was a little bit confusing yesterday, so I'm trying to phrase it in a different way.
• I often say that there are a few basic steps to writing a Java program (assuming that you've already done some design work):
  • Select existing classes
  • Design and implement new classes
  • Build a main method that directs the program
  • Put the main method in some class
• The main method is the director of the ``play'' that is an object oriented program. As such it
  • Creates new objects
  • Tells those objects what to do
• Each object may also create other objects and tell objects what to do.

Your Second Java Program

• Let's now look at a program that involves more than one class.
• We'll create a simple helper class, Computer, that does some computations.

  /**                                      // Introductory class comment
   * A class that can perform some very
   * simple computations.
   */                                      // End of introductory comment
  public class Computer                    // Class declaration
  {                                        // Start body of class
    /**                                    // Method comment
     * Compute the square of X.
     */                                    // End of method comment
    public int square(int X)               // Method declaration
    {                                      // Start of method body
      return X*X;                          // Return a value
    } // int square(int)                   // End of method body
  
    /**
     * Compute the absolute value of X.
     */
    public int absoluteValue(int X)
    {
      if (X < 0)                        // Conditional
        return -X;
      else
        return X;
    } // absoluteValue(int)
  } // class Computer                      // End body of class
  

• A second class, ProgramTwo, uses the a Computer to compute some values.

  import Computer;
  import SimpleOutput;
  /**
   * Print the results of some simple computations.
   */
  public class ProgramTwo
  {
    public static void main(String[] args)
    {
      SimpleOutput output = new SimpleOutput();
      Computer hal = new Computer();
      int value = -5;
      output.println(value + " squared  is " + hal.square(value));
      output.println("|" + value + "| is " + hal.absoluteValue(value));
    } // main(String[])
  } // class ProgramTwo
  

• Note that there is a particular (and important) syntax for calling methods.
  • You must specify an object that supplies the method (output, hal).
  • You follow the object with a dot and the name of the method.
• Note also that you can concatenate strings with the + operation.

Preparation

I'll spend a few minutes taking general questions about the lab before we begin.

• To make a copy of a library class,
  • Click on the link with the right mouse button.
  • Select ``Save Link As ...''
  • Pick an appropriate file name.
• Make sure to store each class in a separate file of the form ClassName.java.
• To compile a file, use
  /home/rebelsky/bin/jc filename
• To execute (interpret) a program, use
  /home/rebelsky/bin/ji filename

Lab J1

Do Lab J1 from Experiments in Java.

Reflection

We'll spend a few minutes reflecting on what we learned today.