Computer Science 1 Top Lectures Assignments Noticeboard Starting Java

Modeling the program

• Our first program is to make an announcement on the screen, that says:
This is my first Java program
but it won't be my last.
• What are we modeling?
• a human being making an announcement
• What are the objects?
• the human being
• the monitor
• What are the behaviors?
• human being sends message to monitor
• monitor has character printing behavior
• In Java, the initiating object is the main program

How to write the program

• We know that there's a predefined object called System.out, which can put characters onto the screen
• The way to get the behavior is to send the object a message
• The first part of the message is the behavior we want, i.e., println
• The second part of the message contains the arguments, which are the added detail or extra information needed to do the job

System.out.println ("This is my first Java program");
System.out.println ("but it won't be my last.");
 

A valid Java main program must have:

• A name (called an identifier)
• Extra notation (which has meaning, but we won't discuss it yet)

  import java.io.*;
  class Program1 {

    public static void main (String[] arg) {
      System.out.println ("This is my first Java program");
      System.out.println ("but it won't be my last.");
    }
  }

• The first three lines and the last two are going to be the same for this week
• Treat them as rules - any discipline has rules

Summary

• We have modeled a real world example and, using our knowledge of predefined objects, written a simple program that displays two lines on the screen.

Java stuff

• There are, however, specific details in Java that we need to know about.

Java rules

• Java has rules that dictate what an acceptable program looks like
• e.g., can't say

System.out("This is my first java program").println
• violating these rules can be the source of many errors in beginners' programs

Identifiers

• Classes, behaviors and objects must have a name - an identifier.
• Rules for identifiers:
• must contain only letters, digits and underscores
• must begin with a letter
• upper and lowercase letters are different
• beware: System.out
• identifiers should be meaningful
• call a spade a spade, and not x.

Keywords

• Are special words that have special meaning in the Java language, e.g.,
import, class, public, static, void
• Do not use keywords as identifiers!

Order of statements

System.out.println ("One, two, three");
System.out.println ("four, five, six");

is different to

System.out.println ("four, five, six");
System.out.println ("one, two, three");

Program format and comments

Format

• Java is very flexible about format - the chief format rule is to put spaces around two adjacent identifiers or keywords, e.g., class Program1, not classProgram1
• There is a readability issue though,
• you looking at your code later on,
• someone else is maintaining your code
• Some general rules:
• Write one statement per line
• Use consistent spacing when indenting (e.g., use the TAB key)
• This can be difficult to pick up, but you should use examples as your guide

Comments

• Comments are notes that are ignored by the computer
• help the human reader to interpret the code
• In Java, anything that starts with /* and ends with */ is a comment
• Don't have to be on the same line
• Also, line comments are lines that begin with //
• Comments enhance the readability of a program, but should not be over used
• Some rules
• Each file containing a program should have the following header comment:

/* Filename: Program1.java
   Author: Bradley Schmerl
   Date: 8/24/98
   Version: 1.0
   Description: This program announces my first Java
                program experience, and signals my
                intent to continue.

*/

Note: All programs your write in this course must contain this header.
• If more than one class per file, place comments just before the class indicating the purpose and behavior of code
• Don't explain how Java works (assume the reader knows this)
• use comments to give insights that aren't readily apparent
• Don't put comments in the middle of statements