Lecture An introduction to computer science using java (2nd Edition): Chapter 2 - S.N. Kamin, D. Mickunas, E. Reingold

Chapter 2 - Classes and methods I. In this chapter we will: describe structure of Java programs, present techniques for creating and using class instances, introduce two classes for doing output: OutputBox for text output, DrawingBox for graphical output.

Chapter 2

Classes and Methods I

Lecture Slides to Accompany

An Introduction to Computer Science

Using Java (2nd Edition)

byS.N Kamin, D Mickunas, E Reingold

Chapter Preview

In this chapter we will:

• describe structure of Java programs

• present techniques for creating and using class instances

• introduce two classes for doing output:

– OutputBox for text output

– DrawingBox for graphical output

Running Java Programs

• Enter the program source code in a data file

called Hitwall.java using an editor

• Compile the source program by typing

javac Hitwall.java

• Execute the compiled program by typing

java Hitwall

Program Elements – Part 1

• white space

– blank lines and spaces includes in program

source listings to make things more readable

• comments

– lines beginning with two slashes //

– single or multiple lines enclosed by /* */

– that allow the programmer to insert notes to help other people understand the program

• documentation

– program comments and data files describing a program’s structure and behavior

Program Elements – Part 2

• import directive

– tells the Java compiler which packages the

program makes use of

• packages

– predefined collections of programs providing services to many programmers (e.g CSLib.* package used throughout the text)

• class heading

– needs to be included at the beginning of every program

– class name must match root of file name

– each class must be stored in a file of its own

– also known as methods

– define operations that may be applied to a Java data object (class instance)

• body

– Java statements that contain the implementations

of classes and their methods

– statement which manipulates variables or

determines program control during execution

• atomic statements

– single Java expressions terminated by a ;

• variable assignment statement

– executable statement which copies a particular

value to a data location

– Must begin with a letter (upper- or lower-case)

– May be followed by any number (including 0) of letters and digits

– The characters $ and _ are considered letters

– Java identifier names are case sensitive

– May not duplicate a Java keyword (e.g class or main)

• void return type

– Means it does not return a value to the caller (client)

– Display text representation of argument and

advance output cursor to the next line

Using OutputBox

import CSLib.*;

public class Forecast {

// Give the weather forecast.

// Author: E Reingold 11/12/00

public void predict() {

OutputBox out;

out = new OutputBox();

out.print(“The temperature will be ”); out.print(-10);

out.println(“ degrees.”);

out.println(“That’s cold, folks!”);

}

}

DrawingBox Window

• A DrawingBox window is divided into a rectangular

grid of picture elements (pixels)

• The size of a pixel depends on the resolution (the

number of pixels in the grid) of your workstation

Using DrawingBox

import CSLib.*;

public class Concentric {

// Draw concentric circles.

Concentric Client Program

import CSLib.*;

public class ConcentricClient {

public static void main(String[] args) {

Color

• DrawingBox defines several symbolic

constants to simplify selecting drawing colors

Color.white, Color.black, Color.red, Color.blue, etc

• You may select the color to draw with by

calling setColor before drawing

g.SetColor(Color.blue);

g.fill Rectangle(0, 0, 50, 25);