Lecture An introduction to computer science using java (2nd Edition): Chapter 5 - S.N. Kamin, D. Mickunas, E. Reingold

Chapter 5 - Classes and methods II. In this chapter we will: formally introduce the class construct as it is used in the Java language, discuss the use of instance variables to facilitate method communication, demonstrate the use of classes to improve program structure.

Chapter 5

Classes and Methods II

Lecture Slides to Accompany

An Introduction to Computer Science

Using Java (2nd Edition)

by S.N Kamin, D Mickunas, E Reingold

Chapter Preview

In this chapter we will:

• formally introduce the class construct as it is used in the Java language

• discuss the use of instance variables to

facilitate method communication

• demonstrate the use of classes to improve program structure

• Using separate methods for each phase can

improve the maintainability of a class or

program

Building Class Definitions

public class classname {

// Author, data, explanation

declarations of instance variables

public void methodName1 (parameter) {

declarations of local variables

executable statements with comments

Instance Variables

• Local variables

– variables declared inside methods

– not accessible to any other method

– cannot be used for communication

Initialization of Instance Variables

• Instance variable declarations can contain

initializers just like local variables

• Unlike local variables, instance variables will

be initialized to default values if no initializers

are found

– integers and doubles are initialized to 0

– characters are initialized to the null

character (ASCII code 0)

– booleans are initialized to false

– object-type variables are initialized to the reference value null

UML Diagram for Hose Class

Variable Scope Rules

1 The scope of an instance variable is the entire class

body unless another identifier is found with the same name

2 The scope of a formal argument in a method header

is the entire method body

3 The scope of a local variable in a method is from the

point of declaration to the end of the method body

4 It is not legal to declare a variable within a method

using the same name as variable in the enclosing block in that method You cannot declare two

instance variables using the same name

Scope Example

Bad Variable Declarations

Class Constructors with

Arguments

• A constructor is a special method that is called

when an object is allocated.

• Writing constructors for programmer defined

classes will be discussed in Chapter 7.

return Statement

• The return statement

– allows a method to return a value to the caller– can appear any where in the method body

– can be conditionally executed

– results in immediate exit from a method when executed

• Form:

return expression;

Clock Class Methods

Example Explanation void setup() Initializes the clock

void getData() Reads and stores the hour and

minute data String toString() Returns string version of time

suitable for printing void setHour(int h) Sets hour to h

void setMinute(int m) Sets minute to m

int getHour() Returns value of hour

int getMinute() Returns value of minute

boolean priorTo(Clock c) Returns true is receiver < c void display

(DrawingBox d,

int x, int y, int r)

Draws the clock with center at (x,y) and radius r, in the DrawingBox referred to by d

Clock Class Outline

Geometry of Clock Drawing

Drawing the Clock Hands

• Assuming (x, y) is the bottom vertex and

recalling that computer graphics coordinates are upside down

TwoClocks Client

public class TwoClocksClient {

public static void main (String[] args){ TwoClock twins = new TwoClocks();

twins.drawClocks();

twins.compareClocks();

}

}

Output from TwoClocks Client

UML Class Diagram for Clock-DrawingBox Composition