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
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

 

 


Building Classes with
Multiple Methods

• Computer programs can be thought of using
phases
– Input
– Computation
– Output

• 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
}
public void methodName2 (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

• Instance variables
– declared outside the methods, but declared inside
the class
– all class methods have access to the class
instance variables
– can be used for communication inside class
 

 


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
 

 


Hose Class Methods
void getData()
// Reads and stores the height and
// weight data.
void compute()
// Computes and stores hose size.
void display()
// Displays the results of the
// computation.
 

 


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.
• We can write
OutputBox out = new OutputBox(“A Title”);

• Instead of
OutputBox out = new OutputBox();
Out.setTitle(“A Title”);

• Writing constructors for programmer defined
classes will be discussed in Chapter 7.
 

 


Return Types
• It is possible to have methods that have
return types other than void
• Example:
public class Clock (
int hour;
public int getHour (){
return hour;
}
}
 

 


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

 

 


Theta Calculations
• For the hour hand
theta =
2*Math.PI*minute/60.0;

• For the minute hand
theta =
2*Math.PI*(hour + minute)/60.0/12.0;

 

 


Drawing the Clock Hands

• Assuming (x, y) is the bottom vertex and
recalling that computer graphics coordinates
are upside down
x1 = x + (int) (r*Math.sin(theta));
y1 = y - (int) (r*Math.cos(theta));
d.drawLine(x, y, x1, y1);

•  For the hour hand
use r*.8 in place of r
 


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