Chapter 6 Methods - SIUEstornar/courses/notes/cs140/06.Methods.pdf · Chapter 6 Methods 1. Class and Method Definitions 2 Class Name Instance Variables Methods. Java is Object Oriented

Chapter 6

Methods

1

Class and Method

Definitions

2

Class Name

Instance Variables

Methods

Java is Object Oriented

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It can model any real world object

A class is a blueprint of what an object will look like

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The object is just an instance of the class

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Object Oriented Programming deals with the creation of

objects

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and their relationships and interactions

Start by defining the class

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Car

- bodyPaintColor: Color- numberOfTires: int

+ getBodyPaintColor(): Color + setBodyPaintColor(Color color): void + getNumberOfTires(): int

Use a UML class diagram

instance variables

instance methods

Code the class definition

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public class Car {

private Color bodyPaintColor; private int numberOfTires; public Color getBodyPaintColor() { return bodyPaintColor; } // end getPaintColor()

public void setBodyPaintColor(Color color) { bodyPaintColor = color; } // end setPaintColor()

public int getNumberOfTires() { return numberOfTires; } // end getNumberOfTires()

public void setNumberOfTires(int tireCount) { numberOfTires = tireCount; } // end setNumberOfTires()} // end Car

instance variables

instance methods

An Object consists of data ...

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bodyPaintColor Color.Green

numberOfTires 4greenCar

object's memoryfootprint

and operations that store and manage the data

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bodyPaintColor Color.Green

numberOfTires 4

greenCar

bodyPaintColor Color.Red

numberOfTires 4

redCar

Class Car methods

methods are shared by all car objects

Each class should be in a separate file

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public class Car { // code omitted} // end Car

Car.java

public class Driver { // code omitted} // end Driver

Driver.java

new Creates an instance of a class

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Car myCar = new Car();

Recap

▪ An object is an instance of class

▪ Use new to create an object

▪ Objects have data (instance variables)

▪ Objects offer functionality (methods)

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There are two types of methods

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Methods that do not return a value (void)

System.out.println("println does not return");

and methods that do return a value

int num = keyboard.nextInt();

Let's see how methods work

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Car myCar = new Car();

First create the object

bodyPaintColor null

numberOfTires 0myCar Default values

You can then call a method to setan instance variable

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myCar.setNumberOfTires(4);

bodyPaintColor null

numberOfTires 4myCar

Receiving Object

or a get method to retrieve an instance variable

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int tireCount = myCar.getNumberOfTires()

bodyPaintColor null

numberOfTires 4myCar

4

this Demystified

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public int getNumberOfTires() { return this.numberOfTires;} // end getNumberOfTires()

public void setNumberOfTires(int tireCount) { this.numberOfTires = tireCount;} // end setNumberOfTires()

Since each method is shared by all the objects, the methods need to be able to identify the receiving object.

this refers to the receiving object, implicitly.

void Method Definition

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public void setNumberOfTires(int tireCount) {

numberOfTires = tireCount;

} // end setNumberOfTires()

Method is accessible by defining class and any other

class

Parameter list can be empty or list

parameters needed

Instance Variable

return Method Definition

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public int getNumberOfTires() {

return numberOfTires;

} // end getNumberOfTires()

return type of int

Parameter list can be empty or list

parameters needed

Instance Variable

Recap

▪ Methods expose a class's functionality

▪ Call a method on a receiving object

▪ this identifies the receiving object inside the method's definition

▪ Each class is stored in its own .java file

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Local variables are defined within a method

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public double updateSumAmount(double amount) {

double newSumAmount += amount;

return newSumAmount;

} // end updateSumAmount()

local variable

Methods can definelocal variables named the same

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public void method1() { double someDouble = 0;

// Code omitted

} // end method1()

public void method2() { double someDouble = 0;

// Code omitted

} // end method2()

local to method1

local to method2

Information Hiding and

Encapsulation

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A method should hide how it is implemented

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I know what the method

does,just not how!

Know "what" a method does, not "how" it does it

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Methods can be public

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These define the class's interface

or private

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These are part of the implementation

Instance Variables are private

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They define the implementation

Accessor methods controlaccess to instance variables

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Getters retrieve instance variables

Setters set instance variables

Recap

▪ Local variables are defined within a method

▪ Know "what" a method does, not "how" it does it

▪ Public methods define the class's interface

▪ Private instance variables/methods are part of the implementation

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Class Deconstructed <Fraction>

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Fraction

- numerator: int- denominator: int

- reduce(): void+ getNumerator(): int + setNumerator(int n): void + getDenominator(): int + setDenominator(int d): void + setNumeratorAndDenominator(int n, int d): void + add(Fraction f): Fraction + subtract(Fraction f): Fraction

Application Deconstructed<Fraction.java>

package fractiondemo;

public class Fraction { private int numerator; private int denominator;

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private void reduce() { int u = numerator; int v = denominator; int temp; while (v != 0) { temp = u % v; u = v; v = temp; }// end while numerator /= u; denominator /= u; }// end reduce()


public int getNumerator() { return numerator; }// end getNumerator()

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public void setNumerator(int n) { setNumeratorAndDenominator(n, denominator); }// end setNumerator()

public int getDenominator() { return denominator; }// end getDenominator()

public void setDenominator(int d) { setNumeratorAndDenominator(numerator, d); }// end setDenominator()


public void setNumeratorAndDenominator(int n, int d) { numerator = n; if (d == 0) { System.err.println("ERROR: Invalid parameter (" + d + ") in setNumeratorAndDenonimator"); System.exit(1); } else { denominator = d; }// end if }// end setNumeratorAndDenominator()

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public Fraction add(Fraction f) { Fraction sum = new Fraction(); sum.setNumeratorAndDenominator(numerator * f.denominator + denominator * f.numerator, denominator * f.denominator); sum.reduce(); return sum;


public Fraction subtract(Fraction f) { Fraction difference = new Fraction(); difference.setNumeratorAndDenominator( numerator * f.denominator - denominator * f.numerator, denominator * f.denominator); difference.reduce(); return difference; }// end subtract()

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public Fraction multiply(Fraction f) { Fraction product = new Fraction(); product.setNumeratorAndDenominator( numerator * f.numerator, denominator * f.denominator); product.reduce(); return product; }// end multiply()


public Fraction divide(Fraction f) { Fraction division = new Fraction(); division.setNumeratorAndDenominator( numerator * f.denominator, denominator * f.numerator); division.reduce(); return division; }// end divide()

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public void show() { System.out.print("(" + numerator + " / " + denominator + ")"); }// end show()}// end Fraction()

Application Deconstructed<FractionDemo.java>

package fractiondemo;

public class FractionDemo { public static void main(String[] args) { Fraction f1 = new Fraction(); Fraction f2 = new Fraction(); Fraction result = new Fraction(); // Set f1 to 1 / 4. f1.setNumeratorAndDenominator(1, 4); // Set f2 to 1 / 2. f2.setNumeratorAndDenominator(1, 2);

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// Output their sum, difference, product and division. result = f1.add(f2); f1.show(); System.out.print(" + "); f2.show(); System.out.print(" = "); result.show(); System.out.println(); result = f1.subtract(f2); f1.show(); System.out.print(" - "); f2.show(); System.out.print(" = "); result.show(); System.out.println();


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result = f1.multiply(f2); f1.show(); System.out.print(" * "); f2.show(); System.out.print(" = "); result.show(); System.out.println(); result = f1.divide(f2); f1.show(); System.out.print(" / "); f2.show(); System.out.print(" = "); result.show(); System.out.println(); }// end main()}// end FractionDemo


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Objects and References

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There are two types of variables

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Value: Stores the actual value

1

Reference: Stores a reference to the actual value

2

Value types store values

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int x = 100; 100x

Reference types store references

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Fraction f = new Fraction();

2040

.

.

fnumerator ?denominator ?

.

.

2040

Lets compare value types

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int x = 100; x 100

int y = 200; y 200

x == y ? false

x = y; x 200

y 200

x == y ? true

Now lets compare reference types

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f1 == f2 ? false

f1 == f2 ? true

Fraction f1 = new Fraction();f1.setNumeratorAndDenominator(1,2);

f1 200numerator 1 denominator 2

200



208

f1 = f2;f1 208


208

The solution to the == problem?

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Define an equals method

Code Deconstructed <equals method>

public boolean equals(Fraction f) {

return this.numerator == f.numerator && this.denominator == f.denominator;

}// end equals()

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Two fractions are equal if both their numerator and denominator values are the same.

Code Deconstructed <equals method>

Fraction f1 = new Fraction();f1.setNumeratorAndDenominator(1, 2);

Fraction f2 = new Fraction();f2.setNumeratorAndDenominator(1, 2);

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if (f1 == f2) System.out.println("Both variables refer to the same object");else System.out.println("Each variable refers to a different object"); if ( f1.equals(f2) ) System.out.println("Both objects have the same value");else System.out.println("Each object has a different value");

Code Deconstructed <Object variables as parameters>

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200



208

if (f1.equals(f2) {...}...public boolean equals(Fraction f) {...}


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f 208

Both the argument (f2) and the parameter (f) point to the same object.

Notice how the parameter (f) refers to the same object as the argument (f2) and thus object can be changed from within the method.

Code Deconstructed <Object variables as parameters>

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200

resetFraction(f1);...

public void resetFraction(Fraction f)


200

f 200

Notice here how the object variable (f) has been assigned a new object, and that f1 stills refers to its original object.

{ f = new Fraction(); f.setNumeratorAndDenominator(1, 1);}


200

f 208numerator 1 denominator 1

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Chapter 6 Methods - SIUEstornar/courses/notes/cs140/06.Methods.pdf · Chapter 6 Methods 1. Class and Method Definitions 2 Class Name Instance Variables Methods. Java is Object Oriented

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