Chapter 6
Methods
1
Class and Method
Definitions
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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
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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()
Application Deconstructed<Fraction.java>
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()
Application Deconstructed<Fraction.java>
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;
Application Deconstructed<Fraction.java>
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()
Application Deconstructed<Fraction.java>
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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Application Deconstructed<FractionDemo.java>
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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();
Application Deconstructed<FractionDemo.java>
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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
Application Deconstructed<FractionDemo.java>
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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 ?
.
.
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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
Fraction f2 = new Fraction();f2.setNumeratorAndDenominator(1,2);
f2 208numerator 1 denominator 2
208
f1 = f2;f1 208
f2 208numerator 1 denominator 2
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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Fraction f1 = new Fraction();f1.setNumeratorAndDenominator(1,2);
f1 200numerator 1 denominator 2
200
Fraction f2 = new Fraction();f2.setNumeratorAndDenominator(1,2);
f2 208numerator 1 denominator 2
208
if (f1.equals(f2) {...}...public boolean equals(Fraction f) {...}
f2 208numerator 1 denominator 2
208
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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Fraction f1 = new Fraction();f1.setNumeratorAndDenominator(1,2);
f1 200numerator 1 denominator 2
200
resetFraction(f1);...
public void resetFraction(Fraction f)
f1 200numerator 1 denominator 2
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);}
f1 200numerator 1 denominator 2
200
f 208numerator 1 denominator 1
208