Slide 1

Slide 2

C OMP 401 O BJECTS Instructor: Prasun Dewan

Slide 3

2 C OMPUTER VS. P ROGRAM M ODEL Processor Compiler Program (source code)

Slide 4

3 S TRUCTURING IN S CRIPTS Script Performer Theater Follows

Slide 5

4 S TRUCTURING IN S CRIPTS Script Introduction Body Conclusion Paragraph 1 Paragraph 2 Sentence 1Sentence 2 Script components are abstract. So are program components!

Slide 6

5 P ROGRAM C OMPONENTS ~ P HYSICAL O BJECTS Natural Objects Manufactured Objects ~ Program Components

Slide 7

6 P ROGRAM O BJECTS ~ M ANUFACTURED O BJECTS manufactured by perform Operations accelerate brake Class Program Object instance of Methods add subtract execute invoke call

Slide 8

7 C LASSIFICATION T HROUGH F ACTORIES manufactured by manufactured by

Slide 9

8 C LASSIFICATION T HROUGH F ACTORIES ASquareCalculator ABMICalculator ASquareCalculator Instance ABMICalculator Instance instance of

Slide 10

9 A S IMPLE I NSTANTIATED C LASS public class ASquareCalculator { public int square( int x) { return x*x; } public class SquareCalculatorTester { public static void main (String[] args) { ASquareCalculator squareCalculator = new ASquareCalculator(); System.out.println (squareCalculator.square(5)); } Object Creation Object Use No static because class will be instantiated No package

Slide 11

10 P ACKAGES package math; public class ASquareCalculator { public int square( int x) { return x*x; } package main; import math.ASquareCalculator; public class SquareCalculatorTester { public static void main (String[] args) { ASquareCalculator squareCalculator = new ASquareCalculator(); System.out.println (squareCalculator.square(5)); } Class in different package must be imported using full name of class ( a la full file name)

Slide 12

11 P ACKAGES package math; public class ASquareCalculator { public int square( int x) { return x*x; } package math; public class SquareCalculatorTester { public static void main (String[] args) { ASquareCalculator squareCalculator = new ASquareCalculator(); System.out.println (squareCalculator.square(5)); } Class in same package need not be imported

Slide 13

12 P ACKAGES package math; public class ASquareCalculator { public int square( int x) { return x*x; } import math.ASquareCalculator; public class SquareCalculatorTester { public static void main (String[] args) { ASquareCalculator squareCalculator = new ASquareCalculator(); System.out.println (squareCalculator.square(5)); } No package means package named default, hence import needed

Slide 14

13 P ACKAGES public class ASquareCalculator { public int square( int x) { return x*x; } package main ; public class SquareCalculatorTester { public static void main (String[] args) { ASquareCalculator squareCalculator = new ASquareCalculator(); System.out.println (squareCalculator.square(5)); } Short name of class in default package same as its full name

Slide 15

14 P ACKAGES public class ASquareCalculator { public int square( int x) { return x*x; } public class SquareCalculatorTester { public static void main (String[] args) { ASquareCalculator squareCalculator = new ASquareCalculator(); System.out.println (squareCalculator.square(5)); } No package means package named default, hence no import needed here

Slide 16

15 L ONG NAME WITH NO IMPORT package math; public class ASquareCalculator { public int square( int x) { return x*x; } package main; public class SquareCalculatorTester { public static void main (String[] args) { ASquareCalculator squareCalculator = new math.ASquareCalculator(); System.out.println (squareCalculator.square(5)); } Can use the full name of class directly

Slide 17

16 package safemath; public class ASquareCalculator { public long square( int x) { return x*x; } W HY IMPORTS / FULL NAME ? package math; public class ASquareCalculator { public int square( int x) { return x*x; } package main; import math.ASquareCalculator; public class SquareCalculatorTester { public static void main (String[] args) { ASquareCalculator squareCalculator = new ASquareCalculator(); System.out.println (squareCalculator.square(5)); } Twice the size of ints Disambiguates

Slide 18

17 package safemath; public class ASquareCalculator { public long square( int x) { return x*x; } A MGIGOUS I MPORT package math; public class ASquareCalculator { public int square( int x) { return x*x; } package main; import math.ASquareCalculator; import safemath.ASquareCalculator; public class SquareCalculatorTester { public static void main (String[] args) { ASquareCalculator squareCalculator = new ASquareCalculator(); System.out.println (squareCalculator.square(5)); } Ambiguous

Slide 19

18 W HY P ACKAGES ? Can create competing implementations of same class. A la creating files Test.java in different assignment directories/folders Can browse related classes A la browsing through all files in an assignment directory/folder. Like directories/folders packages can be hierarchical package math.power; public class ACubeCalculator {}

Slide 20

19 B ROWSING J AVA C LASSES Very useful package

Slide 21

20 L ANGUAGE VS. L IBRARY Built-in classes

Slide 22

21 C HANGING P ARAMETER public class ASquareCalculator { public int square( int x) { return x*x; } public class SquareCalculatorTester { public static void main (String[] args) { ASquareCalculator squareCalculator = new ASquareCalculator(); System.out.println (squareCalculator.square(5)); } Calculates 5*5

Slide 23

22 C HANGING P ARAMETER public class ASquareCalculator { public int square( int x) { return x*x; } public class SquareCalculatorTester { public static void main (String[] args) { ASquareCalculator squareCalculator = new ASquareCalculator(); System.out.println (squareCalculator.square(341)); } Must change code

Slide 24

23 R ERUN P ROGRAM public class ASquareCalculator { public int square( int x) { return x*x; } public class SquareCalculatorTester { public static void main (String[] args) { ASquareCalculator squareCalculator = new ASquareCalculator(); System.out.println (squareCalculator.square(Ineteger.parseInt(args[0])); } Must re-run program How to not re- run program without writing tedious UI code?

Slide 25

24 O BJECT E DITOR package math; public class ASquareCalculator { public int square( int x) { return x*x; } package main; import math.ASquareCalculator; import bus.uigen.ObjectEditor; public class SquareCalculatorTester { public static void main (String[] args) { ASquareCalculator squareCalculator = new ASquareCalculator(); ObjectEditor.edit(squareCalculator ); } ObjectEditor is predefined packaged class

Slide 26

25 E DITING AS QUARE C ALCULATOR I NSTANCE

Slide 27

26 I NVOKING A M ETHOD A ND V IEWING THE R ESULT

Slide 28

27 A NOTHER S IMPLE C LASS : ABMIC ALCULATOR ASquareCalculatorABMICalculator Specification: Given an integer x, calculate the square of x. package math ; public class ASquareCalculator { public int square( int x) { return x*x; } Specification: Given the weight (kg) and height (m) of a person, calculate the persons body mass index a.k.a. BMI. ?

Slide 29

28 ABMIC ALCULATOR package bmi ; public class ABMICalculator { public int calculateBMI( int weight, int height) { return weight/(height*height); } Parameter and return types are integers But height (m) and weight (kg) are expressed as decimals How do we solve the discrepancy?

Slide 30

29 ABMIC ALCULATOR package bmi ; public class ABMICalculator { public int calculateBMI( int weight, int height) { return weight/(height*height); } double Doubles are decimal/real numbers

Slide 31

30 ABMIC ALCULATOR package bmi ; public class ABMICalculator { public double calculateBMI( double weight, double height) { return weight/(height*height); }

Slide 32

31 F ORMAL VS. A CTUAL P ARAMETERS package bmi ; public class ABMICalculator { public double calculateBMI( double weight, double height) { return weight/(height*height); } weight 0 0 height 0 0 variablesmemory Parameters Invoke calculateBMI Actual Parameters Formal Parameters 74.98 1.94 assigned

Slide 33

32 P ROGRAMMED C ALL public class ABMICalculator { public double calculateBMI( double weight, double height) { return weight/(height*height); } public class BMICalculatorTester { public static void main (String[] args) { ABMICalculator bmiCalculator = new ABMICalculator(); System.out.println (bmiCalculator.calculateBMI(75, 1.77)); } Formal Parameters Actual Parameters

Slide 34

33 P ROGRAMMED VS. INTERACTIVE CALL System.out.println(setWeight Called); Target ObjectMethod NameActual Parameters Programmed Call Interactive Call

Slide 35

34 I NTERNAL VS. E XTERNAL M ETHOD C ALLS public class APoundInchBMICalculator { public double calculateBMI( double weightInLbs, double heightInInches) { return ( new ABMICalculator()).calculateBMI( toKgs(weightInLbs), toMetres(heightInInches)); } public double toMetres( double heightInInches) { } public double toKgs( double weightInLbs) { } } APoundInchBMICalculator External Caller and callee methods are in different objects Must specify target object Internal Caller and callee methods are in the same object Target object is implicit ( this ) Actual parameters:

Slide 36

35 E RRORS package bmi ; class ABMICalculator { double calculateBMI( double weight, double height) { return (height*heigh)/weight } Syntax ErrorLogic Error Semantics Error Access Error

Slide 37

36 E RRORS package bmi ; class ABMICalculator { double calculateBMI( double weight, double height) { return (height*heigh)/weight } Syntax ErrorLogic Error Semantics Error Access Error

Slide 38

37 M ETHOD A CCESS E RROR You instantiate a ABMICalculator object but there is no ABMICalculator menu item Reason You have not defined any public methods in ABMICalculator