Object-Oriented Programming

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Beverly Horatio McBride
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1 Objects and Classes Object-Oriented Programming

2 Outline Classes vs. objects Designing a class Methods and instance variables Encapsulation & information hiding Readings: HFJ: Ch. 2, 3, 4. GT: Ch. 3, 4. Đại học Công nghệ - ĐHQG HN Objects and Classes 2

3 Java program public class Greeting { public void greet() { System.out.print("Hi there!"); public class TestGreeting { public static void main(string[] args) { Greeting gr = new Greeting(); gr.greet(); A Java program, at run-time, is a collection of objects. They do things (their methods) and ask other objects to do things (calling methods of others). A Java program, when we write it, is a collection of classes A Java library contains predefined classes that we can use in our programs Đại học Công nghệ - ĐHQG HN Objects and Classes 3

4 Classes vs. objects A class is a blueprint/template that is used to construct objects. Each object is instantiated from a class. That object is called an instance of the class. Đại học Công nghệ - ĐHQG HN Objects and Classes 4

5 Designing a class When you design a class, think about the objects that will be created from that class things the object knows about itself things the object does Đại học Công nghệ - ĐHQG HN Objects and Classes 5

6 Designing a class things the object knows about itself instance variables the object's instance variables represent its state things the object can do methods the object's methods represent its behavior Đại học Công nghệ - ĐHQG HN Objects and Classes 6

7 Writing a class 1. Write the class class Dog { int size; String breed; String name; instance variables a method void bark() { System.out.println("Ruff! Ruff!"); DOG size breed name bark() Đại học Công nghệ - ĐHQG HN Objects and Classes 7

8 Writing a class 2. Write a tester (TestDrive) class with code to test the Dog class dot notation (.) gives access to an object's instance variables and methods public class DogTestDrive { public static void main(string [] args) { Dog d = new Dog(); make a Dog object d.name = "Bruno"; d.bark(); set the name of the Dog call its bark() method Information hiding is not here yet. Đại học Công nghệ - ĐHQG HN Objects and Classes 8

9 Writing a class Instance variables/methods belong to an object. Thus, when accessing them, you MUST specify which object they belong to. dot notation (.) and the object reference public class DogTestDrive { public static void main(string [] args) { Dog d = new Dog(); d.name = "Bruno"; d.bark(); access 'name' of the Dog call its bark() method Đại học Công nghệ - ĐHQG HN Objects and Classes 9

10 Object references 3 steps of object declaration, creation and assignment: 1. Declare a reference variable Dog mydog = new Dog(); 2. Create an object Dog mydog = new Dog(); 3. Link the object and the reference Dog mydog = new Dog(); Đại học Công nghệ - ĐHQG HN Objects and Classes 10

11 Object references Dog mydog = new Dog(); Remember: References are not objects! Đại học Công nghệ - ĐHQG HN Objects and Classes 11

12 Messaging between objects Sending a message to an object is actually calling a method of the object. d.bark() Syntax: <object reference>.<method_name>(<arguments>) recipient message content extra information Đại học Công nghệ - ĐHQG HN Objects and Classes 12

13 Methods How objects behave Objects have state (instance variables) behavior (methods) A method can use instance variables' value and change the object's state. A method can use instance variables so that objects of the same type can behave differently Đại học Công nghệ - ĐHQG HN Objects and Classes 13

14 State affects behavior, behavior affects state class Dog { 1. Write the class int size; String breed; String name; void bark() { if (size > 14) System.out.println("Ruff! Ruff!"); else System.out.println("Yip! Yip!"); void getbigger() { size += 5; State affects behavior. Dogs of different sizes behave differently. size breed name DOG bark() method changes state getbigger() Đại học Công nghệ - ĐHQG HN Objects and Classes 14

15 State affects behavior, behavior affects state class DogTestDrive { 1. Write the class public static void main (String[] args) { Dog one = new Dog(); one.size = 7; Dog two = new Dog(); two.size = 13; two.bark(); two.getbigger(); two.bark () ; one.bark(); Đại học Công nghệ - ĐHQG HN Objects and Classes 15

16 State affects behavior, behavior affects state class DogTestDrive { 1. Write the class public static void main (String[] args) { Dog one = new Dog(); one.size = 7; Dog two = new Dog(); two.size = 13; one Dog object 1 name:null size:0 breed:null two.bark(); two.getbigger(); two.bark () ; %> java DogTestDrive one.bark(); Đại học Công nghệ - ĐHQG HN Objects and Classes 16

17 State affects behavior, behavior affects state class DogTestDrive { 1. Write the class public static void main (String[] args) { Dog one = new Dog(); one.size = 7; Dog two = new Dog(); two.size = 13; one Dog object 1 name:null size: 7 breed:null two.bark(); two.getbigger(); two.bark () ; %> java DogTestDrive one.bark(); Đại học Công nghệ - ĐHQG HN Objects and Classes 17

18 State affects behavior, behavior affects state class DogTestDrive { 1. Write the class public static void main (String[] args) { one Dog object 1 name:null size: 7 breed:null Dog one = new Dog(); one.size = 7; Dog two = new Dog(); two.size = 13; two.bark(); two.getbigger(); two.bark () ; two %> java DogTestDrive Dog object 2 name:null size:13 breed:null one.bark(); Đại học Công nghệ - ĐHQG HN Objects and Classes 18

19 State affects behavior, behavior affects state class DogTestDrive { 1. Write the class public static void main (String[] args) { one Dog object 1 name:null size: 7 breed:null Dog one = new Dog(); one.size = 7; Dog two = new Dog(); two.size = 13; two Dog object 2 name:null size:13 breed:null two.bark(); two.getbigger(); two.bark () ; one.bark(); %> java DogTestDrive Yip! Yip! Đại học Công nghệ - ĐHQG HN Objects and Classes 19

20 State affects behavior, behavior affects state class DogTestDrive { 1. Write the class public static void main (String[] args) { one Dog object 1 name:null size: 7 breed:null Dog one = new Dog(); one.size = 7; Dog two = new Dog(); two.size = 13; two Dog object 2 name:null size:18 breed:null two.bark(); two.getbigger(); two.bark () ; one.bark(); %> java DogTestDrive Yip! Yip! Đại học Công nghệ - ĐHQG HN Objects and Classes 20

21 State affects behavior, behavior affects state class DogTestDrive { 1. Write the class public static void main (String[] args) { one Dog object 1 name:null size: 7 breed:null Dog one = new Dog(); one.size = 7; Dog two = new Dog(); two.size = 13; two Dog object 2 name:null size:18 breed:null two.bark(); two.getbigger(); two.bark () ; one.bark(); %> java DogTestDrive Yip! Yip! Ruff! Ruff! Đại học Công nghệ - ĐHQG HN Objects and Classes 21

22 State affects behavior, behavior affects state class DogTestDrive { 1. Write the class public static void main (String[] args) { one Dog object 1 name:null size: 7 breed:null Dog one = new Dog(); one.size = 7; Dog two = new Dog(); two.size = 13; two Dog object 2 name:null size:18 breed:null two.bark(); two.getbigger(); two.bark () ; one.bark(); %> java DogTestDrive Yip! Yip! Ruff! Ruff! Yip! Yip! %> Đại học Công nghệ - ĐHQG HN Objects and Classes 22

23 Compare size, name vs. bruno, value syntax? scope? class Dog { int size; String name;... void getbigger() { size += 5; public class DogTestDrive { public static void main(string [] ar Dog bruno = new Dog(); bruno.name = "Bruno";... int value = bruno.size; Đại học Công nghệ - ĐHQG HN Objects and Classes 23

24 Instance variables vs. local variables Instance variables belong to an object declared inside a class but NOT within a method have default values (0, 0.0, false, null ) class Dog { int size; String name;... void getbigger() { size += 5; Local variables belong to a method declared within a method MUST be initialized before use public class DogTestDrive { public static void main(string [ Dog bruno = new Dog(); bruno.name = "Bruno";... int size = bruno.size; Đại học Công nghệ - ĐHQG HN Objects and Classes 24

25 Encapsulation Bad Better class Person { String name; Date birthday; String address; // about his/her dog String dogname; String dogbreed; int dogsize; class Dog { int size; String breed; String name;... class Person { String name; Date birthday; String address; Dog petdog; Đại học Công nghệ - ĐHQG HN Objects and Classes 25

26 Encapsulation / information hiding What is wrong with this code? It allows for a supernatural dog Object's data is exposed. class Dog { int size; String breed; String name;... Dog d = new Dog(); d.size = -1; Exposed instance variables can lead to invalid states of object What to do about it? write set methods (setters) for instance variables hide the instance variables to force other code to use the set methods instead of accessing them directly. Đại học Công nghệ - ĐHQG HN Objects and Classes 26

27 Information hiding. Rule of thumb Mark instance variables private. Make getters and setters and mark them public. Don't forget to check data validity in setters. class Dog { private int size; public void setsize(int s) { if (s > 0) size = s; public int getsize() { return size;... Đại học Công nghệ - ĐHQG HN Objects and Classes 27

28 Class access control Access modifiers: public : Accessible anywhere by anyone private : Only accessible within the current class protected : Accessible only to the class itself and to its subclasses or other classes in the same package default (no keyword): accessible within the current package Đại học Công nghệ - ĐHQG HN Objects and Classes 28

object features (instant variables and methods) Usually more involved and may have complex inner

29 Implementation vs. Interface DogTestDrive: a client of Dog Implementation Data structures and code that implement the object features (instant variables and methods) Usually more involved and may have complex inner workings Clients don t need to know Interface The controls exposed to the client by the implementation The knobs on the black box Đại học Công nghệ - ĐHQG HN Objects and Classes 29

30 Encapsulation / information hiding Don t expose internal data structures! Objects hold data and code Neither is exposed to the end user or "client" modules. Interface vs. implementation A cat's look vs. its internal organs A TV's screen & buttons vs. the stuff inside the box Complexity is hidden inside the object Make life easier for clients More modular approach Implementation changes in one component doesn t affect others Less error-prone Đại học Công nghệ - ĐHQG HN Objects and Classes 30

More on Java. Object-Oriented Programming

More on Java. Object-Oriented Programming More on Java Object-Oriented Programming Outline Instance variables vs. local variables Primitive vs. reference types Object references, object equality Objects' and variables' lifetime Parameters passing