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Lecture 2 – Classes
- (^) What is Object Oriented Programming?
- (^) Object oriented Programming languages (OOP for short) include all
the features of structured programming and add more powerful ways
to organize algorithms and data structures.
- (^) There are three key features of OOP languages:
- (^) Encapsulation
- (^) Inheritance
- (^) Polymorphism
- (^) All of them are tied to the notion of a class
- (^) These two lectures focus on encapsulation through the use of classes
and objects - Object Based Programming.
Introduction
Classes
- (^) A class defines a set of variables ( attributes or data members ) that can be associated with the methods which act on an object with the object itself. data member methods
Classes
- (^) A class contains data declarations and method declarations int x, y; char ch; Data declarations Method declarations
Classes
- (^) A class can be viewed as a blueprint of an object.
- (^) It is the model or pattern from which objects are created.
- (^) For example, the String class is used to define String objects.
- (^) Each String object contains specific characters (its state).
- (^) Each String object can perform services (behaviors) such as toUpperCase.
Classes and Objects s1:String "Programming" s2:String "Java" s3:String "ive-ty" String (class)
- (^) The String class can be used to define as many String objects as you like.
- (^) Its reusability is very high.
- (^) The String class is provided for us by the Java standard class library.
- (^) But we can also write our own classes that define specific objects that we need
class TestStringClass { public static void main(String s[]) { String s1 = new String("Programming"); String s2 = new String("Java"); String s3 = new String("ive-ty"); System.out.println(s1.toUpperCase()); System.out.println(s2.toUpperCase()); System.out.println(s3.toUpperCase()); } } Classes and Objects In this example, String is class. s1, s2 and s3 are the objects (or references to objects, precisely speaking). Each of them has different state but same behavior.
Example
- (^) Thus when we create a new object we say we are instantiating the object. Each class exists only once in a program, but there can be many thousands of objects that are instances of that class.
- (^) To instantiate an object in Java we use the new operator. Here's how we'd create a new employee: Employee x = new Employee(); Class Name Object Name Class Name (Again!)
- (^) The instantiation can also be done in two lines: Employee x; x = new Employee();
Example
- (^) The members of an object are accessed using the. (dot) operator, as follows: Employee x = new Employee(); x.setNum( 12651 ); Object Name . (dot) Method Name
Example Employee emp1 = new Employee(); Employee emp2 = new Employee(); Memory emp num : 0 getNum setNum emp num : 0 getNum setNum emp num : 0 getNum setNum
Example emp1.setNum(12651); emp2.setNum(36595); emp1.getNum() returns 12651 emp2.getNum() returns 36595
Note that the object name (emp1 and emp2) is simplified here. See
later slides for details
Memory emp num : 12651 getNum setNum emp num : 12651 getNum setNum emp num : 36595 getNum setNum emp num : 0 getNum setNum
Encapsulation
- (^) As you can see from the above diagram, an object's variables make up the centre or nucleus of the object. Methods surround and hide the object's nucleus from other objects in the program.
- (^) This is called encapsulation. emp num : 0 getNum setNum
Encapsulation
- (^) Member access modifiers
- (^) Control access to class’ instance variables and methods
- (^) public
- (^) Variables and methods accessible to other classes
- (^) private
- (^) Variables and methods not accessible to other classes
- (^) Can be accessed via
- (^) Accessor method (“ get ” method)
- (^) Allow clients to read private data
- (^) Mutator method (“ set ” method)
- (^) Allow clients to modify private data
A BAD Example class PublicDataEmployee { public int num; } class PrintPublic { public static void main(String[] args) { PublicDataEmployee emp1 = new PublicDataEmployee(); PublicDataEmployee emp2 = new PublicDataEmployee(); emp1.num = 12651; emp2.num = 36595; System.out.println("num of emp1 : " + emp1.num ); System.out.println("num of emp2 : " + emp2.num ); } }
java PrintPublic num of emp1 : 12651 num of emp2 : 36595
- (^) The following example shows an incorrect use of public data member.
A GOOD Example class PrivateDataEmployee { private int num; public int getNum() {return num;} public void setNum(int newNum) {num=newNum;} } class PrintPrivate { public static void main(String[] args) { PrivateDataEmployee emp1 = new PrivateDataEmployee(); PrivateDataEmployee emp2 = new PrivateDataEmployee(); emp1.setNum(12651); emp2.setNum(36595); System.out.println("num of emp1 : " + emp1.getNum() ); System.out.println("num of emp2 : " + emp2.getNum() ); } }
java PrintPrivate num of emp1 : 12651 Num of emp2 : 36595
- (^) The following shows how to fix the problem in the previous example.
