Principles of Object Oriented Programming. Lecture 4

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1 Principles of Object Oriented Programming Lecture 4

2 Object-Oriented Programming There are several concepts underlying OOP: Abstract Types (Classes) Encapsulation (or Information Hiding) Polymorphism Inheritance

3 Abstract Data Types Object-oriented languages provide a way to define abstract data types, and then create objects from them It s a template (or cookie cutter ) from which we can create new objects For example, a Car class might have attributes of speed, colour, and behaviours of accelerate, brake, etc

4 Encapsulation The data (state) of an object is private it cannot be accessed directly. The state can only be changed through its behaviour, otherwise known as its public interface or contract This is called encapsulation Private Data Public Interface "The Doughnut Diagram" Showing that an object has private state and public behaviour. State can only be changed by invoking some behaviour

5 Encapsulation Main benefit of encapsulation Internal state and processes can be changed independently of the public interface Limits the amount of large-scale changes required to a system

6 Modifiers Apr

7 Modifiers Public/private are visibility modifiers Used to indicate visibility of methods and attributes

8 Modifiers Java has a range of other modifiers Control ownership of a method or attribute Control when and how variable can be initialised Control inheritance of methods (and whether they can be overridden by a sub-class)

9 Modifiers Public Protected Private Static Final

10 Modifiers Access Specifiers Class Package Child Class Entire Program Public O O O O Protected O O O X (omitted) O O X X Private O X X X

11 Example class A { private int a; int b; public int c; public class Test { public static void main(string args[]) { A obj = new A(); obj.a = 10; //? obj.b = 20; //? obj.c = 30; //?

12 Setters and Getters Setter / Mutator a method that sets up a value of a field format: setxxx() Getter / Accessor a method that returns a value of a field format: getxxx()

13 Setters and Getters These public methods are used to access private fields These methods are also used to check the attempt to change the values of private fields public void setage(int age) { if( age < 0 ) this.age = 0; else this.age = age;

14 LAB# 4-1 When we use an array, if we set up the index out of the size of the array, we will encounter a run-time error. Create a class for a safe array that checks if the given index is within the scope of its index public class SafeArray { private int a[]; public int length; public SafeArray(int size) { a = new int[size]; length = size; public int get(int index) {?? public void put(int index, int value) {??

15 Static static indicates a class variable or class method. It s not owned by an individual object This means we don t have to create an object to use it Arrays.sort and System.arrayCopy are static methods

16 Static -- Example public class MyClass { public static void utilitymethod() { public void othermethod() { //using the above: MyClass.utilityMethod(); MyClass objectofmyclass = new MyClass(); objectofmyclass.othermethod(); objectofmyclass.utilitymethod(); //this is illegal: MyClass.otherMethod();

17 Final final to make a variable that can have a single value Can be assigned to once and once only Useful to ensure a variable isn t changed once its assigned. final int count; count = 10; //the following will cause an error count = 20;

18 Defining Constants Unlike other languages, Java has no const keyword Must use a combination of modifiers to make a constant static to indicate its owned by the class final to make sure it can t be changed (and initialise it when its declared) Naming convention for constants is to use all capitals

19 Constants An Example public class MyClass { public static final int COUNT = 0; public static final boolean SWITCHED_ON = false; //example usage: if (MyClass.COUNT > 0) { if (MyClass.SWITCHED_ON) {

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