Abstract class & Interface

Transcription

1 Islamic University of Gaza Faculty of Engineering Computer Engineering Department Computer Programming Lab (ECOM 2124) Lab 3 Abstract class & Interface Eng. Mohammed Abdualal

2 Abstract class 1. An abstract class is one that cannot be instantiated. 2. It contains abstract methods, which are implemented in concrete subclasses. Declaring Abstract class: Use the abstract keyword to declare a class abstract. The keyword appears in the class declaration somewhere before the class keyword. public abstract class Person{ private String Name; private int Age; public Person(String Name, int Age) { this.name = Name; this.age = Age; public String getname() { return Name; public void setname(string Name) { this.name = Name; public int getage() { return Age; public void setage(int Age) { this.age = Age; public String tostring() { return getname()+" "+getage(); If you try to make instance from an abstact class you will see compilation error Like the following code.

3 class Student extends Person{ private double grade; public Student(double grade, String Name, int Age) { super(name, Age); this.grade = grade; public double getgrade() { return grade; public void setgrade(double grade) { this.grade = grade; public String tostring() { return super.tostring(); From above classes (Person superclass and abstact,student subclass ) Testing: class Testing{ public static void main(string[] args) { Student s = new Student(90, "Mohammed S. Abdualal", 24); System.out.println("Grade : "+s.getgrade()); Person p = new Student(90, "Mohammed S. Abdualal", 24); System.out.println("Age :"+p.getage()); System.out.println("Name + Age :"+p.tostring()); Output :

4 Abstract Methods: If you want a class to contain a particular method but you want the actual implementation of that method to be determined by child classes, you can declare the method in the parent class as abstract. The abstract keyword is also used to declare a method as abstract. An abstract method consists of a method signature, but no method body. Abstract method would have no definition, and its signature is followed by a semicolon, not curly braces as follows : public abstract class Employee { private String name; private String address; private int number; public abstract double computepay(); //Remainder of class definition Declaring a method as abstract has two results: 1. The class must also be declared abstract. If a class contains an abstract method, the class must be abstract as well. 2. Any child class must either override the abstract method or declare itself abstract. You have to override abstract method in Employee computepay() class Salary extends Employee{ private double salary= ; public double computepay() { return salary/50;

5 Java Interfaces An interface is a collection of abstract methods. A class implements an interface, thereby inheriting the abstract methods of the interface. An interface is not a class. they are two different concepts: 1. A class describes the attributes and behaviors of an object. 2. An interface contains behaviors that a class implements. The class that implements the interface is abstract, all the methods of the interface need to be defined in the class. An interface is different from a class in several ways: o You cannot instantiate an interface. o An interface does not contain any constructors. o All of the methods in an interface are abstract. o An interface cannot contain instance fields. The only fields that can appear in an interface must be declared both static and final. o An interface is not extended by a class; it is implemented by a class. o An interface can extend multiple interfaces. Declaring Interfaces: The interface keyword is used to declare an interface. Example: interface Animal{ public void eat(); public void travel(); Interfaces have the following properties: An interface is implicitly abstract. Each method in an interface is also implicitly abstract Methods in an interface are implicitly public.

6 Implementing Interfaces: class Dog implements Animal { public void eat() { System.out.println("Meat and Fish :) "); public void travel() { System.out.println("Travel from Egypt to Gaza"); Testing: package test_interface; public class Test_Interface { public static void main(string[] args) { // TODO code application logic here Dog dog = new Dog(); dog.eat(); dog.travel(); Output: When implementation interfaces there are several rules: o A class can implement more than one interface at a time. o A class can extend only one class, but implement many interfaces. o An interface can extend another interface, similarly to the way that a class can extend another class.

7 Extending Interfaces: An interface can extend another interface. The extends keyword is used to extend an interface The child interface inherits the methods of the parent interface. Example: The following Sports interface is extended by Hockey and Football interfaces. interface Sports { public void sethometeam(string name); public void setvisitingteam(string name); interface Football extends Sports { public void hometeamscored(int points); public void visitingteamscored(int points); public void endofquarter(int quarter); interface Hockey extends Sports { public String homegoalscored(); public String visitinggoalscored(); class Barcelona implements Hockey { private String HomeTeam; private String VisitingTeam; public String homegoalscored() { return HomeTeam; public String visitinggoalscored() { return VisitingTeam; public void sethometeam(string HomeTeam) { this.hometeam = HomeTeam; public void setvisitingteam(string VisitingTeam) { this.visitingteam = VisitingTeam;

8 Testing: package test_interface; public class Test_Interface { public static void main(string[] args) { // TODO code application logic here Barcelona barcelona = new Barcelona(); barcelona.sethometeam("barcelona"); barcelona.setvisitingteam("realmadried"); System.out.println(""+barcelona.homeGoalScored()); System.out.println(""+barcelona.visitingGoalScored()); Output: Extending Multiple Interfaces: 1. A Java class can only extend one parent class. Multiple inheritance is not allowed. 2. An interface can extend more than one parent interface. 3. The extends keyword is used once, and the parent interfaces are declared in a comma-separated list. Example: public interface Hockey extends Sports,Football. Best wishes