Interfaces

1. Chapter 05 Interfaces Prepared by Miss Simab Shahid (s.shahid@uoh.edu.sa) Lecturer computer Science and Software Engineering department, University of Hail

2. Interface • An interface is something like an extreme case of an abstract class • An abstract class can have non abstract methods • But interface must have abstract methods only. • However an interface is not a class • It is a type that can be satisfied by any class that implements the interface Simab Shahid UOH, Girls Branch

3. Interface Syntax • The syntax for defining an interface is similar to that of defining a class • Except the word interface is used in place of class • An interface specifies a set of methods that any class that implements the interface must have Simab Shahid UOH, Girls Branch

4. Interface example interfaceFirstHelloInterface { void sayHello(); } Important Point: Method declared in interface is implicitly public and abstract Simab Shahid UOH, Girls Branch

5. Important about interface • We cant create object of an interface just like we cant create object of an abstract class • Because both the interface and abstract class have abstract methods • Rather we create object of a class that implements the interface. Simab Shahid UOH, Girls Branch

6. Class implementing an interface • If the class is not implementing all the abstract methods of given interface, then the class must be declared as abstract class. • In the example below Cat is declared as abstract, coz it is not implementing the abstract method “void eat()” interface Animal { void talk( ); // abstract method void eat( ); // abstract method } public abstract class Cat implements Animal { void talk( ) // class is implementing this method { Sytem.out.println(“ Cat says Mao Mao”); } abstract void eat( ); // abstract method } Simab Shahid UOH, Girls Branch

7. Class implementing an interface • If the class is implementing all the abstract methods of given interface, then the class will be declared as concrete class. • In the example below Cat is declared as concrete class, because it is implementing all the abstract methods of interface “Animal” interface Animal { void talk( ); // abstract method void eat( ); // abstract method } public class Cat implements Animal { void talk( ) // class is implementing this method { Sytem.out.println(“ Cat says Mao Mao…..”); } void eat( ) { Sytem.out.println(“ Cat eats meat…..”); } } Simab Shahid UOH, Girls Branch

8. Interface • Interface contains method headings and constant definitions only interface shape {   Stringbaseclass = "shap"; // constantpublic void Draw(); // method heading  } Simab Shahid UOH, Girls Branch

9. Interfaces • Interface contains neither instance variables nor any complete method definitions interfaceshap  { public  String baseclass;// wrong Shap(){ } // wrong not allowed  public void Draw() { System.out.println(“Implementation of Draw() Method”);//wrong } } Simab Shahid UOH, Girls Branch

10. Interfaces • Interface cannot be instantiated interface A{ public void display( String s); } publicclassInterfaceExample { publicstaticvoidmain( Stringargs[] ) { A objOfA = new A();//WRONG: not allowed to create object of A(Interface) } } Simab Shahid UOH, Girls Branch

11. Interface • Interface variable is implicitly public, static and final • Interface method is implicitly publicand abstract (is not implemented by this class) • A class can implement one or more interfaces • An interface can be implemented by several classes • Interface name can be used as the type of a variable i.e interface reference Simab Shahid UOH, Girls Branch

12. Interface • An interface and all of its method headings should be declared public • They cannot be given private, protected, or package access • When a class implements an interface, it must make all the methods in the interface public • Because an interface is a type, a method may be written with a parameter of an interface type • That parameter will accept as an argument any class that implements the interface Simab Shahid UOH, Girls Branch

13. Interface (Examples) interface Communicate { int LOUD = 0; int SOFT = 1; int OFF = 2; void talk(); voidlisten(); } class Telephone implements Communicate { //implementation of talk() method of interface Communicate publicvoid talk() { System.out.println(“Talk Method”); } //implementation of listen() method of Communicate interface publicvoid listen(){ System.out.println(“Listen Method”); } //implementation of call() method ,call() method is not member of //Communicate interface publicvoid call( String number) { System.out.println(“Call Method”); } } Simab Shahid UOH, Girls Branch

14. Interface (Example) class Professor implements Communicate { publicvoid talk() { System.out.println(“Talk Method”); } publicvoid listen() { System.out.println(“Listen Method”); } // other methods implemented void Lecture( String topic) { } } • The keyword implements indicates that the class implements one or more interfaces. Simab Shahid UOH, Girls Branch

15. Exercise(Interface) classInterfaceRefVariable { publicstaticvoid main( String[] args) { B b =new D0(); b.display(); b = new D1(); b.display(); b =new D2(); b.display(); } } // What compile-time error generated for this program? interface B { void display(); } class D0 { } class D1 implements B { publicvoid display() { System.out.println( "D1" ); } } class D2implementsB { publicvoid display() { System.out.println( "D2" ); } } Simab Shahid UOH, Girls Branch

16. // Interface extends one or more interfaces • interface J { • inti=200; • int J1(); • } • interface K { double K1(); } • interface L extends J, K • { • boolean L1(); • } • class I implements L { • publicint J1() • { • return 4; • } • public double K1() • { • return7.98; • } • public boolean L1() • { • returntrue; • } • } classInterfaceInheritance { publicstaticvoid main( String[]args) { I a =new I(); System.out.println(a.i); System.out.println(a.J1()); System.out.println(a.K1()); System.out.println(a.L1()); } } Simab Shahid UOH, Girls Branch

17. Interface • The keyword extends is used to define an inheritance relationship between interfaces. • An interface may directly extend multiple interfaces. interface L1 { void f(); void g(); } interface L2 extends L1 { void f(); int g(); } class CE { publicstaticvoid main(String [] args) { System.out.println( "Hello" ); } } • The following program generates a compile-time error. • Interface L1 declares f() and g(). • Interface L2 declares the same except return type of g() is different. The compiler identifies it as an error. Simab Shahid UOH, Girls Branch

18. Instanceof Operator • The instanceof operator is used to determine if an object is of a particular class or implements a specific interface. • Syntax: varName instanceof type • varName is an object reference variable • type is the name of either a class or an interface • The expression evaluates to true if varName is a type. Otherwise, it evaluates to false. Simab Shahid UOH, Girls Branch

19. Instanceof Operator (Example) abstract class Fish { abstractvoid display(); } abstract class SaltWtrFish extends Fish { } abstract class FreshWtrFish extends Fish { } class Trout extends FreshWtrFish { void display() { System.out.println( "Trout" ); } } class Tuna extends SaltWtrFish { void display() { System.out.println( "Tuna" ); } } class InstantofOperator { publicstaticvoid main( String[] arg) { Fish f[] =new Fish[3]; f[0] =new Trout(); f[1] =new Tuna(); f[2] =new Trout(); for(int j = 0; j < 3; j++) if ( f[j] instanceof Trout ) f[j].display(); } } Simab Shahid UOH, Girls Branch Output: Trout Trout

20. Instanceof Operator (Example) interface Vehicle { void drive(); } abstractclass Mammal { } class Bear extends Mammal { } class Elephant extends Mammal implements Vehicle { publicvoid drive() { System.out.println( "Elephant: Drive" ); } } class Horse extends Mammal implements Vehicle { publicvoid drive() { System.out.println( "Horse:Drive" ); } } class Lion extends Mammal{ } class InstantofInterface { publicstaticvoid main( String[] ar) { Mammal m[] =new Mammal[4]; m[0]=new Elephant(); m[1]=new Bear(); m[2]=new Horse(); m[3]=new Lion(); for( intj = 0; j < 4; j++) { if ( m[j] instanceof Vehicle) { Vehicle v = (Vehicle)m[j]; v.drive(); } } } } Simab Shahid UOH, Girls Branch Output: Elephant: Drive Horse: Drive

21. Exercise (Instanceof operator) • Write a program that demonstrate the instanceof operator. Declare interface I1 and I2. Interface I3 extends both of these interface. Also declare interface I4. Class X implements I3. Class W extends X and implements I4. Create an object of class W. Use the instanceof operator to test if object implements each of the interface and is of type X. Simab Shahid UOH, Girls Branch

22. Comparable Interface • Chapter 4 discussed the Merge Sort and the quick sort algorithms, and examined a method for sorting an array into increasing order • This code could be modified to sort into decreasing order, or to sort strings instead • Each of these methods would be essentially the same, but making each modification would be a nuisance. • The only difference would be the types of values being sorted, and the definition of the ordering. • Using the Comparable interface could provide a single sorting method that covers all these cases. Simab Shahid UOH, Girls Branch

23. Comparable Interface • The Comparable interface is in the java.lang package, and so is automatically available to any program. • It has only the following method heading that must be implemented: • public int compareTo(Object other); • It is the programmer's responsibility to follow the semantics of the Comparable interface when implementing it. Simab Shahid UOH, Girls Branch

24. import java.lang ; public class Employee implementsComparable { intEmpID; String Ename; doubleSal; publicEmployee( String Ename, double Sal) { EmpID ++; this. Ename =Ename; this. Sal = Sal; } public int compareTo(Object o1) { if (this.Sal == ((Employee) o1).Sal) { return0; }else if ((this.Sal) > ((Employee) o1).Sal) { return1; } else return-1; } } Simab Shahid UOH, Girls Branch

25. Class Test { publicstaticvoid main( String[]arg) { Employee Obj1 =new Employee(“Clary ” , 4000 ); Employee Obj2 =new Employee(“Amna ” , 6000 ); intreturnValue = Obj2.compareTo(Obj1) if (returnValue ==0 ) System.out.println(“ Both the employees have equal salaries ”); if (returnValue >0 ) System.out.println(“Employee 1 salary is high ”); if (returnValue < 0 ) System.out.println(“Employee 2 salary is high ”); } } Simab Shahid UOH, Girls Branch

26. Comparable Interface • The method compareTo must return: • A negative number if the calling object "comes before" the parameter other. • A zero if the calling object "equals" the parameter other. • A positive number if the calling object "comes after" the parameter other. • If the parameter other is not of the same type as the class being defined, then a ClassCastException should be thrown. Simab Shahid UOH, Girls Branch

27. Inner classes • Inner classes are classes defined within other classes. • The class that includes the inner class is called the outer class. public class University { String name; String city; private class Student { private String ID ; private int Age ; private String Grade ; } } • Here University is Outer class & Student is Inner class Simab Shahid UOH, Girls Branch

28. Placement of inner class in Outer Class • There is no particular location where the definition of the inner class (or classes) must be place within the outer class. • Placing it first or last, however, will guarantee that it is easy to find. Simab Shahid UOH, Girls Branch

29. Example 01 Example 02 public class Person { • //Good to write Inner class in the start, //because it is easy to find it • public class Name{ • String f_name; • String m_name; • String l_name; • } private double age; Person() { age = 0.0; } public void setAge( double age) { this.age = age; } } public class Person { private double age; Person() { age = 0.0; } public void setAge( double age) { this.age = age; } //Good to writeInner class at the //End, because it is easy to find it public class Name{ String f_name; String m_name; String l_name; } } Simab Shahid UOH, Girls Branch

30. Example 03 public class Person { private double age; Person() { age = 0.0; } //Not Good to write Inner class in the middle, because it is difficult to find it public class Name{ String f_name; String m_name; String l_name; } public void setAge( double age) { this.age = age; } } Simab Shahid UOH, Girls Branch

31. Inner Class • An inner class definition is a member of the outer class in the same way that the instance variables and methods of the outer class are members. • In the previous example the class Person has three members • private variable “age” • public function setAge() • And inner class Name. • An inner class is local to the outer class definition. • The name of an inner class may be reused for something else outside the outer class definition. • If the inner class is private, then the inner class cannot be accessed by name outside the definition of the outer class. Simab Shahid UOH, Girls Branch

32. Uses of Inner classes • Within the definition of a method of an inner class: • It is legal to reference a private instance variable of the outer class • It is legal to invoke a private method of the outer class • Within the definition of a method of the outer class • It is legal to reference a private instance variable of the inner class on an object of the inner class • It is legal to invoke a (non static) method of the inner class as long as an object of the inner class is used as a calling object • Within the definition of the inner or outer classes, the modifiers public and private are equivalent. Simab Shahid UOH, Girls Branch

33. public class University{ private class Student { private String ID ; private intAge ; private String Grade ; public Student (String I, int A, String G) { this.ID = I ; this.Age = A ; this.Grade = G ; } public StringgetGrade( ) { return Grade ; } public StringgetStudentInfo( ) { return ID +” ” +Age; } } // end of inner class private Student S ; public String getInfo( ) { return S.Grade + “ “ + S.getStudentInfo() ; } } // end of outerclass Inner Classes Example Simab Shahid UOH, Girls Branch

34. Advantages Inner Classes • There are two main advantages to inner classes. • They can make the outer class more self-contained since they are defined inside a class. • Both of their methods have access to each other's private methods and instance variables. • Using an inner class as a helping class is one of the most useful applications of inner classes. • If used as a helping class, an inner class should be marked private. Simab Shahid UOH, Girls Branch