Interface

Advanced Programming

SUT • Spring 2019

Outline

  • interface

  • Multiple Inheritance

Interface

Abstract Example

Abstract Method

  • All subclasses have the method
  • But we can not implement the method in super-class
  • So why we define it in super-class?
    • Polymorphism

Interface

  • Sometimes we have an abstract class, with no concrete method
  • interface: pure abstract class
  • no implemented method

Interface

interface Shape {
    int getColor();
    double getArea();
    double getPrimeter();
}

Interface

  • All the methods are implicitly abstract

    • No need to abstract specifier

  • All the methods are implicitly public

    • No need to public specifier

interface Shape {
    int getColor();
    double getArea();
    double getPrimeter();
}

Interface Implementation

  • Some classes may inherit abstract classes

  • Some classes may implement interfaces

  •  

  • Is-a relation exists

  • If a class implements an interface

  • But does not implement all the methods

  • What will happen?

    • The class becomes an abstract class

class Rectangle implements Shape {...}

Example

class Rectangle implements Shape {
    private double width, length;
    private int color;
    public Rectangle(double width, double length, int color) {
        this.width = width;
        this.length = length;
        this.color = color;
    }
    public int getColor() {
        return color;
    }

    public double getArea() {
        return width * length;
    }

    public double getPrimeter() {
        return 2 * (width + length);
    }
}

Multiple Inheritance in Java

  • A class can inherit one and only one class

  • A class may implement zero or more interfaces

interface CanFight {
    void fight();
}

interface CanSwim {
    void swim();
}

interface CanFly {
    void fly();
}

class ActionCharacter {
    private String name;
    public String getName() {
        return name;
    }
}

class Hero extends ActionCharacter implemenets CanFight, CanSwim, 
                   CanFly {
    public void fly()   {}
    public void swim()  {}
    public void fight() {}
}

A Question

  • Why multiple inheritance is not supported for classes?

  • Why multiple inheritance is supported for interfaces?

What About Name Collision?

  • The return types are incompatible for the inherited methods B.f(), A.f()

interface A {
    int f();
}

interface B {
    void f();
}

public abstract class TestCase implements A, B {};
interface CanFight {
    void fight();
    void move();
}

interface CanSwim {
    void swim();
}

interface CanFly {
    void fly();
    void move();
}

class ActionCharacter {
    public void fight() {}
}

class Hero extends ActionCharacter implemenets CanFight, CanSwim, 
                   CanFly {}

Example

Interface Extension

  • Interfaces may inherit other interfaces

    • Code reuse

    • Is-a relation

interface CanSee{
    void move();
    void see();
}

interface CanMove{
    void move();
}

interface CanFight{
    void fight();
}

Interface properties

  • No member variable

    • Variables: implicitly final and static

    • Usually interfaces declare no variable

  • Only operations are declared

  • No constructor

    • Why?

Example

interface A {
    int a = 5;
    int f();
}

public class TestClass implements A {
    public static void main(String[] args) {
        System.out.println(a);
    }
}

Interfaces Applications

  • Pure Abstract classes

  • Describe the design

    • The architect designs interfaces, the programmer implements them

    • Only interfaces are delivered to code consumers

  • More powerful inheritance and polymorphism

Default Method (Java 8)

public interface Interface {
    void method(String str);

    default void log(String str) {
        System.out.println("logging::" + str);
        Interface.print("abc");
    }
}

Default Method (Java 8)

public interface OtherInterface {
    void otherMethod();

    default void log(String str) {
        System.out.println("Other logging::" + str);
    }
}

Default Method (Java 8)

public class Concrete implements Interface, OtherInterface {
    @Override
    public void method(String str) {
    }

    @Override
    public void otherMethod() {
    }

    @Override
    public void log(String str) {
        System.out.println("MyClass logging::" + str);
    }
}

Static method

public interface Interface {
    void method(String str);

    default void log(String str) {
        System.out.println("logging::" + str);
        Interface.print("abc");
    }

     static void print(String str) {
        System.out.println("Interface Print " + str + ".");
    }
}

Private Static method

public interface Interface {
    void method(String str);

    default void log(String str) {
        System.out.println("logging::" + str);
        Interface.print("abc");
    }

     private void print(String str) {
        System.out.println("Interface Print " + str + ".");
    }
}

Title Text

Interface

By Behnam Hatami

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Interface

Interface / Advanced Programming Course @ SUT, Spring 2019

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