Interface
Advanced Programming
SUT • Spring 2019
Outline
-
interface
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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
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All the methods are implicitly abstract
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No need to abstract specifier
-
-
All the methods are implicitly public
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No need to public specifier
-
interface Shape {
int getColor();
double getArea();
double getPrimeter();
}
Interface Implementation
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Some classes may inherit abstract classes
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Some classes may implement interfaces
-
-
Is-a relation exists
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If a class implements an interface
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But does not implement all the methods
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What will happen?
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The class becomes an abstract class
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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
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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
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Why multiple inheritance is not supported for classes?
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Why multiple inheritance is supported for interfaces?
What About Name Collision?
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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
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Interfaces may inherit other interfaces
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Code reuse
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Is-a relation
-
interface CanSee{
void move();
void see();
}
interface CanMove{
void move();
}
interface CanFight{
void fight();
}
Interface properties
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No member variable
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Variables: implicitly final and static
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Usually interfaces declare no variable
-
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Only operations are declared
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No constructor
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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
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Pure Abstract classes
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Describe the design
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The architect designs interfaces, the programmer implements them
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Only interfaces are delivered to code consumers
-
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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
Interface
Interface / Advanced Programming Course @ SUT, Spring 2019
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