bung Softwareentwicklung 2 fr Wirtschaftsinformatik

1
Übung Softwareentwicklung 2für
Wirtschaftsinformatik
UE03Interfaces
2
Introduction

• An interface is a device or a system that
  unrelated entities use to "interact"
• remote control, English language
• In Java, an interface is a device that unrelated
  objects use to interact with each other
• analogous to a protocol
• An interface is a contract in the form of a
  collection of method and constant declarations.
• When a class implements an interface, it promises
  to implement all of the methods

3
What is an interface?

• An interface is related to an abstract class in
  that it has no implementation.
• An interface is a type definition.
• We use inheritance when we want to say that a
  class is a certain kind.
• We use interfaces when we want to say that a
  class has a behavior.
• An interface defines functionality which we want
  to make available through polymorphism.

4
How interfaces differ from classes?

• public abstract class Shape public abstract
  double area() public double circumference()
• class Circle extends Shape protected double
  r public Circle() r 1.0 public double
  area() return PI r r ...
• class Rectangle extends Shape protected
  double w, h public Rectangle() w 1.0
  h 1.0 public double area() return w
  h ...

Shape
Circle
Rectancle
5
How interfaces differ from classes?

• class DrawableCircle extends Shape protected
  double r public DrawableCircle() r
  1.0 public double area() return PI r
  r public void draw() ...
• class DrawableRectangle extends Shape
  protected double w, h public
  DrawableRectangle() w 1.0 h 1.0
  public double area() return w h
  public void draw() ...

Shape
Circle
Rectancle
DrawableCircle
DrawableRectancel
6
How interfaces differ from classes?

• But this is not a good abstraction we need to
  isolate the notion of a class of drawable
  objects.
• abstract class Drawable public void draw()
  public void setColour() ...

• class DrawableRectangle extends Shape, Drawable
  protected double w, h public
  DrawableRectangle() w 1.0 h 1.0
  public double area() return w h
  public draw() ...

7
How interfaces differ from classes?

• Drawable is a "description of a behavior" we want
  Rectangles and Circles to have.
• interface Drawable public void draw()
  public void setColour() ...
• class DrawableRectangle extends Shape
  implements Drawable
• public void draw()
• ...

definition of an interface instead of an abstract
class
8
How interfaces differ from classes?

• A class can implement any number of interfaces
• class DrawableRectangle extends Shape
  implements Drawable, Cloneable public void
  draw()
• ...

9
What goes in an interface?

• Interfaces may contain ordinary abstract methods
• Interfaces may not contain static methods.
• interface Sortable // so thispublic boolean
  lessThan (Sortable a)
• // not thispublic static boolean lessThan
  (Sortable a Sortable b)
• Interfaces may contain constants which must have
  an implementation
• interface Drawable
• ...
• static final Color DEFAULT_COLOR RED
• ...

10
More interface syntax

• Interfaces may be extended and there may be
  multiple extensions.
• public interface Transformable extends Scalable,
  Rotatable
• Interfaces can be used as types
• Drawable items new Drawable5
• An interface can be declared as a marker
  interface with no methods or constants.
• The Cloneable interface is used like this. A
  class that implements the interface should have a
  sensible implementation of the clone() method.

11
When to use interfaces?

• Interfaces should be used whenever you see code
  in the following form.
• if (x is of type 1)
• action1(x)
• else if (x is of type 2)
• action2(x)
• We use inheritance when we want to say that a
  class is a certain kind.
• We use interfaces when we want to say a class has
  a behavior.