Session 4 Interfaces

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Session 4Interfaces
Übung Softwareentwicklung 2für
Wirtschaftsinformatik
Dr. Ismail Khalil Ibrahim, MScDr. Wieland
Schwinger, MSc
Some code in this lecture is based on code from
the book Java Structures by Duane A. Bailey
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

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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.

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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
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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
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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() ...

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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
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How interfaces differ from classes?

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

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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
• ...

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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.

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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.