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The object contractThe object contract
Abstract classes Abstract Vs ConcreteAbstract Vs. Concrete Reference type Vs. Object type Multiple Inheritance Interfaces
The object contractThe object contract
z The superclass defines how the objects are to be used (the contract).
- Functionality can be overridden, but the method name/purpose
defines the contract.
z Example: Say we wish to keep track of a bunch of 2D shapes to bounce
around a Jpanel
z All shapes need: constructor, draw(), move()
Shape Shape (x, y, dx, dy, color)
97 Wright State University, College of Engineering Dr. T. Doom, Computer Science & Engineering
CS 241 Computer Programming II
z constructor and move() is the same all shapes
z draw() is different code for each shape
z Polymorphism allows us to treat all objects similarly
- object.move()
- object.draw()
z This is the “contract” – how to use the object
- The contract is the same for all subclasses
move() draw()
Triangle draw ()
Circle draw ()
Square draw ()
Polymorphism as a contract:Polymorphism as a contract:
Using the Shape ClassUsing the Shape Class
ArrayList shapeList; … public void makeShapes () { shapeList = new ArrayList(); shapeList.add(new Circle (0,0,5,3,Color.Red)); shapeList.add(new Square (10,10,-1,-5,Color.Green)); } // end makeShapes
98 Wright State University, College of Engineering Dr. T. Doom, Computer Science & Engineering
CS 241 Computer Programming II
public void paintComponent (Graphics g) { for (Shape shape : shapeList) { shape.move(); shape.draw(); } } // end method paintComponent …
How do I define Shape?How do I define Shape?
z What should the default code be for draw()?
z What should happen if I try to make a new Shape()?
Shape Shape (x, y, dx, dy, color)
Arra List shapeList
99 Wright State University, College of Engineering Dr. T. Doom, Computer Science & Engineering
CS 241 Computer Programming II
move() draw()
Triangle draw ()
Circle draw ()
Square draw ()
ArrayList shapeList; … public void makeShapes () { shapeList = new ArrayList(); shapeList.add( new Circle(0,0,5,3,Color.Red)); shapeList.add( new Shape(10,10,1,-5,Color.Green)); } // end makeShapes …
Abstract classesAbstract classes
z Some classes should never be instantiated
- Some superclasses exists only to define a contract
- You may never want to allow anyone to create one!
z Marking a class abstract tells the compiler to create an instance of the
class. Its only use is to be extended! Shape
abstract class Shape {^ Shape (x, y, dx, dy, color)
100 Wright State University, College of Engineering Dr. T. Doom, Computer Science & Engineering
CS 241 Computer Programming II
move() draw()
Triangle draw ()
Circle draw ()
Square draw ()
p { Shape (int x, int y, int dx, int dy, Color color) { … // my code to inherit here } // end constructor Shape void move () { … // my code to inherit here } // end method move void draw () { … // what should go here? } // end method draw } // end class Shape
Abstract Vs. Concrete classesAbstract Vs. Concrete classes
z Abstract class: A class that cannot be
instantiated
- An abstract class has virtually no use,
no value, and no purpose
- It can have static members
- It can define a contract (this is, it can be
Shape Shape (x, y, dx, dy, color)
ABSTRACT
UML Note: Use italics to indicate abstract
101 Wright State University, College of Engineering Dr. T. Doom, Computer Science & Engineering
CS 241 Computer Programming II
extended)
z Concrete class: A class that is not abstract
- The objects created and doing the work
at runtime are concrete
- These objects may be instances of a
subclass of an abstract class
move() draw()
Triangle draw ()
Circle draw ()
Square draw () CONCRETE CONCRETE CONCRETE
The object contractThe object contract
z For an object of type Triangle
- Everything accessible in class Triangle defines part of your contract
- Everything accessible in class Shape defines part of your contract
- Everything accessible in class Object defines part of your contract
z Accessible: public or protected Shape
Shape (x, y, dx, dy, color)
105 Wright State University, College of Engineering Dr. T. Doom, Computer Science & Engineering
CS 241 Computer Programming II
move() draw ()
Triangle draw () rotate()
Circle draw ()
Square draw () rotate()
Object Object() equals (object ) toString () clone(object) getClass() …
Shape Shape() move() draw()
Triangle
draw() rotate()
t
Multiple inheritance IMultiple inheritance I
z What if I want to create an object that fulfills the contracts of two
separate (unrelated) existing classes?
z This would allow us to use all the existing code that works for either
method
z Example: GUI_Card
- It’s a card, and can be used with existing
106 Wright State University, College of Engineering Dr. T. Doom, Computer Science & Engineering
CS 241 Computer Programming II
Cardgame code
- It also needs to be displayed on the GUI,
and is essentially just a Square
z We want the following to be true:
- GUI_Card IS-A Card
- GUI_Card IS-A Square
Square draw () rotate()
Card show()
GUI_Card draw () rotate()
Multiple inheritance: Here be dragons!Multiple inheritance: Here be dragons!
z What if both superclasses have a member with the same name?
z How do we avoid ambiguity?
- Extra syntax?
- How do we help check for errors?
- Basically, this is hard
107 Wright State University, College of Engineering Dr. T. Doom, Computer Science & Engineering
CS 241 Computer Programming II
z Some languages allow this (C++, for one)
- C++ is all about handling hard
z Some languages don’t (Java, for one)
- Java is all about the simple
z Java does provide limited multiple inheritance
Square draw () rotate()
Card show() draw()
GUI_Card draw () Which draw() superclass method do I inherit?!? rotate()
InterfacesInterfaces
z Java allows multiple inheritance to interface classes
z Interface classes are:
- 100% pure abstract class
- They contain only abstract, public methods
z Thus there is no ambiguity. The interface defines the contract but forces
the subclass object to define the one and only one implementation.
108 Wright State University, College of Engineering Dr. T. Doom, Computer Science & Engineering
CS 241 Computer Programming II
the subclass object to define the one and only one implementation.
Shape draw () move ()
Card show() draw()
GUI_Card draw () rotate() move()
public interface Shape { public abstract void move (); public abstract void draw (); } // end interface Shape
public class GUI_Card extends Card implements Shape { // Implementation must override move/draw } // end class GUI_Card
Contract StyleContract Style
z When do you make a class, a subclass, an abstract class, or an interface?
z Class
- Appropriate for objects that don’t extend anything
- Fails the IS-A test for all other types (except Object)
z Subclass (extend a class)
- Only when you need to make a more specific version of a class
109 Wright State University, College of Engineering Dr. T. Doom, Computer Science & Engineering
CS 241 Computer Programming II
- Only when you need to make a more specific version of a class
- Override or add new behaviors
z Abstract class
- Define a template/contract for a group of subclasses
- Has some implementation code that all subclasses can use
- Guarantee that nobody can make an object of that type
z Interface
- Define a role that other classes can play, regardless of their type
