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COS 260 DAY 22
Tony Gauvin
1
Agenda
• Questions?
• 9th Mini Quiz corrected
– Good results
• Assignment 5 Not corrected yet
• Assignment 6 Posted (one more)
– Due Dec 3
• Finish Discussion on Further Abstraction
techniques
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Final Countdown
• Today
– Finish Chapter 10
• Nov 30
– 10th Mini quiz
– Begin Chap 11
– Capstone Progress
Report
• Dec 3
– Finish Chapter 11
– Assignment 6 due
Copyright © 2014 Pearson Education, Inc.
• Dec 7
– 11th mini quiz
– Begin Chapter 12
• Dec 10
– Finish Chapter 12
– Assignment 7 Due
• Dec 14 @ 10 AM
– 12th mini quiz
– Capstone Presentations
Slide 1-3
3
Further abstraction
techniques
Abstract classes and interfaces
5.0
Main concepts to be covered
• Abstract classes
• Interfaces
• Multiple inheritance
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
5
Configuration of foxes
• Similar simplifications to rabbits.
• Hunting and eating could be modeled
in many different ways.
– Should food level be additive?
– Is a hungry fox more or less likely to
hunt?
• Are simplifications ever acceptable?
• Ex 10.13 to 10.21
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
6
The Simulator class
• Three key components:
– Setup in the constructor.
– The populate method.
• Each animal is given a random starting age.
– The simulateOneStep method.
• Iterates over separate populations of foxes
and rabbits.
• Two Lists for each animal are used :
– rabbits and newRabbits
– foxes and newFoxes
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
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The update step
for(Iterator<Rabbit> it = rabbits.iterator();
it.hasNext(); ) {
Rabbit rabbit = it.next();
rabbit.run(newRabbits);
if(! rabbit.isAlive()) {
it.remove();
}
}
…
for(Iterator<Fox> it = foxes.iterator();
it.hasNext(); ) {
Fox fox = it.next();
fox.hunt(newFoxes);
if(! fox.isAlive()) {
it.remove();
}
}
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
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Room for improvement
• Fox and Rabbit have strong
similarities but do not have a
common superclass.
• The update step involves similarlooking code.
• The Simulator is (too) tightly
coupled to specific classes.
– It ‘knows’ a lot about the behavior of
foxes and rabbits.
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
9
Fixing The simulator
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
10
The <abstract> Animal
superclass
• Place common fields in Animal:
– age, alive, location
• Method renaming to support
information hiding:
– run and hunt become act.
• Simulator can now be
significantly decoupled.
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
11
Revised (decoupled) iteration
for(Iterator<Animal> it = animals.iterator();
it.hasNext(); ) {
Animal animal = iter.next();
animal.act(newAnimals);
// Remove dead animals from simulation
if(! animal.isAlive()) {
it.remove();
}
}
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
12
The act method of Animal
• Static type checking requires
an act method in Animal.
• There is no obvious shared
implementation.
• Define act as abstract:
abstract public void act(List<Animal> newAnimals);
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
13
Abstract classes and methods
• Abstract methods have abstract in the
signature.
• Abstract methods have no body.
• MUST be overridden in Subclass
• Abstract methods make the class abstract.
• Abstract classes cannot be instantiated.
• Concrete subclasses complete the
implementation.
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
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The Animal class
public abstract class Animal
{
fields omitted
/**
* Make this animal act - that is: make it do
* whatever it wants/needs to do.
*/
abstract public void act(List<Animal> newAnimals);
other methods omitted
}
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
15
Abstract Hierarchy
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
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Problem 1 Ex 10.43-10.49  Bonus 10.50
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
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Further abstraction
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
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Multiple inheritance
• Having a class inherit directly from
multiple ancestors.
• Each language has its own rules.
– How to resolve competing definitions?
• Java forbids it for classes.
• Java permits it for interfaces.
– No competing implementation.
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
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Selective drawing
(multiple inheritance)
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
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An Actor interface
public interface Actor
{
/**
* Perform the actor's regular behavior.
* @param newActors A list for storing newly created
*
actors.
*/
void act(List<Actor> newActors);
/**
* Is the actor still active?
* @return true if still active, false if not.
*/
boolean isActive();
}
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
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Classes implement an
interface
public class Fox extends Animal implements Drawable
{
...
}
public class Hunter implements Actor, Drawable
{
...
}
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
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Interfaces as specifications
• Strong separation of functionality from
implementation.
– Though parameter and return types are
mandated.
• Clients interact independently of the
implementation.
– But clients can choose from alternative
implementations.
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
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Interfaces as types
• Implementing classes do not inherit
code, but ...
• ... implementing classes are subtypes
of the interface type.
• So, polymorphism is available with
interfaces as well as classes.
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
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Features of interfaces
•
•
•
•
All methods are abstract.
There are no constructors.
All methods are public.
All fields are public, static and final.
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
25
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
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Alternative implementations
http://docs.oracle.com/javase/7/docs/api/java/util/List.ht
ml
Problem 2 Ex 10.58
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
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The Class class
• A Class object is returned by
getClass() in Object.
• The .class suffix provides a Class
object:
Fox.class
• Used in SimulatorView:
Map<Class, Color> colors;
• String getName() for the class name.
• http://docs.oracle.com/javase/7/docs/ap
i/java/lang/Class.html
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
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Review
• Inheritance can provide shared
implementation.
– Concrete and abstract classes.
• Inheritance provides shared type
information.
– Classes and interfaces.
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
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Review
• Abstract methods allow static type
checking without requiring
implementation.
• Abstract classes function as
incomplete superclasses.
– No instances.
• Abstract classes support
polymorphism.
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
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Review
• Interfaces provide specification
without implementation.
– Interfaces are fully abstract.
• Interfaces support polymorphism.
• Java interfaces support multiple
inheritance.
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
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