COE318 Lecture Notes Week 9 (Week of Oct 29, 2012)

Transcription

1 COE318 Lecture Notes: Week 9 1 of 14 COE318 Lecture Notes Week 9 (Week of Oct 29, 2012) Topics The final keyword Inheritance and Polymorphism The final keyword Zoo: Version 1 This simple version models a Zoo that contains an arbitrary number of Lions. There is nothing new here. It's just a starting point for exploring, using and understanding some important features of object-oriented programming. But, some notes... The instance variables of Lion are private and final. Consequently, Lion objects are (for the moment) immutable. public class Lion { private final String name; private final int birthyear; public Lion(String name, int year) { this.name = name; birthyear = year;

2 COE318 Lecture Notes: Week 9 2 of 14 public int age() { return birthyear; public String getname() { return name; public String noise() { return "GGGrowl"; public boolean eatspeople() { return true; import java.util.arraylist; public class Zoo { private final ArrayList<Lion> lions = new ArrayList<Lion>(); public void add(lion lion) { lions.add(lion); public String getnames() { String s = ""; for(lion leo : lions) { s += leo.getname() + "\n"; return s; public static void main(string[] args) { Zoo zoo = new Zoo(); Lion a = new Lion("Alex", 2010);

3 COE318 Lecture Notes: Week 9 3 of 14 zoo.add(a); zoo.add(new Lion("Betty", 2009)); System.out.println(zoo.getNames()); Zoo Version 2: A First Look at Inheritance The zoo wants to have more animals than just lions. But proceeding as before would be very tedious if, say, there were 100 different kinds of animals: We'd need to create Pig, Cat, Crocodile, Elephant, Hummingbird, etc. classes and they would all have just about the same code; We'd need separate ArrayLists for each type of animal in the Zoo class. It would be difficult to treat all the animals as a group: to sort them alphabetically by name or by age, for example. Inheritance (subclassing) to the rescue! We can put practically all the code for Lion into another class we call AbstractAnimal. (Why not call it Animal instead of AbstractAnimal? Good question! To be explained in the next version.) We can then rewrite the Lion class to extend the AbstractAnimal class; all of its methods are inherited by Lion and do not have to be copied and pasted. The Lion class now looks like: public class Lion extends AbstractAnimal { public Lion(String name, int year) { super(name, year); But what's this super(name, year); stuff?

4 COE318 Lecture Notes: Week 9 4 of 14 It invokes the constructor (with the same signature) of its superclass. It is not always necessary to invoke the superclass constructor as is done here; however, If your do, it must be the first statement in the subclass's constructor. The superclass is AbstractAnimal and here's what it looks like: public class AbstractAnimal { private final String name; private final int birthyear; public AbstractAnimal(String name, int year) { this.name = name; birthyear = year; public int age() { return birthyear; public String getname() { return name; public String noise() { return "GGGrowl"; public boolean eatspeople() { return true; We can now write the Hedgehog class. However, the inherited methods eatspeople() and noise() give the wrong answer for a hedgehog. An inherited method can be overridden by simply re-writing it. (A subclass almost always overrides at least one method from its superclass.) Note: The line is not required. It can be useful for the compiler and some tools. Netbeans recommends you put it in. It also reminds programmers that they are modifying an inherited method.) In general, lines beginning are called annotations. Programmers can define their own annotations but that is beyond the scope of this course.)

5 COE318 Lecture Notes: Week 9 5 of 14 public class Hedgehog extends AbstractAnimal { public Hedgehog(String name, int year) { super(name, year); public boolean eatspeople() { return false; How do we declare the data type of a Lion or Hedgehog? In the past (before inheritance), this question would appear foolish. For example, we would write things like: Resistor r; r = new Resistor(10); r.setvoltage(10); There would be no other choice. But with inheritance we can declare a Lion as either of type Lion or type AbstractAnimal: Lion p = new Lion( Leo ); AbstractAnimal q = new Lion( Simba ); There is absolutely no difference between how p and q behave. They are both Lions and behave as such no matter how their data types were declared. And there is no difference in runtime efficiency. But declaring Lions or Hedgehogs or Pigs to be AbstractAnimals has big advantages. For example, the Zoo class can keep all of the animals in a single ArrayList<AbstractAnimal>. Below is the new, improved Zoo class: import java.util.arraylist; public class Zoo { private final ArrayList<AbstractAnimal> animals = new ArrayList<AbstractAnimal>();

6 COE318 Lecture Notes: Week 9 6 of 14 public void add(abstractanimal animal) { animals.add(animal); public String getnames() { String s = ""; for (AbstractAnimal a : animals) { s += a.getname() + "\n"; return s; public AbstractAnimal[] getanimals() { AbstractAnimal[] ts = new AbstractAnimal[0]; return animals.toarray(ts); public static void main(string[] args) { Zoo zoo = new Zoo(); Lion a = new Lion("Leo", 2010); zoo.add(a); zoo.add(new Lion("Simba", 2009)); zoo.add(new Hedgehog("Sonic", 2010)); System.out.println(zoo.getNames()); AbstractAnimal[] animals = zoo.getanimals(); for (AbstractAnimal an : animals) { System.out.println(an.getClass().getName() + " " + an.getname() + ": " + (an.eatspeople()? "eats" : "does not eat") + " people"); Zoo Version 3: Abstract classes and methods AbstractAnimal is just a normal class and it is legal to write: AbstractAnimal a = new AbstractAnimal( Sue );

7 COE318 Lecture Notes: Week 9 7 of 14 This should not be allowed! It makes no sense to create an animal when we don't know what kind of animal it is. AbstractAnimal exists only for the purpose of subclassing. We can make this clear by declaring it to be abstract. The compiler will now disallow: AbstractAnimal a = new AbstractAnimal( Sue ); We now refer to AbstractAnimal as an abstract class that cannot be instantiated. But Lion and Hedgehog are concrete classes that can be instantiated. We can also declare methods to be abstract. If we do so, we must declare the class to be abstract. Furthermore, when we extend an abstract class as a concrete class, the compiler will insist that we write implementation code for any abstract methods. The code for our classes is given below. Note: the Zoo class needs no changes. The AbstractAnimal class is now declared abstract as is its noise() method. Both Lion and Hedgehog now have to implement (and not just inherit) the noise() method. public abstract class AbstractAnimal { private final String name; private final int birthyear; public AbstractAnimal(String name, int year) { this.name = name; birthyear = year; public int age() { return birthyear; public String getname() { return name; public abstract String noise(); public boolean eatspeople() { return true;

8 COE318 Lecture Notes: Week 9 8 of 14 public class Hedgehog extends AbstractAnimal { public Hedgehog(String name, int year) { super(name, year); public boolean eatspeople() { return false; public String noise() { return "Chirp...chirp"; public class Lion extends AbstractAnimal { public Lion(String name, int year) { super(name, year); public String noise() { return "Rrroar (yum yum)"; Zoo Version 4: Interfaces The zoo owner is happy with version 3; lots and lots of concrete animal classes can be easily added; no changes needed to the Zoo or AbstractAnimal classes. But then one day he says, I love my fridge. I want to have it as one of the amimals in my zoo.

9 COE318 Lecture Notes: Week 9 9 of 14 The guy may be a lunatic, but we have to respect his wishes. Tell him just to subclass Refrigerator from AbstractAnimal. But he can't do that; Refriger ator is already subclassed from ElectricalAppliance. OK, we'll make an interface called Animal. He just has to implement the interface. What is an interface? An interface is simply a collection of zero or more public abstract methods. A class can implement as many interfaces as it wants. It just has to ensure that all the abstract methods are actually implemented. (And the compiler will ensure that this is done.) We also take advantage of the Animal interface and have AbstractAnimal implement it. Objects can be declared as their own type or as any of their superclass's type. In addition, an object can be declared as any interface type implemented by the actual class. Consequently, we can now declare all the animals (and the fridge) as type Animal. We also have AbstractAnimal implement a standard imterface, Comparable<Animal>. Note (lab 5): You are provided with an interface in lab 5: UserInterface which describes what the methods should do. The reason for this so that any user interface has the same mthods and can be easily plugged into the existing code. (A graphical user interface would have buttons and show the cards in a graphical way. A networked interface coudl also be written.) interface Animal { String noise(); boolean eatspeople(); int age(); String getname(); public abstract class AbstractAnimal implements Animal, Comparable<Animal> { private final String name; private final int birthyear; public AbstractAnimal(String name, int year) { this.name = name; birthyear = year;

10 COE318 Lecture Notes: Week 9 10 of 14 public int age() { return birthyear; public String getname() { return name; public abstract String noise(); public boolean eatspeople() { return this instanceof Dangerous; public int compareto(animal other) { return getname().compareto(other.getname()); public boolean equals(object obj) { if (obj == null) { return false; if (getclass()!= obj.getclass()) { return false; final AbstractAnimal other = (AbstractAnimal) obj; if ((this.name == null)? (other.name!= null) :!this.name.equals(other.name)) { return false; return true; public int hashcode() { int hash = 7; hash = 83 * hash

11 COE318 Lecture Notes: Week 9 11 of 14 + (this.name!= null? this.name.hashcode() : 0); return hash; public interface Dangerous { public class Lion extends AbstractAnimal implements Dangerous{ public Lion(String name, int year) { super(name, year); public String noise() { return "GGGrowl"; public class Hedgehog extends AbstractAnimal { public Hedgehog(String name, int year) { super(name, year); public String noise() { return "Chirp chirp";

12 COE318 Lecture Notes: Week 9 12 of 14 import java.util.arraylist; public class Zoo { private final ArrayList<Animal> animals = new ArrayList<Animal>(); public void add(animal animal) { animals.add(animal); public String getnames() { String s = ""; for (Animal a : animals) { s += a.getname() + "\n"; return s; public Animal[] getanimals() { Animal[] ts = new AbstractAnimal[0]; return animals.toarray(ts); public static void main(string[] args) { Zoo zoo = new Zoo(); Animal a = new Lion("Alex", 2010); zoo.add(a); zoo.add(new Lion("Betty", 2009)); zoo.add(new Hedgehog("Charlie", 2010)); System.out.println(zoo.getNames()); Animal[] animals = zoo.getanimals(); for (Animal an : animals) { System.out.println(an.getClass().getName() + " " + an.getname() + ": " + (an.eatspeople()? "eats" : "does not eat") + " people");

13 COE318 Lecture Notes: Week 9 13 of 14 Zoo Version 5: Exceptions import java.util.arraylist; public class Zoo { private final ArrayList<Animal> animals = new ArrayList<Animal>(); public void add(animal animal) { if (animals.contains(animal)) { throw new IllegalArgumentException("Duplicate name not allowed"); animals.add(animal); public String getnames() { String s = ""; for (Animal a : animals) { s += a.getname() + "\n"; return s; public Animal[] getanimals() { Animal[] ts = new AbstractAnimal[0]; return animals.toarray(ts); public static void main(string[] args) { Zoo zoo = new Zoo(); Animal a = new Lion("Alex", 2010); zoo.add(a); zoo.add(new Lion("Betty", 2009)); Animal charlie1 = new Hedgehog("Charlie", 2010); zoo.add(charlie1);

14 COE318 Lecture Notes: Week 9 14 of 14 System.out.println(zoo.getNames()); Animal charlie2 = new Lion("Charlie", 2007); added"); try { zoo.add(charlie2); catch (IllegalArgumentException ex) { System.err.println("Duplicate name not Animal[] animals = zoo.getanimals(); for (Animal an : animals) { System.out.println(an.getClass().getName() + " " + an.getname() + ": " + (an.eatspeople()? "eats" : "does not eat") + " people");