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Object-Oriented ProgrammingMore Inheritance
Ewan Klein
School of Informatics
Inf1 :: 2009/10
Ewan Klein (School of Informatics) OOP: More Inheritance Inf1 :: 2009/10 1 / 45
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.. .1 Inheritance
Flat HierarchyLayered Hierarchy
.. .2 Polymorphism
.. .3 Overriding and Overloading
.. .4 Abstract Classes
.. .5 Recap
Ewan Klein (School of Informatics) OOP: More Inheritance Inf1 :: 2009/10 2 / 45
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Flat Animal Hierarchy
sleep()makeNoise()roam()
Animal
makeNoise()
Lion
makeNoise()
Cat makeNoise()
Wolf
makeNoise()
Dog
Ewan Klein (School of Informatics) OOP: More Inheritance Inf1 :: 2009/10 3 / 45
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Animals Example, 1
Our base class: Animal.Animal..
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public class Animal {public void sleep() {
System.out.println(”Sleeping: Zzzzz”);}public void makeNoise() {
System.out.println(”Noises...”);}public void roam() {
System.out.println(”Roamin’ on the plain.”);}
}
Ewan Klein (School of Informatics) OOP: More Inheritance Inf1 :: 2009/10 4 / 45
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Animals Example, 2
...1 Lion IS-A Animal
...2 Override the makeNoise() method.
.Lion..
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public class Lion extends Animal {public void makeNoise() {
System.out.println(”Roaring: Rrrrrr!”);}
}
Ewan Klein (School of Informatics) OOP: More Inheritance Inf1 :: 2009/10 5 / 45
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Animals Example, 3
...1 Cat IS-A Animal
...2 Override the makeNoise() method.
.Cat..
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public class Cat extends Animal {public void makeNoise() {
System.out.println(”Miaowing: Miaooo!”);}
}
Ewan Klein (School of Informatics) OOP: More Inheritance Inf1 :: 2009/10 6 / 45
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Animals Example, 4
...1 Wolf IS-A Animal
...2 Override the makeNoise() method.
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public class Wolf extends Animal {public void makeNoise() {
System.out.println(”Howling: Ouooooo!”);}
}
Ewan Klein (School of Informatics) OOP: More Inheritance Inf1 :: 2009/10 7 / 45
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Animals Example, 5
...1 Dog IS-A Animal
...2 Override the makeNoise() method.
.Dog
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public class Dog extends Animal {public void makeNoise() {
System.out.println(”Barking: Woof Woof!”);}
}
Ewan Klein (School of Informatics) OOP: More Inheritance Inf1 :: 2009/10 8 / 45
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Animals Example, 6.The Launcher..
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public class AnimalLauncher {public static void main(String[] args) {
System.out.println(”\nWolf\n=====”);Wolf wolfie = new Wolf();wolfie.makeNoise() ; // from Wolfwolfie.roam(); // from Animalwolfie.sleep(); // from Animal
System.out.println(”\nLion\n=====”);Lion leo = new Lion();leo.makeNoise(); // from Lionleo.roam(); // from Animalleo.sleep(); // from Animal
}}
Ewan Klein (School of Informatics) OOP: More Inheritance Inf1 :: 2009/10 9 / 45
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Animals Example, 7
.Output..
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Wolf=====Howling: Ouooooo!Roamin’ on the plain.Sleeping: Zzzzz
Lion=====Roaring: Rrrrrr!Roamin’ on the plain.Sleeping: Zzzzz
Ewan Klein (School of Informatics) OOP: More Inheritance Inf1 :: 2009/10 10 / 45
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Nested Animal Hierarchy
Lions and cats can be grouped together into Felines, with common roam()behaviours.
Dogs and wolves can be grouped together into Canines, with commonroam() behaviours.
Ewan Klein (School of Informatics) OOP: More Inheritance Inf1 :: 2009/10 11 / 45
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Nested Animal Hierarchy
sleep()makeNoise()roam()
Animal
makeNoise()
LionmakeNoise()
Cat
makeNoise()
Wolf
makeNoise()
Dog
roam()
Feline
roam()
Canine
Ewan Klein (School of Informatics) OOP: More Inheritance Inf1 :: 2009/10 12 / 45
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Animals Example, 1
Same as before.
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public class Animal {public void sleep() {
System.out.println(”Sleeping: Zzzzz”);}public void makeNoise() {
System.out.println(”Noises...”);}public void roam() {
System.out.println(”Roamin’ on the plain.”);}
}
Ewan Klein (School of Informatics) OOP: More Inheritance Inf1 :: 2009/10 13 / 45
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Animals Example, 2
The new class Feline
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public class Feline extends Animal {public void roam() {
// Override roam()System.out.println(”Roaming: I’m roaming alone.”);
}}
Ewan Klein (School of Informatics) OOP: More Inheritance Inf1 :: 2009/10 14 / 45
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Animals Example, 3
The new class Canine
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public class Canine extends Animal {public void roam() {
// Override roam()System.out.println(”Roaming: I’m with my pack.”);
}}
Ewan Klein (School of Informatics) OOP: More Inheritance Inf1 :: 2009/10 15 / 45
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Animals Example, 4
...1 Lion IS-A Feline
...2 Override the makeNoise() method.
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public class Lion extends Feline {public void makeNoise() {
System.out.println(”Roaring: Rrrrrr!”);}
}
Similarly for Cat.
Ewan Klein (School of Informatics) OOP: More Inheritance Inf1 :: 2009/10 16 / 45
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Animals Example, 5
...1 Wolf IS-A Canine
...2 Override the makeNoise() method.
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public class Wolf extends Canine {public void makeNoise() {
System.out.println(”Howling: Ouooooo!”);}
}
Similarly for Dog.
Ewan Klein (School of Informatics) OOP: More Inheritance Inf1 :: 2009/10 17 / 45
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Which method gets called?
sleep()makeNoise()roam()
Animal
makeNoise()
Wolf
roam()
Canine
1. Wolf wolfie = new Wolf();
2. wolfie.makeNoise(); 3. wolfie.roam();
4. wolfie.sleep();
Ewan Klein (School of Informatics) OOP: More Inheritance Inf1 :: 2009/10 18 / 45
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Animals Example, 6.The Launcher..
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public class AnimalLauncher {public static void main(String[] args) {
System.out.println(”\nWolf\n=====”);Wolf wolfie = new Wolf();wolfie.makeNoise(); // from Wolfwolfie.roam(); // from Caninewolfie.sleep(); // from Animal
System.out.println(”\nLion\n=====”);Lion leo = new Lion();leo.makeNoise(); // from Lionleo.roam(); // from Felineleo.sleep(); // from Animal
}}
Ewan Klein (School of Informatics) OOP: More Inheritance Inf1 :: 2009/10 19 / 45
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Animals Example, 7
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Wolf=====Howling: Ouooooo!Roaming: I’m with my pack.Sleeping: Zzzzz
Lion=====Roaring: Rrrrrr!Roaming: I’m roaming alone.Sleeping: Zzzzz
Ewan Klein (School of Informatics) OOP: More Inheritance Inf1 :: 2009/10 20 / 45
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Typing and Polymorphism
polymorphism (= ‘many shapes’): the same piece of code can be assignedmultiple types.
A class defines a type, namely the signatures of its methods.
S is a subtype of T, written S <: T, if a value of type S can be used in anycontext where a value of type T is expected.
The relation <: is reflexive: T <: TThe relation <: is transitive: if S <: T and T <: U, then S <: U.
NB: We say T is a supertype of S if S is a subtype of T.
Inclusion polymorphism: objects of different types S1, S2, …may be treateduniformly as instances of a common supertype T.
Ewan Klein (School of Informatics) OOP: More Inheritance Inf1 :: 2009/10 21 / 45
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Declaring and Initializing a Reference Variable
Wolf wolfie = new Wolf();
create a Wolf object
Ewan Klein (School of Informatics) OOP: More Inheritance Inf1 :: 2009/10 22 / 45
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Declaring and Initializing a Reference Variable
Wolf wolfie = new Wolf();
declare a reference variable
Ewan Klein (School of Informatics) OOP: More Inheritance Inf1 :: 2009/10 23 / 45
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Declaring and Initializing a Reference Variable
Wolf wolfie = new Wolf();
link the object to the reference
Ewan Klein (School of Informatics) OOP: More Inheritance Inf1 :: 2009/10 24 / 45
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Declaring and Initializing a Reference Variable
Animal wolfie = new Wolf();
supertypeobject of subtype
Reference type can be supertype of the object type.
E.g., Wolf <: Animal.
Ewan Klein (School of Informatics) OOP: More Inheritance Inf1 :: 2009/10 25 / 45
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Polymorphic ArrayList
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public class AnimalLauncher2 {public static void main(String[] args) {
Wolf wolfie = new Wolf();Lion leo = new Lion();Cat felix = new Cat();Dog rover = new Dog();ArrayList< Animal > animals = new ArrayList<Animal>();animals.add(wolfie);animals.add(leo);animals.add(felix);animals.add(rover);for ( Animal a : animals) {
a.makeNoise();}
}}
Ewan Klein (School of Informatics) OOP: More Inheritance Inf1 :: 2009/10 26 / 45
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Polymorphic Arrays
ArrayList<Animal> is polymorphic.
animals.add(wolfie)add an object of type Wolf. OK since Wolf <: Animal.
for (Animal a : animals)for each object a of type T such that T <: Animal …
a.makeNoise()if a is of type T, use T’s makeNoise() method.
Ewan Klein (School of Informatics) OOP: More Inheritance Inf1 :: 2009/10 27 / 45
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Method Overriding, 1
If a class C overrides a method m of superclass D, then:
Parameter lists must be same and return type must be compatible:...1 signature of m in C must be same as signature of m in D; i.e. same name, same
parameter list, and...2 return type S of m in C must such that S <: T, where T is return type of m in D.
m must be at least as accessible in C as m is in D
Ewan Klein (School of Informatics) OOP: More Inheritance Inf1 :: 2009/10 28 / 45
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Method Overriding, 2
.method in Animal..
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public void makeNoise() {...}
.Wrong: method in Wolf..
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public void makeNoise( int volume ) {...}
.Wrong: method in Wolf..
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private void makeNoise() {...}
Ewan Klein (School of Informatics) OOP: More Inheritance Inf1 :: 2009/10 29 / 45
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Method Overloading, 1
Overloading: two methods with same name but different parameter lists..Overloaded makeNoise..
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public void makeNoise() {...}public void makeNoise(int volume) {...}
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System.out.println(3); // intSystem.out.println(3.0); // doubleSystem.out.println((float) 3.0); // cast to floatSystem.out.println(”3.0”); // String
Ewan Klein (School of Informatics) OOP: More Inheritance Inf1 :: 2009/10 30 / 45
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Method Overloading, 2
...1 Return types can be different.
...2 You can’t just change the return type — gets treated as an invalid override.
...3 Access levels can be varied up or down.
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public String makeNoise() {String howl = ”Ouooooo!”;return howl;}Exception in thread ”main” java.lang.Error: Unresolved compila-tion problem:
The return type is incompatible with Animal.makeNoise()
at week06.Wolf.makeNoise(Wolf.java:15)at week06.AnimalLauncher.main(AnimalLauncher.java:11)
Ewan Klein (School of Informatics) OOP: More Inheritance Inf1 :: 2009/10 31 / 45
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Animal Objects?
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Wolf wolfie = new Wolf();
Animal leo = new Lion();
Animal weird = new Animal();
Animal class is meant to contain information that all animals have incommon.
But this is not enough to define any one specific animal.
Ewan Klein (School of Informatics) OOP: More Inheritance Inf1 :: 2009/10 32 / 45
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Concrete vs. Abstract
Concrete
Examples: Cat, WolfSpecific enough to beinstantiated.
Abstract
Examples: Animal, FelineNot intended to haveinstances.
Only useful if extended.
Any ’instances’ will have tobe instances of a subclass ofthe abstract class.
Ewan Klein (School of Informatics) OOP: More Inheritance Inf1 :: 2009/10 33 / 45
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The Abstract Animal, 1
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abstract class Animal {public void sleep() {
System.out.println(”Sleeping: Zzzzz”);}public void makeNoise() {
System.out.println(”Noises...”);}public void roam() {
System.out.println(”Roamin’ on the plain.”);}
}
Just put the keyword abstract before the class declaration.
Ewan Klein (School of Informatics) OOP: More Inheritance Inf1 :: 2009/10 34 / 45
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The Abstract Animal, 2
An abstract class can be extended by other abstract classes.Canine and Feline can (and should) both be abstract.
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abstract class Animal {public void sleep() {
System.out.println(”Sleeping: Zzzzz”);}public void makeNoise() {
System.out.println(”Noises...”);}public void roam() {
System.out.println(”Roamin’ on the plain.”);}
}
Just put the keyword abstract before the class declaration.Ewan Klein (School of Informatics) OOP: More Inheritance Inf1 :: 2009/10 35 / 45
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The Abstract Animal, 2
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abstract class Animal {public void sleep() {
System.out.println(””Sleeping: Zzzzz””);public abstract void roam();public abstract void makeNoise();}
}
Now has abstract methods!
Ewan Klein (School of Informatics) OOP: More Inheritance Inf1 :: 2009/10 36 / 45
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The Abstract Animal, 3
roam() and makeNoise() are abstract methods (we’ve already seen these ininterfaces):
I no body;I must be implemented in any concrete subclass (implemented ∼ overriden);I don’t have to be implemented by an abstract subclass;I can only be declared in an abstract class;
sleep() is not abstract, so can be straightforwardly inherited.
Ewan Klein (School of Informatics) OOP: More Inheritance Inf1 :: 2009/10 37 / 45
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Abstract Classes in Animal Hierarchy
sleep()makeNoise()roam()
<<Abstract>>
Animal
makeNoise()
LionmakeNoise()
Cat
makeNoise()
Wolf
makeNoise()
Dog
roam()
<<Abstract>>
Feline
roam()
<<Abstract>>
Canine
Ewan Klein (School of Informatics) OOP: More Inheritance Inf1 :: 2009/10 38 / 45
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Using Abstract Classes
Use an abstract class when you have several similar classes that:I have a lot in common — the implemented parts of the abstract classI have some differences — the abstract methods.
Ewan Klein (School of Informatics) OOP: More Inheritance Inf1 :: 2009/10 39 / 45
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Digression: Constructor Chaining
Constructor of the immediate superclass invoked with super()If you don’t explicitly call this, the compiler will.
Case 1 No constructor explicitly declared. Compiler inserts
public Foo() {super();
}Case 2 You have already declared one or more constructors. Compiler
inserts the following as first statement in each one:
super();
Ewan Klein (School of Informatics) OOP: More Inheritance Inf1 :: 2009/10 40 / 45
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Recap: Implementing/Overriding, 1
If a class C implements or overrides a method m of superclass D, then:
Parameter lists must be same and return type must be compatible:...1 signature of m in C must be same as signature of m in D; i.e. same name, same
parameter list, and...2 return type S of m in C must such that S <: T, where T is return type of m in D.
Ewan Klein (School of Informatics) OOP: More Inheritance Inf1 :: 2009/10 41 / 45
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Method Implementation, 1
.Abstract method in Animal.... ..
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.public abstract void makeNoise();
.Wrong: method in Wolf..
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public void makeNoise( int volume ) {...}
Ewan Klein (School of Informatics) OOP: More Inheritance Inf1 :: 2009/10 42 / 45
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Method Overloading
Overloading: two methods with same name but different parameter lists..Overloaded makeNoise..
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public void makeNoise(int volume) {...}public void makeNoise() {
makeNoise(int 1);}
Ewan Klein (School of Informatics) OOP: More Inheritance Inf1 :: 2009/10 43 / 45
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Method Implementation, 2
You cannot implement or override a method by providing just an overloadedmethod..Declaration of sleep() in Animal..
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public void sleep() {System.out.println(”Sleeping: Zzzzz”);
}
.Right: overriding sleep() in Wolf..
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public void sleep() {System.out.println(”Sleeping: XXXXX”);
}
Ewan Klein (School of Informatics) OOP: More Inheritance Inf1 :: 2009/10 44 / 45
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Method Implementation, 3
.Wrong: overloading sleep() in Wolf..
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public void sleep(int duration) {String sleepNoise = ”Sleeping: Zzzzz”;if (duration > 2)
sleepNoise = ”Still Sleeping: XXXXX”;System.out.println(sleepNoise);
}
.Right: overloading and overriding..
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public void sleep(int duration) {...
}public void sleep() {
System.out.println(”Sleeping: XXXXX”);}
Ewan Klein (School of Informatics) OOP: More Inheritance Inf1 :: 2009/10 45 / 45
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