cop 3503 fall 2012 shayan javed lecture 6
DESCRIPTION
COP 3503 FALL 2012 Shayan Javed Lecture 6. Programming Fundamentals using Java. Interfaces. Animal picture food sleep() roam() makeNoise () eat(). Animal picture food sleep() roam() makeNoise () eat(). Canine roam(). Feline roam(). Animal picture food sleep() roam() - PowerPoint PPT PresentationTRANSCRIPT
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COP 3503 FALL 2012SHAYAN JAVED
LECTURE 6
Programming Fundamentals using Java
1
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Interfaces
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ANIMALpicturefoodsleep()roam()makeNoise()eat()
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ANIMALpicturefoodsleep()roam()makeNoise()eat()
FELINE
roam()
CANINE
roam()
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ANIMALpicturefoodsleep()roam()makeNoise()eat()
FELINE
roam()
CANINE
roam()
LIONmakeNoise()eat()
CATmakeNoise()eat()
WOLFmakeNoise()eat()
DOGmakeNoise()eat()
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Problem
Need to add Pet behaviors to the pet animal classes (Cat and Dog):
• play()
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Option 1
Add concrete methods: play() to Animal class
Animal
Feline Canine
Lion Cat Wolf Dog
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Option 1
Add concrete methods: play() to Animal class
What’s the problem?
Animal
Feline Canine
Lion Cat Wolf Dog
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Option 1
Add concrete methods: play() to Animal class
What’s the problem?
Also exist for Lion/Wolf
Animal
Feline Canine
Lion Cat Wolf Dog
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Option 2
Add abstract methods: play() to Animal classAnimal
Feline Canine
Lion Cat Wolf Dog
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Option 2
Add abstract methods: play() to Animal class
Provide empty body for it in Lion and Wolf classes
Animal
Feline Canine
Lion Cat Wolf Dog
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Option 2
Add abstract methods: play() to Animal class
Provide empty body for it in Lion and Wolf classes
Provide non-empty body for it in Cat and Dog classes
Animal
Feline Canine
Lion Cat Wolf Dog
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Option 3
Add concrete methods: play() to Cat and Dog classes
Animal
Feline Canine
Lion Cat Wolf Dog
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What we need
Pet behaviors just in Pet classes (like Dog and Cat)
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What we need
Pet behaviors just in Pet classes (like Dog and Cat)
Consistency - Guarantee that all Pet classes use the same signature for Pet methods
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What we need
Pet behaviors just in Pet classes (like Dog and Cat)
Consistency - Guarantee that all Pet classes use the same signature for Pet methods
Take advantage of Polymorphism
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Multiple Inheritance?
Animal
Feline Canine
Lion Cat Wolf Dog
Pet
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Multiple Inheritance?
Animal
Feline Canine
Lion Cat Wolf Dog
Pet
But Java does not allow multiple inheritance! Why?
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Multiple Inheritance?
Not allowed
Mainly to avoid complexity/confusion.
Want to keep it simple.
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The Diamond Problem
SuperClass
method()
A
method()B
method()
C
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The Diamond Problem
SuperClass
method()
A
method()B
method()
C
What happens when method() is called in C?
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Solution: Interfaces
Animal
Feline Canine
Lion Cat Wolf Dog
Petabstract play();
Make Pet an interface and put all pet behaviors in it as abstract methods
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Interfaces
Classlike construct• contains only constants and abstract methods.
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Interfaces
Classlike construct• contains only constants and abstract methods. • cannot contain variables or concrete methods.
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Interfaces
Classlike construct• contains only constants and abstract methods. • cannot contain variables or concrete methods.
Declaration:public interface <InterfaceName> { // constant declarations; // method signatures;}
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Interface Pet
public interface Pet {public void play();
}
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Implementing Pet interface
class Dog extends Canine implements Pet{
public void play(){ System.out.println(“Catch ball”);
} }
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Implementing Pet interface
class Dog extends Canine implements Pet{
public void play(){ System.out.println(“Catch ball”);
} }
class Cat extends Feline implements Pet {
public void play(){ System.out.println(“Roll ball”);
} }
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Interface Characteristics
Interfaces are defined in their own file
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Interface Characteristics
Interfaces are defined in their own file
Methods are abstract. No implementations. May omit the abstract keyword.
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Interface Characteristics
Interfaces are defined in their own file
Methods are abstract. No implementations. May omit the abstract keyword.
Do not have instance fields.
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Interface Characteristics
Interfaces are defined in their own file
Methods are abstract. No implementations. May omit the abstract keyword.
Do not have instance fields.
Can have constant fields. Automatically treated as public, static, final; may omit any of these keywords
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Example
public interface Interface1 {int aNumber = 0;
public void aMethod();}
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Example
public interface Interface1 {int aNumber = 0;
public void aMethod();}
Same as:public interface Interface1 {
public static final int aNumber = 0;
public abstract void aMethod();}
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Interface Characteristics
A class can implement any number of interfaces
public class aClass implements I1, I2, I3 {
// methods from I1, I2, I3...
}
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Interface Characteristics
A class can implement any number of interfaces
public class aClass implements I1, I2, I3 {
// methods from I1, I2, I3...
}
Interfaces can not be instantiated
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Interface Characteristics
A class can implement any number of interfaces
public class aClass implements I1, I2, I3 {
// methods from I1, I2, I3...
}
Interfaces can not be instantiated
But can be used as a data type for a variable
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Using the interface
Pet[] pets = new Pet[2];pet[0] = new Dog();pet[1] = new Cat();
for(int i = 0; i < pets.length; i++)
pet[i].play();
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Using the interface
Pet[] pets = new Pet[2];pet[0] = new Dog();pet[1] = new Cat();
for(int i = 0; i < pets.length; i++)
pet[i].play();
Similar to abstract classes – advantage of polymorphism
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Animal
Feline Canine
Lion Cat Wolf Dog
Petabstract play();
Robot
RoboDog Roomba
Classes from different inheritance hierarchy can implement same interface!
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Abstract Classes vs. Interfaces
Variables Constructors Methods
Abstract class
No restrictions Constructors are invoked by subclasses through constructor chaining.
An abstract class cannot be instantiated using the new operator.
No restrictions.
Interface All variables must be public static final
No constructors.
An interface cannot be instantiated using the new operator.
All methods must be public abstract instance methods
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Abstract Classes vs. Interfaces
Variables Constructors Methods
Abstract class
No restrictions Constructors are invoked by subclasses through constructor chaining.
An abstract class cannot be instantiated using the new operator.
No restrictions.
Interface All variables must be public static final
No constructors.
An interface cannot be instantiated using the new operator.
All methods must be public abstract instance methods
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Abstract Classes vs. Interfaces
Variables Constructors Methods
Abstract class
No restrictions Constructors are invoked by subclasses through constructor chaining.
An abstract class cannot be instantiated using the new operator.
No restrictions.
Interface All variables must be public static final
No constructors.
An interface cannot be instantiated using the new operator.
All methods must be public abstract instance methods
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Abstract Classes vs. Interfaces
Cannot inherit from multiple classes but, Can can implement multiple interfaces
Interfaces = model “can-do” type relationships Inheritance = model “is-a” type of relationship
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Next Lecture
More on interfaces