COMP200 INTERFACES. OOP using Java, from slides by Shayan Javed

Transcription

1 1 1 COMP200 INTERFACES OOP using Java, from slides by Shayan Javed

2 Interfaces 2

3 ANIMAL picture food sleep() roam() makenoise() eat() 3

4 ANIMAL picture food sleep() roam() makenoise() eat() 4 roam() FELINE roam() CANINE

5 ANIMAL picture food sleep() roam() makenoise() eat() 5 roam() FELINE roam() CANINE LION makenoise() eat() CAT makenoise() eat() WOLF makenoise() eat() DOG makenoise() eat()

6 6 Problem Need to add Pet behaviors to the pet animal classes (Cat and Dog): play()

7 7 Option 1 Animal Add concrete methods: play() to Animal class Feline Canine Lion Cat Wolf Dog

8 8 Option 1 Animal Add concrete methods: play() to Animal class Feline Canine What s the problem? Lion Cat Wolf Dog

9 9 Option 1 Animal Add concrete methods: play() to Animal class Feline Canine What s the problem? Lion Cat Wolf Dog Also exist for Lion/Wolf

10 10 Option 2 Animal Add abstract methods: play() to Animal class Feline Canine Lion Cat Wolf Dog

11 11 Option 2 Animal Add abstract methods: play() to Animal class Feline Canine Provide empty body for it in Lion and Wolf classes Lion Cat Wolf Dog

12 12 Option 2 Animal Add abstract methods: play() to Animal class Feline Canine Provide empty body for it in Lion and Wolf classes Lion Cat Wolf Dog Provide non-empty body for it in Cat and Dog classes

13 13 Option 3 Feline Animal Canine Add concrete methods: play() to Cat and Dog classes Lion Cat Wolf Dog

14 14 What we need Pet behaviors just in Pet classes (like Dog and Cat)

15 15 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

16 16 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

17 17 Multiple Inheritance? Animal Pet Feline Canine Lion Cat Wolf Dog

18 18 Multiple Inheritance? Animal Pet Feline Canine Lion Cat Wolf Dog But Java does not allow multiple inheritance! Why?

19 19 Multiple Inheritance? Not allowed Mainly to avoid complexity/confusion. Want to keep it simple.

20 20 The Diamond Problem SuperClass method() A method() method() B C

21 21 The Diamond Problem SuperClass method() A method() method() B C What happens when method() is called in C?

22 22 Solution: Interfaces Make Pet an interface and put all pet behaviors in it as abstract methods Animal Pet abstract play(); Feline Canine Lion Cat Wolf Dog

23 23 Interfaces Classlike construct contains only constants and abstract methods.

24 24 Interfaces Classlike construct contains only constants and abstract methods. cannot contain variables or concrete methods.

25 25 Interfaces Classlike construct contains only constants and abstract methods. cannot contain variables or concrete methods. Declaration: public interface <InterfaceName> { // constant declarations; } // method signatures;

26 26 Interface Pet public interface Pet { } public void play();

27 27 Implementing Pet interface class Dog extends Canine implements Pet{ } public void play(){ System.out.println( Catch ball ); }

28 28 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 ); }

29 29 Interface Characteristics Interfaces are defined in their own file

30 30 Interface Characteristics Interfaces are defined in their own file Methods are abstract. No implementations.! May omit the abstract keyword.

31 31 Interface Characteristics Interfaces are defined in their own file Methods are abstract. No implementations.! May omit the abstract keyword. Do not have instance fields.

32 32 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

33 33 Example public interface Interface1 { int anumber = 0; } public void amethod();

34 34 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();

35 35 Interface Characteristics A class can implement any number of interfaces public class aclass implements I1, I2, I3 { // methods from I1, I2, I3... }

36 36 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

37 37 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

38 38 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();

39 39 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

40 40 Classes from different inheritance hierarchy can implement same interface! Robot Feline Animal Canine Pet abstract play(); RoboDog Roomba Lion Cat Wolf Dog

41 41 Abstract Classes vs. Interfaces Variables Constructors Methods Abstract class No restrictions Constructors are invoked by subclasses through constructor chaining. No restrictions. An abstract class cannot be instantiated using the new operator. 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

42 42 Abstract Classes vs. Interfaces Variables Constructors Methods Abstract class No restrictions Constructors are invoked by subclasses through constructor chaining. No restrictions. An abstract class cannot be instantiated using the new operator. 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

43 43 Abstract Classes vs. Interfaces Variables Constructors Methods Abstract class No restrictions Constructors are invoked by subclasses through constructor chaining. No restrictions. An abstract class cannot be instantiated using the new operator. 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

44 44 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

45 45 Next Lecture More on interfaces