COMP200 INTERFACES OOP using Java, from slides by Shayan Javed 2 - - PowerPoint PPT Presentation

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COMP200 INTERFACES OOP using Java, from slides by Shayan Javed 2 - - PowerPoint PPT Presentation

Jan 18, 2024 512 likes •977 views

1 1 COMP200 INTERFACES OOP using Java, from slides by Shayan Javed 2 Interfaces 3 ANIMAL picture food sleep() roam() makeNoise() eat() 4 ANIMAL picture food sleep() roam() makeNoise() eat() CANINE FELINE roam() roam() 5

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COMP200


INTERFACES

OOP using Java, from slides by Shayan Javed

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Interfaces

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3 ANIMAL

picture food sleep() roam() makeNoise() eat()

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4 ANIMAL

picture food sleep() roam() makeNoise() eat()

FELINE roam() CANINE roam()

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5 ANIMAL

picture food sleep() roam() makeNoise() eat()

FELINE roam() CANINE roam() LION makeNoise() eat() CAT makeNoise() eat() WOLF makeNoise() eat() DOG makeNoise() 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 class

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

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 Pet abstract 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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40 Animal Feline Canine Lion Cat Wolf Dog Pet abstract 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