A6: Inheritance

Learning Goals

To use inheritance to form type hierarchies
- These allow us to record similarity between types
To use inheritance to reuse code
- Between two different implementations
To use inheritance to refine or override code

Inheritance

A class can be declared to extend exactly one other class

public class Poodle extends Dog {

}

Becomes a Subtype

Becomes a Supertype

Supertype can be substituted with any of its subtypes
- Just like with interfaces

Why Inheritance?

Remember: object-orientation models the real world!

...

Standard Poodle

Miniature Poodle

Toy Poodle

...

German
Shepherd

Border
Collie

Poodle

Dog

public class Dog {
  public void bark() {
    System.out.println("Wow wow");
  }
}

public class Poodle extends Dog {
  public void bark() {
    System.out.println("Wiiif Wiiif");
  }
}

public class BorderCollie extends Dog {
  public void bark() {
    System.out.println("Wuuuf Wuuuf");
  }
}

public class GermanShepherd extends Dog {
  public void bark() {
    System.out.println("Waaaf Waaaf");
  }
}

Implementing

Dog

public class Dog {
  public void bark() {
    System.out.println("Wow wow");
  }
}

public class Poodle extends Dog {
  public void bark() {
    System.out.println("Wiiif Wiiif");
  }
}

public class BorderCollie extends Dog {
  public void bark() {
    System.out.println("Wuuuf Wuuuf");
  }
}

public class GermanShepherd extends Dog {
  public void bark() {
    System.out.println("Waaaf Waaaf");
  }
}

Dog dog = new Dog();
Dog poodle = new Poodle();
Dog borderCollie = new BorderCollie();
Dog germanShepherd = new GermanShepherd();

dog.bark(); // ???
poodle.bark(); // ???
borderCollie.bark(); // ???
germanShepherd.bark(); // ???

public class UnclassifiedDog extends Dog {

}

Dog unclassified = new UnclassifiedDog();
unclassified.bark(); // ???

Examples

Lecture Ticket Review (1)

public class TransitPass {
  // REQUIRES: amount > 0
  // MODIFIES: this 
  // EFFECTS: adds amount to the balance on this pass
  void reload(int amount) {...}
  // EFFECTS: returns balance on this pass
  int getBalance() {...}
  // REQUIRES: pass has sufficient balance to pay fare 
  // MODIFIES: this 
  // EFFECTS: taps in to pay fare for transit trip 
  void tapIn() {...}
}

public class TransitPassTripTracker 
                extends TransitPass {
  // REQUIRES: pass has sufficient balance to pay fare 
  // MODIFIES: this 
  // EFFECTS: taps in to pay fare for transit trip and 
  //          adds one to number of trips
  // paid for with this pass
  void tapIn() {...}
  // EFFECTS: returns number of trips paid for using 
  //          this transit pass
  int getNumTrips() {...}
}

Lecture Ticket Review (2)

public class TransitPass {
  // REQUIRES: amount > 0
  // MODIFIES: this 
  // EFFECTS: adds amount to the balance on this pass
  void reload(int amount) {...}
  // EFFECTS: returns balance on this pass
  int getBalance() {...}
  // REQUIRES: pass has sufficient balance to pay fare 
  // MODIFIES: this 
  // EFFECTS: taps in to pay fare for transit trip 
  void tapIn() {...}
}

public class TransitPassTripTracker 
                extends TransitPass {
  // REQUIRES: pass has sufficient balance to pay fare 
  // MODIFIES: this 
  // EFFECTS: taps in to pay fare for transit trip and 
  //          adds one to number of trips
  // paid for with this pass
  void tapIn() {...}
  // EFFECTS: returns number of trips paid for using 
  //          this transit pass
  int getNumTrips() {...}
}

And Part 3, 4, 5

protected

protected int age;

private < default < protected < public

= private + subclasses

Visibility

super

Used in Two Contexts

public Poodle() {
  super(); // First statement!
  //...
}

public int getAge() {
  return super.age;
}

public int getAge() {
  return super.getAge();
}

Call constructor of immediate superclass

Access fields/methods of a superclass

Keywords

super

protected

method override

supertype

subtype

declared type

actual type

method dispatch

method overload

= apparent type

= instantiated type

Lecture Lab

edX > A6 > Lecture Lab

https://github.students.cs.ubc.ca/CPSC210/SimpleDrawingPlayer-Starter

A6: Inheritance

Learning Goals

Inheritance

Why Inheritance?

Implementing

Examples

Lecture Ticket Review (1)

Lecture Ticket Review (2)

Keywords

Lecture Lab

A6: Inheritance

The End - Thank You!