A7: Abstract Classes & Method Overloading
CPSC 210
Learning Goals
-
To use abstract classes to
- record the similarity between other types
- avoid code duplication
- store common implementations to be inherited by subclasses
- require specialized behaviour from subclasses
- To describe and use method overloading in the design of an interface or class
Assume all mammals walk, eat, and make noises
public interface Mammal {
void walk();
void makeNoise();
void eat();
}Assume all dogs walk and eat the same way
public class Dog implements Mammal {
@Override
public void walk() {
System.out.println("waaalk...");
}
@Override
public void eat() {
System.out.println("mmmmpf...");
}
}@Override
public void makeNoise() {
System.out.println("NOT SURE"); // ???
}But different dogs make completely different noises
Why Abstract Classes?
public class Dog implements Mammal {
// ...
@Override
public void makeNoise() {
System.out.println("NOT SURE"); // ???
}
}public class Poodle extends Dog {
}"Not Sure"?
- How do I make any subclass implement its own makeNoise method?
- How do I implement all methods except makeNoise in the Dog class and make all subclasses implement makeNoise?
Abstract Classes
- Defines a new type of data
- Can optionally not provide impls for methods
- Cannot be instantiated
Abstract Class
Abstract Classes (2)
- Must provide impls for all abstract methods
"Concrete"Class
Abstract Classes (3)
public class Dog implements Mammal {
// ...
@Override
public void makeNoise() {
System.out.println("NOT SURE");
}
}public abstract class Dog implements Mammal {
// ...
@Override
public abstract void makeNoise();
}public class Poodle extends Dog {
@Override
public void makeNoise() {
System.out.println("Wiiif wiiif");
}
}OR (next level!)
public abstract class Dog implements Mammal {
}public class Poodle extends Dog {
}public abstract void bark();@Override
public void makeNoise() {
bark();
}@Override
public void bark() {
System.out.println("Wiiif wiiif");
}Type Substitution
- Everything we have learned about type substitution also applies to type hierarchies with abstract classes
- You can use any subtype for a super type
- Except..
Dog dog = new Dog();
Dog poodle = new Poodle();
Dog borderCollie = new BorderCollie();
Dog germanShepherd = new GermanShepherd();
dog.bark(); // Woof Woof (generic)
poodle.bark(); // Wiiif Wiiif
borderCollie.bark(); // Wuuuf Wuuuf
germanShepherd.bark(); // Waaaf Waaafno creating instances of abstract classes
Method Overloading
public class Dog {
// ...
void bark();
void bark(int times);
}- Two methods with same name but different parameter lists
- We can reuse a name to describe similar operations that take different parameters as input
- As far as Java is concerned:
totally unrelated methods - Methods can also call each other
public class Poodle extends Dog {
@Override
public void bark() {
System.out.println("Wiiif wiiif");
}
@Override
public void bark(int times) {
for (int i = 0; i < times; i++) {
bark();
}
}
}Lecture Ticket Review
public abstract class Barista {
public void serveCoffee(){
greet();
giveCoffee();
sayGoodbye();
}
protected _________ void giveCoffee()
protected _________ void greet()
protected _________ void sayGoodbye()
}
public class HappyBarista {
// greet and say goodbye in a cheery way
}
public class GrumpyBarista {
// greet and say goodbye in a grumpy way
}What methods should be abstract?
What methods should be implemented in HappyBarista and GrumpyBarista?
Lecture Lab
Get Oval and Rectangle Working at the Same Time
Lecture Lab (A)
Lecture Lab (B1)
Lecture Lab (B2)
Lecture Lab (C)
A7: Abstract Classes & Method Overloading
The End - Thank You!
CPSC210 - A7 Abstract Classes & Method Overloading
By Steven Wolfman
