Advanced OOP

Abstract Classes

An abstract class is a class that cannot be instantiated (you cannot create objects of it).

• Used to provide a common base for subclasses.
• Can contain abstract methods (method without body) and regular methods.

abstract class Animal {
  public abstract void animalSound(); // No body
  public void sleep() {
    System.out.println("Zzz");
  }
}

Interfaces

An interface is a completely "abstract class" that is used to group related methods with empty bodies.

• Keyword: interface.
• Implementation Keyword: implements.
• Support Multiple Inheritance: A class can implement multiple interfaces.

interface Animal {
  public void animalSound(); // Interface method (no body)
  public void run();
}

class Pig implements Animal {
  public void animalSound() { System.out.println("Wee wee"); }
  public void run() { System.out.println("Pig running"); }
}

[!NOTE] Java does not support multiple inheritance with classes (extending two classes), but you can implement multiple interfaces!

Abstract Class vs Interface

Interactive Code

Try implementing an interface!

Quiz

Next Steps

We've covered the structure. Now let's handle text and collections of data in Module 6: Strings & Arrays.