Inheritance is an important idea in object-oriented design. It helps us understand how abstract classes and interfaces work. Both of these tools help us hide complex details, but they have different purposes and features. Let's break it down!

Abstract Classes

• What They Are: Abstract classes act like a base for other classes. They can have two types of methods:
  • Abstract methods: These are like plans. They tell what should happen but don’t do anything yet.
  • Concrete methods: These actually do something.
• How Inheritance Works: A class can only inherit from one abstract class at a time. This is called single inheritance. Here’s a simple example:

  abstract class Animal {
      abstract void makeSound(); // No code yet, just a promise
      void breathe() { // This one does something
          System.out.println("Breathing...");
      }
  }
  
  class Dog extends Animal {
      void makeSound() {
          System.out.println("Bark"); // This is where we fill in the promise
      }
  }
  

Interfaces

• What They Are: Interfaces are like contracts. They say what methods a class should have. All the methods in an interface are abstract by default, which means they don’t do anything unless the class that uses them fills in the details. Until Java 8, interfaces couldn’t have any working methods.

• How Inheritance Works: A class can use multiple interfaces, which is called multiple inheritance. Here’s how that looks:

  interface CanFly {
      void fly(); // Still just a plan
  }
  
  interface CanSwim {
      void swim(); // Another plan
  }
  
  class Bird implements CanFly, CanSwim {
      public void fly() {
          System.out.println("Flying"); // This is how a bird flies
      }
      public void swim() {
          System.out.println("Swimming"); // And this is how it swims
      }
  }
  

Summary

To sum it up, choosing between an abstract class and an interface depends on what you need.

• Use abstract classes if you want to share code between classes that are quite similar.

• Use interfaces if you want to create a contract that many different classes can follow. This way, you can mix and match methods to give your classes more flexibility.

Keep these differences in mind when designing your structures!