In the world of Object-Oriented Programming (OOP), it's important to know the difference between abstract classes and interfaces. This knowledge helps you organize your code better. Let's break down how they are different.

An abstract class is special because it can have two kinds of methods:

1. Abstract methods: These don’t have any code in them. They just tell you what the method should do.
2. Concrete methods: These have complete code that tells what to do.

This means that an abstract class can provide some ready-to-use features, and also force other classes to fill in the missing pieces. For example, think about a class called Animal. It could have a concrete method called sleep() that tells what happens when an animal sleeps. It can also have an abstract method called makeSound() that doesn’t have any code; it just says that each animal needs to provide its own sound. This way, abstract classes offer some help while also making sure that specific details are filled in by other classes.

Now, let’s discuss an interface. An interface is like a rulebook. It can only list methods, but it does not give any details on how to do those methods. When a class says it follows an interface, it must provide the actual code for all the listed methods. For example, if you have an interface called Readable, it might list a method named readPage(). Any class that uses Readable has to create its own version of readPage(). This is useful because interfaces make sure different classes follow the same rules without stating how to work.

Key Differences:

• Implementation: Abstract classes can have both kinds of methods (abstract and concrete), while interfaces can only list methods.
• Multiple Inheritance: A class can inherit from just one abstract class, but it can implement many interfaces. This gives more options when you design your code.
• State: Abstract classes can remember things using instance variables, but interfaces can’t remember any information.

When to Use Them:

• Use abstract classes when you want a basic class that shares some behavior with other classes but also needs those classes to do certain tasks.
• Use interfaces when you want to set a rule that many classes need to follow, even if they don’t share a common background.

In summary, deciding between an abstract class and an interface depends on how much flexibility and guidance you want in your code. Knowing these differences helps developers create stronger and easier-to-maintain code in OOP.