In the world of programming, there are important ideas called abstract classes and interfaces. These two concepts are very important, but they are not the same. Knowing the differences between them can help programmers build better software.

What Are Abstract Classes and Interfaces?

An abstract class is like a blueprint. You can't use it on its own; instead, you take it and make other classes from it. An abstract class can have methods that are either fully defined or not defined at all. This means it can share some code while letting the subclasses fill in the gaps.

On the other hand, an interface is like a set of rules. It tells classes what methods they must have but doesn’t provide any instructions on how to do them. When a class agrees to follow an interface, it must include all the methods that the interface lists. Interfaces help make different parts of a program work together smoothly.

Main Differences

Here are some key differences between abstract classes and interfaces:

1. Instantiation:

   • Abstract Classes: You can’t create an object from an abstract class directly. You need to create a subclass that gives specific details for any missing methods.
   • Interfaces: You also can’t create objects from interfaces. They only exist to guide classes on what to include.

2. Inheritance:

   • Abstract Classes: A class can only extend one abstract class because of how many programming languages are structured.
   • Interfaces: A class can implement many interfaces. This lets a class take on different features from various places.

3. Method Implementation:

   • Abstract Classes: Can have methods that are not defined and methods that are fully defined. This allows for a mix of shared and specific functionality.
   • Interfaces: Mostly just have method names without any details on how they work (although some recent languages let you have default methods). They mainly focus on method names.

4. State:

   • Abstract Classes: Can have variables that hold data. These can be set when you create the abstract class.
   • Interfaces: Cannot have variables you can change. Any variables in an interface are always constants.

5. Access Modifiers:

   • Abstract Classes: Can use different access rules to control who can see the methods and properties.
   • Interfaces: All methods are open for everyone by default, and you can’t use access rules in interfaces.

6. Use Cases:

   • Abstract Classes: Best to use when you have a common base that should enforce certain behaviors across multiple subclasses. This is helpful for sharing some logic or data.
   • Interfaces: Great for setting common rules that different classes can follow, no matter where they are in the class structure. This helps keep everything separate but still working together.

Real-Life Examples

Let’s look at a banking app to see how these ideas work:

• Abstract Class Example: Imagine you have an abstract class called Account. This class has methods like deposit() and withdraw() that everyone uses, but it also has an abstract method calculateInterest(). Each type of account, like SavingsAccount or CheckingAccount, can use the shared parts while figuring out how to calculate interest their own way.

• Interface Example: Now, think about an interface called Transaction. This interface says that any class must have methods like execute() and revert(). This means that any class, whether it’s for Transfer, Deposit, or Withdraw, has to offer these methods. This ensures they all follow the same pattern.

Conclusion

In short, abstract classes and interfaces are important parts of programming that help organize and shape software. Abstract classes help create a clear structure where some code is shared, while interfaces allow for more freedom and flexibility in how classes interact.

Choosing whether to use an abstract class or an interface depends on what your application needs and how you plan to design it. In good designs, you often find both abstract classes and interfaces being used together to balance structure and flexibility.