Interface vs Abstract Class?

Interface:

• An interface is a structure that defines only the signatures (method signatures) of the methods a class must implement. In other words, it does not contain the method bodies (implementation), only specifying what needs to be done.
• A class can implement one or more interfaces.
• A class that implements an interface must implement all the methods defined in that interface.
• It is used to solve the problem of multiple inheritance.

---

Abstract Class:

• An abstract class is a class that can contain both abstract and concrete methods. This means some methods may be bodiless (abstract), while others may be implemented.
• A class can inherit from only one abstract class (single inheritance).
• Abstract classes cannot be directly instantiated (no instances can be created), but they can be extended by subclasses.
• They can contain both abstract and regular methods.
• Variables can be defined with any access modifier (public, private, protected).
• Subclasses are required to implement the abstract methods.
• Inheritance from multiple abstract classes is not possible.

---

Differences Between Interface and Abstract Class:

Criterion	Interface	Abstract Class
Method Type	Generally only method signatures (except default/static methods after Java 8).	Can have both abstract and concrete methods.
Inheritance	Multiple interfaces can be implemented.	Only one abstract class can be inherited.
Variables	Only public static final (constants).	Variables of any type can be defined.
Constructor	Cannot have a constructor.	Can have a constructor.
Access Modifier	All methods are public by default.	Methods can have different access modifiers.
Purpose	Provides a fully abstract structure, typically used to define a set of behaviors.	Provides partial implementation, ideal for sharing common code.

---

When Should We Use Which Structure?

• Use an Interface:

  • When you want to define a common behavior across multiple classes.
  • When you want to achieve full abstraction between classes.
  • When multiple inheritance is needed.
  • Example: General behaviors like Comparable, Runnable.

• Use an Abstract Class:

  • When you want to share common code (e.g., if some methods will have the same implementation).
  • When you want to create a hierarchy between classes.
  • When you need to use variables or concrete methods.

---

Thanks for reading.

Burak Boduroglu