Use abstract classes to share code and define non-static fields, while interfaces define contracts and support multiple inheritance. 1. Abstract classes allow both abstract and concrete methods, interfaces only abstract (before Java 8) or default/static methods (Java 8 ). 2. Abstract classes can have non-static, non-final fields; interfaces have public static final fields. 3. Abstract classes support all access modifiers; interface members are public by default. 4. Abstract classes use extends for inheritance; a class can implement multiple interfaces. 5. Abstract classes can have constructors; interfaces cannot. Choose abstract classes for shared implementation, interfaces for behavioral contracts across unrelated classes.

In Java, interfaces and abstract classes are both used to achieve abstraction, but they serve different purposes and have distinct features. Choosing between them depends on what you're trying to model in your design.

When to Use an Abstract Class

An abstract class is best when you want to share code among closely related classes. It can contain both abstract methods (without implementation) and concrete methods (with implementation). This makes it a good fit when you have a base class that provides some default behavior, but also expects subclasses to fill in the blanks.

• If you need to define non-static or non-final fields, an abstract class is the way to go.
• You can use constructors and instance initialization blocks.
• It supports access modifiers like private, protected, etc., for its members.

For example, if you're building a set of shapes like Circle, Rectangle, and Triangle, and they all share some common logic for area calculation setup, an abstract class could handle that shared part while leaving the actual formula to each subclass.

When to Use an Interface

An interface is more about defining a contract. It's ideal when you want unrelated classes to implement a specific behavior without forcing a class hierarchy. Since Java 8, interfaces can also have default and static methods, which gives them more flexibility than before.

• Interfaces support multiple inheritance — a class can implement multiple interfaces.
• They are great for defining APIs or behaviors that can be implemented across different types.
• From Java 9 onward, interfaces can even have private methods to help organize default method implementations.

For instance, something like a Logger interface can be implemented by various components in your system — a FileLogger, DatabaseLogger, or CloudLogger — each doing logging differently but adhering to the same method signature.

Key Differences at a Glance

Here’s a quick breakdown:

• Method Implementation:

  • Abstract class: Can have both abstract and concrete methods.
  • Interface: Before Java 8, only abstract methods; now can have default and static methods.

• Fields:

  • Abstract class: Can have non-static, non-final fields.
  • Interface: Fields are implicitly public static final.

• Access Modifiers:

  • Abstract class: Supports all modifiers.
  • Interface: Members are public by default.

• Inheritance:

  • Abstract class: Uses class inheritance (extends).
  • Interface: A class can implement multiple interfaces.

• Constructors:

  • Abstract class: Yes, can have constructors.
  • Interface: No constructors allowed.

Design Considerations

If you're designing a library or framework, think carefully:

• Use an abstract class when you're creating a base class that offers shared functionality and expects subclasses to extend and refine it.
• Use an interface when you're defining a capability that different classes should provide, regardless of their place in the class hierarchy.

Also, keep in mind that starting from Java 8 , the gap between interfaces and abstract classes has narrowed — but they still aren't interchangeable. The choice often comes down to whether you need multiple inheritance of type (use interface) or want to share implementation details among a group of classes (use abstract class).

That's pretty much it. Not too hard once you get the pattern, but easy to mix up if you're not paying attention.

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