Abstract classes in Java are classes that cannot be instantiated on their own and are meant to be subclassed. They can contain abstract methods (methods without a body) that must be implemented by subclasses, as well as concrete methods (methods with a body). The purpose of abstract classes is to provide a common base for subclasses to share code and define a common interface.

• Key Points:
  • Abstract classes are declared using the `abstract` keyword.
  • They can have both abstract and concrete methods.
  • They are used to achieve abstraction and define a template for subclasses.
  • Abstract classes cannot be instantiated directly.

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An inner class in Java is a class defined within another class or interface. It allows for logically grouping related classes, enhancing encapsulation and improving code readability. Inner classes can access all members (variables and methods) of their outer class, which makes them useful for certain design patterns.

In Java, inner classes are also known as non-static nested classes, and they cannot have static members. This structure helps in organizing code and aligning it with real-world object-oriented design principles.

1. Instantiation:
   • Abstract Class: Cannot be instantiated directly. It serves as a blueprint for subclasses.
   • Interface: Cannot be instantiated; it defines a contract that implementing classes must fulfill.
2. Method Implementation:
   • Abstract Class: Can contain both abstract methods (without implementation) and concrete methods (with implementation). This allows for shared behavior among subclasses.
   • Interface: All methods are abstract by default (except for default and static methods introduced in Java 8). Interfaces cannot provide method implementations until Java 8, which allows for default methods.
3. Multiple Inheritance:
   • Abstract Class: A class can inherit from only one abstract class.
   • Interface: A class can implement multiple interfaces, allowing for greater flexibility in design.
4. Variable Declarations:
   • Abstract Class: Can have instance variables with any access modifier (public, protected, private).
   • Interface: Variables are implicitly public, static, and final (constants).

• Unrelated Classes: Use an interface when you want to define a contract that can be implemented by classes that are not related by inheritance. For example, a `Flyable` interface can be implemented by `Bird` and `Airplane` classes.
• Multiple Behaviors: If you need to implement multiple behaviors from different sources, interfaces are the way to go. They allow for polymorphism and flexibility in your design.
• Future Compatibility: Interfaces are less flexible when it comes to adding new methods. If you anticipate needing to add methods in the future, consider using an abstract class instead.

• Shared Code: Use an abstract class when you want to provide a common base with shared functionality. For example, if you have a base class `Animal` with common methods like `eat()` and `sleep()`, you can define these in an abstract class.
• Partial Implementation: If you want to provide some default behavior while still requiring subclasses to implement specific methods, an abstract class is ideal. This allows you to define both abstract and concrete methods.
• Versioning: Abstract classes allow for better forward compatibility. You can add new methods without breaking existing implementations, which is not possible with interfaces.

In summary, use interfaces to define capabilities and contracts for unrelated classes, while use abstract classes to provide a common base with shared functionality. Understanding these distinctions will help you make informed design decisions in your Java applications.