How do interfaces differ from abstract classes in Java?

In Java, both interfaces and abstract classes are used to define abstract types, but they have key differences in how they function and are used in object-oriented programming.

1. Definition and Purpose:

Interfaces are contracts that define methods without implementing them. A class that implements an interface must provide implementations for all the methods declared in the interface.

Abstract classes, on the other hand, can have both abstract methods (without implementation) and concrete methods (with implementation). An abstract class allows you to provide some default behavior while leaving other methods abstract for subclasses to implement.

2. Multiple Inheritance:

Interfaces support multiple inheritance, meaning a class can implement more than one interface. This allows for greater flexibility in designing systems where a class can inherit behavior from multiple sources.

Abstract classes do not support multiple inheritance. A class can only extend one abstract class, which might limit flexibility in some cases.

3. Constructor:

Interfaces cannot have constructors because they do not hold state or any implementation details.

Abstract classes can have constructors that can be called by subclasses to initialize state.

4. Fields and Constants:

Interfaces can only declare constants (static final variables) but cannot have instance variables.

Abstract classes can have both constants and instance variables, which gives more flexibility when defining shared behavior or state.

5. Default Methods:

Interfaces (since Java 8) can now have default methods, which allow them to provide default implementations for methods, making it easier to evolve interfaces without breaking existing implementations.

Abstract classes always have the ability to provide method implementations.

In summary, interfaces are more about defining a contract, while abstract classes are about shared behavior with room for customization. For those aiming to become a full stack Java developer by The IoT Academy, understanding these distinctions is crucial as it influences how you structure the design of complex applications.

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