In Java, both interfaces and abstract classes can be used to define abstract types, which cannot be directly instantiated. However, there are some key differences between them:
- Method implementation: In an interface, all methods are abstract and do not have an implementation. In contrast, an abstract class can have both abstract and non-abstract methods, and it can provide default implementations for some of them.
- Inheritance: A class can implement multiple interfaces, but it can only inherit from one abstract class. This means that interfaces are more flexible than abstract classes when it comes to designing the inheritance hierarchy.
- Constructors: An interface cannot have constructors, while an abstract class can.
- Access modifiers: All methods in an interface are implicitly public, whereas an abstract class can have methods with any access modifier.
- Variables: An interface can only define constants (final variables), while an abstract class can have instance variables.
In summary, an interface is a pure abstract type, which defines a set of methods that a class must implement. An abstract class, on the other hand, can provide some common functionality and state for its subclasses, but it still leaves some methods to be implemented by its concrete subclasses.