What is the difference between abstract classes and interfaces in Java?
In Java, abstract classes and interfaces are two distinct concepts that have the following differences:
- An abstract class is a class that can contain abstract methods, which are methods declared but not implemented within the class. Subclasses must implement these abstract methods in order to be instantiated. Interfaces are a completely abstract type where methods are only declared, not implemented.
- A class can inherit from another abstract class and must implement the abstract methods in the abstract class. A class can implement multiple interfaces, but interfaces themselves cannot implement any methods.
- Abstract classes can have both non-abstract methods and fields, while interfaces can only contain constants and abstract methods.
- Interfaces can be implemented by multiple classes, achieving polymorphism, whereas abstract classes can only inherit one superclass.
- Interfaces can be used to define types, while abstract classes cannot.
In general, abstract classes are used to represent an “is a” relationship, while interfaces are used to represent a “can do” relationship. When designing, it is important to choose between abstract classes and interfaces based on the specific situation.