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:

  1. 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.
  2. 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.
  3. Abstract classes can have both non-abstract methods and fields, while interfaces can only contain constants and abstract methods.
  4. Interfaces can be implemented by multiple classes, achieving polymorphism, whereas abstract classes can only inherit one superclass.
  5. 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.

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