Difference Between Abstract Classes and Interfaces In Java - PlanetLearning

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Difference Between Abstract Classes and Interfaces In Java

Difference Between Abstract Class and Interface

Feature	Abstract Class	Interface
Instantiation	Cannot be instantiated directly (only through subclassing).	Cannot be instantiated (must be implemented by a class).
Methods	Can have both abstract and concrete methods.	- Prior to Java 8: Can only have abstract methods. - From Java 8: Can have default and static methods. - From Java 9: Can have private methods for internal use within the interface. By default, methods are public and abstract unless otherwise specified.
Variables	Can have instance variables (need not be `public`, `static`, or `final`).	By default, all variables are `public static final`. Must be initialized at the time of declaration.
Fields	Can have instance variables, just like a regular class.	Can only have constants (`static final` fields).
Constructors	Can have constructors (used to initialize instance variables).	Cannot have constructors (as they cannot instantiate objects).
Multiple Inheritance	Can extend only one class (abstract or concrete).	Can implement multiple interfaces (a class can implement many interfaces).
Inheritance	Use the `extends` keyword for subclassing.	Use the `implements` keyword for implementing an interface.
Default Behavior	Can provide default behavior for subclasses using concrete methods.	Cannot provide default behavior prior to Java 8. From Java 8, interfaces can provide default methods.
Access Modifiers	Methods can have any access modifier (`private`, `protected`, `public`).	Methods are implicitly public unless declared as `private` (Java 9+). Interfaces cannot have `protected` methods.
Object Creation	Can create reference variables and objects of the subclass.	Can only create reference variables, not objects. Cannot instantiate an interface.
Level of Sharability	Provides less sharability than interfaces.	Provides more sharability. Interfaces allow a class to inherit behaviors from multiple sources, providing more flexibility in multiple inheritance.
Fields Initialization	Instance variables are not required to be initialized at the time of declaration.	All variables in an interface must be initialized at the time of declaration.
Modifiers Restrictions	No restrictions for method modifiers. Methods can be `private`, `protected`, `final`, `static`, `synchronized`, etc.	Interface methods cannot have modifiers like `private`, `protected`, `final`, `static`, `synchronized`, or `strictfp` (except from Java 9+, which allows `private` methods for reuse within the interface).
Usage Scenarios	Used when some default behavior or common state needs to be shared between classes.	Used for defining a contract (i.e., what the implementing class must do) without dictating how it should be done.