Difference Between Abstract Class and Interface in Java

Abstraction is considered to be one of the most vital features of object-oriented programming. It helps to hide the implementational complexities by offering functionalities through some easy interfaces. For achieving abstraction in Java abstract class and interface are widely used. The application of abstract class and interface is different from each other.

Abstract Class vs Interface in Java

The main difference between abstract class and interface in java is that abstract class refers to a user-defined type that works as a blueprint to create an object. An abstract class has methods and properties that represent the object’s behaviors and states. On the other hand, the interface refers to a user-defined type that can have a set of method signatures and field constants overridden by classes implemented by the interface.

Abstract Class and Interface in Java

Abstract class refers to a class that includes the abstract keyword in its declaration. It helps to develop the blueprint for concrete classes. It should have a minimum of one abstract method, like a method that doesn’t have a body. An abstract class may have several numbers of concrete methods. It allows its users to reuse codes.

Interface refers to a blueprint that is used to implement a class. Any concrete methods or methods with code are not included in the interface. Only abstract methods or methods without any code are included in an interface. Instantiation of an interface is not possible, but interface classes implementing classes can be easily instantiated.

Comparison Table Between Abstract Class and Interface in Java

Parameters of Comparison Abstract Class in JavaInterface in Java
KeywordFor declaring the abstract class, the abstract keyword is used.     For declaring the interface, the interface keyword is used.
MethodsBoth non-abstract and abstract methods can be present in an abstract class.     Only the abstract method can be presented in the interface. It also can have static and default methods since Java 8.
VariablesAn abstract class can have variables like non-static, non-final, static, and final variables.     Only final and static variables can be there in an interface.
InheritanceMultiple inheritances aren’t supported by an abstract class.     Multiple inheritances are supported by the interface.
ExtensionAnother java can be extended and multiple java interfaces can be implemented by an abstract class.     Only another interface in Java has the opportunity to be extended by an interface.

What is Abstract Class in Java?

In Java, an abstract class is a class that is declared by the abstract keyword. Both the non-abstract and abstract methods can be included in an abstract class in Java. Generally, an abstract class refers to a template that is used to store the useful programming data methods and members. In Java instantiation of the abstract class is not possible directly. Instead of that, the abstract class can be subclassed. 

An abstract class provides the developers with an option of hiding the code implementation which helps them to execute the data abstraction process smoothly. It also saves time in the coding. An abstract method can be called wherever while coding and it also avoids the repetition of the code.

While using an abstract class in Java the most important rules are-

  • While declaring an abstract class it is very important to have the “abstract” keyword.
  • In Java, instantiation of the abstract class is not possible directly.Minimum one abstract method is required in an abstract class.
  • Final methods are also included in an abstract class.
  • Static and constructor methods can be included in an abstract class.

The main advantages of an abstract class in Java are that it is very helpful in writing short-length codes and it also omits the chances of code duplication. It also enables the reusability of code. 

What is Interface in Java?

Like an abstract class, an abstract in Java can have variables and methods. But in an interface, the declared methods are by default abstract. It specifies how and what an abstract class must do. It is considered as the blueprint of the abstract class. It has a collection of methods that class may not or may apply.

Interface in Java is used for achieving total abstraction. It also can have multiple inheritances which are not supported in abstract classes. An interface is also used for achieving loose coupling. Interfaces are widely used in design patterns of Java and can perform a function. The interface methods do not have anybody. It is a mechanism that is mainly used for achieving multiple inheritances and abstractions in Java.

The main properties of a Java interface are –

  • An interface in Java is completely abstract. Abstract keywords are not required while declaring an interface in Java.
  • Each of the interface methods is also completely abstract. As a result, there is no need to use abstract keywords while declaring interface methods.
  • Each interface method is completely public.

The main advantage of an interface in Java is that it can be used for achieving data abstraction and also can be used for supporting multi-inheritance functionalities in Java. It is also used in loose coupling.

Main Differences Between Abstract Class and Interface in Java

  1. Both non-abstract and abstract methods can be present in an abstract class. On the other hand, only abstract methods can be presented in the interface. It also can have static and default methods since Java 8.
  2. An abstract class can have non-static, static, non-final, and final variables. Only final and static variables can be there in an interface.
  3. For declaring the abstract class, the abstract keyword is used. For declaring the interface, the interface keyword is used.
  4. Multiple inheritances aren’t supported by an abstract class. Multiple inheritances, on the contrary, are supported by the interface.
  5. Another java can be extended and multiple java interfaces can be implemented by an abstract class but only another interface in Java has the opportunity to be extended by an interface.

Conclusion

An abstract class and an interface in Java are considered to be the main pillars of the Java Programming Language. Both abstract class and interface in Java are widely used for achieving abstraction where the abstract methods can be declared. The instantiation of the abstract class and interface in Java is not possible. 

The main difference between these two is the use of keywords. For the creation of abstract class abstract keyword is used, while for the creation of interface in Java the interface keyword is used. The best approach to designing a system is combining both abstract class and interface in Java.

References

  1. https://scholar.google.com/scholar?hl=en&as_sdt=0%2C5&q=abstract+class+in+java&btnG=#d=gs_qabs&u=%23p%3DW3WgwQb0NOAJ
  2. https://scholar.google.com/scholar?hl=en&as_sdt=0%2C5&q=interface+in+java&btnG=#d=gs_qabs&u=%23p%3Dzhwh3VcdqSkJ
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