Abstract Class vs Interface in Java: Difference and Comparison

Abstraction is considered to be one of the most vital features of object-oriented programming. It helps hide the implementational complexities by offering functionalities through accessible interfaces.

For achieving abstraction in Java, abstract classes and interfaces are widely used. The application of abstract class and the interface is different from each other.

Key Takeaways

1. Abstract classes allow partial implementation, whereas interfaces provide no implementation.
2. A class can inherit only one abstract class but implement multiple interfaces.
3. Abstract classes can have constructors, fields, and non-abstract methods, while interfaces cannot.

Abstract Class vs Interface in Java

The difference between abstract class and interface in JJava 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 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 impossible, but interface classes implementing classes can be easily instantiated.

Comparison Table

Parameters of Comparison	Abstract Class in Java	Interface in Java
Keyword	For declaring the abstract class, the abstract keyword is used.	For declaring the interface, the interface keyword is used.
Methods	Both 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.
Variables	An abstract class doesn’t support multiple inheritances.	Only final and static variables can be there in an interface.
Inheritance	The interface supports multiple inheritances.	Another java can be extended and an abstract class can implement multiple java interfaces.
Extension	Only another interface in Java can be extended by an interface.	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 used to store 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 allows the developers to hide the code implementation,, which helps them execute the data abstraction process smoothly.

It also saves time in the coding. An abstract method can be called wherever while coding and avoids the code’s repetition.

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

• While declaring an abstract class, having the “abstract” keyword is very important.

• In Java, instantiation of the abstract class is not possible directly. A 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 Java design patterns 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. An abstract class doesn’t support multiple inheritances. 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. Still, only another interface in Java has the opportunity to be extended by an interface.

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