Difference Between Abstract Class and Interface In Java (With Table)

JAVA is an object-oriented programming language that uses various key features of object-oriented programming. Abstraction is one of the features that allows users to hide the complexities of implementations and provides simpler implementation techniques. In Java, it is attained by using either an interface or an abstract class.

Abstract class and Interface are related to abstraction but cannot be used interchangeably; hence, it is important to note when to use an interface and when to use an abstract class and choose the right option.

Abstract Class vs Interface

The main difference between abstract classes and interfaces is that a functional interface consists of only one abstract method apart from static and default methods. This feature can restrict the number of abstract methods to be declared, whereas it is never possible to restrict the number of abstract methods declared in abstract classes.

Abstract classes are simply classes that have been declared using the abstract keyword. It forces subclasses to implement all the declared methods and enables us to allow method signatures to be written using the abstract keyword. Assume that if a class has an abstract method, the class must be abstract as well.

Interfaces serve as blueprints for the class implementation. There are no concrete methods in interfaces, and all methods are abstract. An interface cannot be instantiated, but on the other hand, classes that implement interfaces can be instantiated. Instance variables are never used in interfaces. However, public static final variables can be used.

Comparison Table Between Abstract Class and Interface

What is Abstract Class?

The keyword ‘abstract’ is used to declare an abstract class. At least one abstract method should be present in abstract classes. It may have a variety of concrete methods. You can use abstract classes to generate blueprints for concrete classes. However, the abstract method must be implemented by the inheriting class. It is not possible to instantiate abstract classes.

The benefits of using abstract classes are:

• When attempting to utilize the inheritance concept in code, by giving common base class methods that the subclasses override.
• If one needs to have non-final or non-static methods to adjust the states of an object.
• Helps in code reusability.
• Fulfils specified requirements and provides fractional implementation details.

Abstraction in any programming language refers to hiding non-essential elements from the user so that they may focus on the important ones, thereby increasing efficiency and lowering complexity.

 An abstract class captures common characteristics of subclasses and may or may not contain any abstract method. It cannot be instantiated but can be only used as a superclass by its subclasses.

Syntax:

abstract classname {

    //Statements

}

What is Interface?

One of the fundamental concepts of Java is the interface. Java Interface is a fundamental component of the Java programming language, and it’s utilized extensively in the JDK and in Java design patterns. The majority of frameworks strongly rely on the Java interface. It is created using the keyword ‘interface.’

The benefits of using the interface are:

• Interfaces are used to achieve absolute abstraction.
• Because all interface properties are public, static, and final by default, we don’t need to add access modifiers, but if we do, the compiler won’t complain.
• Designed to support dynamic method resolution at run time.
• It is helpful when a problem needs to be solved using multiple inheritances and is composed of different class hierarchies.

It can have both methods and variables, just like a class. However, the methods declared in an interface are abstract by default.

Syntax:

interface name{

//methods

}

Main Differences Between Abstract Class and Interface In Java

1. The most important distinction between an abstract class and an interface is that because an interface lacks instance variables, it cannot have a state that non-abstract methods may alter.
2. Constructors may be specified within an abstract class but not within an interface in Java, another semantic distinction between an interface with default methods and an abstract class.
3. In the case of the Abstract class, one can take benefit of the default implementation, whereas in the interface, finding all the implementors and implementing is a nightmare.
4. Where abstract class defines the identity of a class, an interface helps to define the peripheral abilities.
5. Abstract classes can inherit a class and multiple interfaces, whereas interfaces can inherit multiple interfaces only, not classes.
6. Constructors and destructors can be defined in an abstract class but not an interface.
7. Another distinction between the abstract class and the interface is that the abstract class may be expanded with the ‘extends’ keyword. The keyword ‘implements’ can be used to implement an interface class.

Conclusion

In Java, the distinction between an abstract class and an interface is that an abstract class implements abstraction, whereas an interface implements both abstraction and multiple inheritances.

The two approaches to designing a class that contains certain, although not all, methods for the class to execute are abstract class and interface. These two are statically distinct and are attained in separate ways. However, in terms of dynamic usage, they are identical.

Abstract classes are analogous to interfaces in a few ways, i.e., users can’t instantiate. Secondly, they both might contain a set of methods declared and defined with or without their implementation.

References

1. https://link.springer.com/content/pdf/10.1007/978-1-4302-0140-3_12.pdf
2. https://ieeexplore.ieee.org/abstract/document/654728/
3. https://ieeexplore.ieee.org/abstract/document/7503306/