Difference between Abstract Class and Interface in Java
As we know that abstraction refers to hiding the internal implementation of the feature and only showing the functionality to the users. i.e. showing only the required features, and hiding how those features are implemented behind the scene. Whereas, an Interface is another way to achieve abstraction in java. Both abstract class and interface are used for abstraction, henceforth Interface and Abstract Class are required prerequisites.
Abstract class vs Interface
- Type of methods: Interface can have only abstract methods. Whereas, an abstract class can have abstract method and concrete methods. From Java 8, it can have default and static methods also. From Java 9, it can have private concrete methods as well.
- Note : Concrete methods are those methods which has their complete definition but they can also be overriden in the inherited class. However, if we make the concrete method as “FINAL” it cannot be overrided in the inherited class because declaring a method as final means – its implementation is complete.
- Final Variables: Variables declared in a Java interface are by default final. An abstract class can contain non-final variables.
- Type of variables: Abstract class can have final, non-final, static and non-static variables. The interface has only static and final variables.
- Implementation: Abstract class can provide the implementation of the interface. Interface can’t provide the implementation of an abstract class.
- Inheritance vs Abstraction: A Java interface can be implemented using the keyword “implements” and an abstract class can be extended using the keyword “extends”.
- Multiple implementations: An interface can extend one or more Java interfaces; an abstract class can extend another Java class and implement multiple Java interfaces.
- Multiple Inheritance: Multiple inheritance can be partially achieved by the use of interfaces , whereas the same can’t be done by the use of abstract classes. Because in Java, one class can implement multiple Interfaces, but one class cannot extend from multiple other classes because that’s just not possible in java as that would lead to the diamond problem.
- Accessibility of Data Members: Members of a Java interface are public by default. A Java abstract class can have class members like private, protected, etc.
Example 1 : (For Abstract Class )
avinash 21 222.2
Example 2 : (For Abstract Class )
Area of rectangle: 6.0 Rectangle has been moved to x = 1 and y = 2 Area of circle: 12.56 Circle has been moved to x = 2 and y = 4
What if we don’t have any common code between rectangle and circle then go with the interface.
Example 1 : (For Interface )
Area of rectangle: 6.0 Area of circle: 12.56
Example 2 : (For Interface )
When to use what?
Consider using abstract classes if any of these statements apply to your situation:
- In the java application, there are some related classes that need to share some lines of code then you can put these lines of code within the abstract class and this abstract class should be extended by all these related classes.
- You can define the non-static or non-final field(s) in the abstract class so that via a method you can access and modify the state of the object to which they belong.
- You can expect that the classes that extend an abstract class have many common methods or fields, or require access modifiers other than public (such as protected and private).
Consider using interfaces if any of these statements apply to your situation:
- It is a total abstraction, all methods declared within an interface must be implemented by the class(es) that implements this interface.
- A class can implement more than one interface. It is called multiple inheritances.
- You want to specify the behavior of a particular data type but are not concerned about who implements its behavior.
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