Sealed classes and Interfaces in Java

 

   

• The above Test class is final . It means no other class can extend it. 

    

• When we write a class in Java, any other class  can extend. We can't add a condition to allow only some classes can extend our class. 
• We can now leverage the sealed classes feature to define list of classes which can extend our class.
• We can declare sealed class with sealed keyword and  We need specify list sub classes using permits keyword.

• Classes in permit clause must extend the sealed class otherwise we get compile time error.
• Classes which are not in permit clause try to extend Test class, we get compile time error.
• Sealed class's subclasses must be final or sealed or non-sealed.

• Test classes defined Test1, Test 2 as its direct sub classes. 
• We have declared Test1 as non-sealed . It means any number of classes can extend Test1. We no need to use final, sealed , non-sealed keywords in Test1 subclasses declarations. These are not subclasses for Test class.
• We have declared Test2 as sealed. All sealed class rules are applicable here.  
• We can't use non-sealed keyword for a class which doesn't extend any sealed class. 

• Sealed classes and its sub classes must be in same module. if the sealed class is in unnamed module, its sub classes also should  be in same unnamed module. Sub classes can be in different package.

Sealed Interfaces:

• Sealed interface rules are same like sealed classes.
• We use sealed keyword to declare sealed interface.
• We can specify list of classes which can implement sealed interface and list of interfaces which can extend sealed interface.

 

• Classes and interfaces which are in permits clause only must implement or extend sealed interface . Other classes and interfaces should not implement or extend the sealed interface.
• Implemented classes must be final or sealed or non-sealed. 
• Sub Interfaces must be either sealed or non-sealed.
• A class can extend a sealed class and implement a sealed interface.

Conclusion:

This feature is useful to define definite level of inheritance for our classes and interfaces. When our developing rest services , We can use this feature for interface to specify what all classes can implement it. It is mainly useful when we are developing some library or framework.

This is all about sealed classes and interfaces . If you have any questions, please comment below. Happy Coding 😀.