Sealed Classes

Question 1

How do we define sealed classes?

Answer 1

public sealed class Name permits SomeClass {}

Each class that is permitted must extend class that permits it:

public final class SomeClass extends Name {}

Instead of “final”, there can be “sealed” and “non-sealed”

Both classes need to be in the same package.

Question 2

What is the purpose of “final” in classes that extend some sealed class?

Answer 2

It is there to block that class of being extended.

Question 3

What is the purpose of “non-sealed” in classes that extend some sealed class?

Answer 3

It opens sealed classes to other classes.

Question 4

When can “permits” be omitted?

Answer 4

When class that would be permitted is in the same java file, nested class or in same named module.

Question 5

How do we define sealed interfaces?

Answer 5

All rules apply as to sealed classes, but difference is that interfaces can permit other interfaces and classes

Question 6

Can sealed classes permit interfaces?

Answer 6

No, because interfaces don’t implement classes.