Java 17
Sealed Classes
A way to put organization domain concepts into code.
With Sealed classes you define who can extend them using sealed to define the list of classes that can extends the current class, and non-sealed to permit extend any child class from a parent sealed class.
Sealed inheritance
Let’s create a parent abstract class named Animal.
package org.acme;
public abstract class Animal {
}Then create the first sub-class of Animal named Reptile sealed to Lizard and Turtle which means that only Lizard and Turtle classes can extend Reptile class.
package org.acme;
public abstract sealed class Reptile permits Lizard, Turtle {
}The Lizard class is a final class, there cannot be inheritance from Lizard:
package org.acme;
public final class Lizard extends Reptile {
}The Turtle class is a none sealed class which means that it’s not closed and you can extend it with no restrictions.
package org.acme;
public non-sealed class Turtle extends Reptile {
}Inner Sealed Classes
When you define an inner class and extending from the outter a sealed class, then permits is not necessary as it’s implecitly set.
Create an Amphibian class containing an inner class:
package org.acme;
public sealed class Amphibian {
public final class Frog extends Amphibian {
}
}