The release of Java SE 15 in Sept 2020 will introduce "sealed classes" as a preview feature. A sealed class is a class or interface which restricts which other classes or interfaces may extend it.

Kotlin developers can now use a Kotlin class as a record in Java by making it a data class and marking it with the @JvmRecord annotation. Sealed interfaces provide more control over inheritance.

The Java 2 SDK organizes its vast collection of classes and interfaces into a tree-like hierarchy of packages within packages, which is equivalent to directories within directories.