Finalized in Java 17 (JEP 409). This is the headline language feature of the Java 17 LTS release. Previous previews: Java 15 (JEP 360) and Java 16 (JEP 397). The Problem: Open Hierarchies Are Hard to Reason About In Java, any class can be extended by default. This openness is flexible but has costs. Consider a Shape interface used in a drawing application: // Java 11 — anyone can implement Shape public interface Shape { double area(); } At runtime, a Shape instance could be a Circle, a Rectangle, a Triangle, or anything else in the classpath.