javademos/jep513/src/OverridableCall.java

// The classic superclass-constructor trap, and the fix JEP 513 makes possible.
public class OverridableCall {
    static class Base {
        Base() {
            IO.println("  Base constructor calls describe(): " + describe());
        }
        String describe() { return "base"; }
    }

    // The bug: describe() runs from Base's constructor, before Broken's own field has been assigned.
    static class Broken extends Base {
        private final String label;
        Broken(String label) {
            super();
            this.label = label;
        }
        @Override String describe() { return "label=" + label; }
    }

    // The fix: assign the field in the prologue, before super() runs Base's constructor.
    static class Fixed extends Base {
        private final String label;
        Fixed(String label) {
            this.label = label;   // allowed in the prologue: a plain assignment to a field of THIS class
            super();
        }
        @Override String describe() { return "label=" + label; }
    }

    public static void main(String[] args) {
        IO.println("Broken (assign after super()):");
        new Broken("hello");
        IO.println("Fixed (assign before super()):");
        new Fixed("hello");
    }
}