How to observe the mutation of a final field before its initialization.

NullMess.java

class A {
    public A() {
        IO.println(message());
    }
    public String message() {
        return "Hello World!";
    }
}

class B extends A {
    private final String message;
    public B(String message) {
        this.message = message;
    }
    public String message() {
        return this.message;
    }
}

void main() {
    var b = new B("Bonjour le monde !");
}

This problem is fixed by adding an explicit call to super() on line 14, after the initialization of message (Java 25 and later).