// The three classic ways to hold a value that is expensive to build. Plain Java, no preview features.
public class OldWays {
    static class Expensive {
        final String how;
        Expensive(String how) { this.how = how; System.out.println("  [init] building Expensive via " + how); }
    }

    // 1. Eager: built when OldWays is initialised, which is before main runs, whether or not anyone uses it.
    static final Expensive EAGER = new Expensive("eager static final");

    // 2. Holder idiom: the JVM initialises Holder the first time it is touched, and does so exactly once.
    static class Holder { static final Expensive VALUE = new Expensive("holder idiom"); }
    static Expensive holder() { return Holder.VALUE; }

    // 3. Double-checked locking: correct only because the field is volatile.
    private static volatile Expensive dcl;
    static Expensive doubleChecked() {
        Expensive local = dcl;
        if (local == null) {
            synchronized (OldWays.class) {
                local = dcl;
                if (local == null) dcl = local = new Expensive("double-checked locking");
            }
        }
        return local;
    }

    public static void main(String[] args) {
        System.out.println("main starts (the eager value already exists)");
        System.out.println("first holder() call:");   holder();
        System.out.println("second holder() call:");  holder();
        System.out.println("first doubleChecked() call:");  doubleChecked();
        System.out.println("second doubleChecked() call:"); doubleChecked();
    }
}