Object Initialisation Order

Key points

• Static blocks and static fields run once, when the class is first loaded, parent before child.
• Then per object: the parent chain runs fully, then the child field initialisers and instance blocks in text order.
• The child constructor body is always last. That is why a parent constructor sees child fields still unset.
• A field with no initialiser is already zeroed: 0, false, or null. Never garbage.
• Static blocks are for expensive one-time setup. Anything that can throw there gives you an ExceptionInInitializerError.
• Instance blocks are rare in real code. Prefer putting the work in the constructor where readers expect it.

Example

public class Main {
    static class Base {
        static { System.out.println("1. Base static block   (once, at class load)"); }
        { System.out.println("3. Base instance block"); }
        Base() { System.out.println("4. Base constructor"); }
    }

    static class Child extends Base {
        static { System.out.println("2. Child static block  (once, at class load)"); }

        private final String field = trace("5. Child field initialiser");
        { System.out.println("6. Child instance block"); }

        Child() { System.out.println("7. Child constructor -> " + field); }

        static String trace(String s) { System.out.println(s); return "ready"; }
    }

    public static void main(String[] args) {
        System.out.println("--- first new Child() ---");
        new Child();
        System.out.println("--- second new Child() ---");
        new Child();                     // static blocks do NOT run again
    }
}

Statics load once; per object the parent finishes before the child body starts.