Careeroza

Initialization Order

What Runs When

When you write new Child(), Java runs several pieces of code in a fixed order. Knowing this order helps you debug strange null values and is a common interview question.

  1. Static blocks and static field initializers (once per class, parent class first).
  2. Instance field initializers and instance initializer blocks { ... }.
  3. The constructor body.

See the Order

Example

public class Main {
    public static void main(String[] args) {
        System.out.println("-- first object --");
        new Demo();
        System.out.println("-- second object --");
        new Demo();
    }
}

class Demo {
    static int s = log("static field");
    int x = log("instance field");

    static {
        log("static block");
    }

    {
        log("instance block");
    }

    Demo() {
        log("constructor");
    }

    static int log(String msg) {
        System.out.println(msg);
        return 0;
    }
}

Output

-- first object --
static field
static block
instance field
instance block
constructor
-- second object --
instance field
instance block
constructor

Note: Static parts run only for the first object. Instance parts run for every object, and the constructor body always runs last.