Are fields initialized before constructor code is run in Java?

Question

Can anyone explain the output of following program? I thought constructors are initialized before instance variables. So I was expecting the output to be \"XZYY\".

Answer 1

To clarify the misconceptions with static - I will simply refer to this small piece of code:

public class Foo {
  { System.out.println("Instance Block 1"); }
  static { System.out.println("Static Block 1"); }
  public static final Foo FOO = new Foo();
  { System.out.println("Instance Block 2"); }
  static { System.out.println("Static Block 2 (Weird!!)"); }
  public Foo() { System.out.println("Constructor"); }
  static public void main(String p[]) {
    System.out.println("In Main");
    new Foo();
  }
}

Surprise is that the output is as follows:

Static Block 1
Instance Block 1
Instance Block 2
Constructor
Static Block 2 (Weird!!)
In Main
Instance Block 1
Instance Block 2
Constructor

Note that we have a static {} that is called after two instance {}. this happens because we throw in the constructor in the middle, interjecting execution order the first time the constructor is called.

Discovered this when I was working on this answer - https://stackoverflow.com/a/30837385/744133.

Basically we observe this to happen:

1. During the first time an object is initialized, Initialize current object for both static and instance initialization intermixed based on order of occurrence

2. For all next initializations, only do the instance initialization in the order of occurrence, as static initialization already happened.

I need to research how the mix of inheritance, and both explicit and implicit calls to super and this will affect this, and will update with findings. It would be likely similar to the other supplied answers, except that they got it wrong with the static initialization.