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

If you look at the decompiled version of the class file

class X {
    Y b;

    X() {
        b = new Y();
        System.out.print("X");
    }
}

class Y {
    Y() {
        System.out.print("Y");
    }
}

public class Z extends X {

    Y y;

    Z() {
        y = new Y();
        System.out.print("Z");
    }

    public static void main(String args[]) {
        new Z();
    }
}

You can find that the instance variable y is moved inside the constructor, so the execution sequence is as follows

1. Call the constructor of Z
2. It triggers the default constructor of X
3. First line of X constructor new Y() is called.
4. Print Y
5. Print X
6. Call the first line in constructor Z new Y()
7. Print Y
8. Print Z

All the instance variables are initialized by using constructor statements.