继承构造器答疑

class X {
Y b = new Y();

X() {
System.out.print("X");
}
}

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

public class Z extends X {
Y y = new Y();

Z() {
System.out.print("Z");
}

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