The output shows that the instance initialization clause is executed before either one of the constructors.

// housekeeping/Mugs.java
// (c)2017 MindView LLC: see Copyright.txt
// We make no guarantees that this code is fit for any purpose.
// Visit http://OnJava8.com for more book information.
// Instance initialization

class Mug {
  Mug(int marker) {
    System.out.println("Mug(" + marker + ")");
  }
}

public class Mugs {
  Mug mug1;
  Mug mug2;

  { // [1]
    mug1 = new Mug(1);
    mug2 = new Mug(2);
    System.out.println("mug1 & mug2 initialized");
  }

  Mugs() {
    System.out.println("Mugs()");
  }

  Mugs(int i) {
    System.out.println("Mugs(int)");
  }

  public static void main(String[] args) {
    System.out.println("Inside main()");
    new Mugs();
    System.out.println("new Mugs() completed");
    new Mugs(1);
    System.out.println("new Mugs(1) completed");
  }
}
/* Output:
Inside main()

Mug(1)
Mug(2)
mug1 & mug2 initialized

Mugs()
new Mugs() completed

Mug(1)
Mug(2)
mug1 & mug2 initialized

Mugs(int)
new Mugs(1) completed
*/
 

ps: blank lines in comments are used to highlight constructor initialize oder.

see static explicit data initialization

references:

1. On Java 8 - Bruce Eckel

2. https://github.com/wangbingfeng/OnJava8-Examples/blob/master/housekeeping/Mugs.java