Order of field initialization

Question

Consider the following example:

public class Constructor
{
     Constructor(int i)
     {
            System.out.println(i);
     }
}

public class Test
{

       Constructor c1 = new Constructor(1);

       Constructor c2 = new Constructor(2);

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

This outputs:

1
2

Please explain why this happens and whether this behavior is consistent.

Answer 1

The initializers for static and instance fields are class are executed in the order that they appear in the source code.

In your example, the c1 declaration is before the c2 declaration, so it is executed first, and indicated by the output you see.

Why?

1. Because the JLS says so:

   "4) Execute the instance initializers and instance variable initializers for this class, assigning the values of instance variable initializers to the corresponding instance variables, in the left-to-right order in which they appear textually in the source code for the class."

   (If that doesn't make sense to you, refer to the JLS to read the sentence in its context.)

2. Because it makes sense to do it that way:

   • an unspecified order of initialization would be bad for code portability,
   • any other order (e.g. reverse order, lexical order) would be counter-intuitive.