Creating an object in a static way

Question

Could anyone explain how Java executes this code? I mean the order of executing each statement.

public class Foo {     boolean flag = sFlag;     static Foo foo = new Foo();     static boolean sFlag = true;      public static void main(String[] args)     {         System.out.println(foo.flag);     } } 

OUTPUT:

false 

Answer 1

• Class initialization starts. Initially, foo is null and sFlag is false
• The first static variable initializer (foo) runs:
  • A new instance of Foo is created
  • The instance variable initializer for flag executes - currently sFlag is false, so the value of flag is false
• The second static variable initializer (sFlag) executes, setting the value to true
• Class initialization completes
• main runs, printing out foo.flag, which is false

Note that if sFlag were declared to be final it would be treated as a compile-time constant, at which point all references to it would basically be inlined to true, so foo.flag would be true too.

Answer 2

foo is instantiated during the static initialization of the class, and before sFlag was initialized, and the default value of a boolean is false.

1. The class is loaded

2. Foo is initialized to the instance

   2.a The instance member flag is initialized to the value of sFlag (false by default)

3. sFlag is initialized to true

Please refer to JLS §12.4 for more details.