Your informant was mistaken about initialisers running the constructor. That is done by the JVM when the new operator is used. You can find out how it is done in the Java Language Specification (I hope I quoted the right section).
Unlike in the rest of the class, the order of members is significant to initialisers. The field must be declared and definitely assigned before it is available to the initialiser. So your first example, where the initialiser preceded the field, is invalid.