In Java, can a final field be initialized from a constructor helper?

Method #2 is your best option. The problem is that if you have an assignment in a private method there is nothing preventing other code in the class outside the constructor calling it, which would then create an issue with an attempted second assignment to the final field.

Java has no construct of a separate method that can only be called during construction.

For completeness, we can make a third option, where you assign the map at initialization and then have the helper method fill it:

 private final HashMap<String, String> myMap = new HashMap<String, String();

And then:

 MyConstructor (String someThingNecessary)
 {
    initializeMyMap(someThingNecessary);

    // other initialization stuff unrelated to myMap
 }


 // helper doesn't work since it can't modify a final member
 private void initializeMyMap(String someThingNecessary)
 {

     myMap.clear();
    myMap.put("blah","blahblah");
    // etc...
  }

And if you really want to be confusing you can use an initializer instead of a constructor, but you should not do that, so unless you really need to know, I won’t expand on that.