Initializing List in constructor or field declaration

Question

I was wondering whether there is a difference in initializing objects like ArrayList<> and stuff in field declaration or constructor.

Is there a difference in memory usage, performance or anything like that or is it completely the same?

Option 1:

class MyClass {
     private List<String> strings = new ArrayList<String>();
}

Option 2:

class MyClass {
    private List<String> strings;
    public MyClass() {
        strings = new ArrayList<String>();
    }
}

It may be a stupid question, or a very basic one, but I like to build from the start, I like to understand all that I see.

Answer 1

There is a difference: when initialization occurs. Fields are initialized first, then the constructor fires.

In your trivial example, there would be no practical difference, but if another field depended on the List field for initialization, the constructor version would explode with a NPE.

Consider:

 private List<String> strings = Arrays.asList("foo", "bar");
 private String stringsDescription = strings.toString();

If you moved initialization of strings to the constructor, the initialization of stringsDescription would explode with a NPE.