Java Enhanced For Loop - Editing Original Array Values

Question

I'd like to know, in detail, how the Enhanced For Loop works in Java (assuming i do get how the basic usage of this loop is and how it works in general).

Given the following code:

String[] a = {"dog", "cat", "turtle"};

for (String s : a) {
  out.println("String: " + s);
  s = in.readLine("New String? ");
}

It doesn't actually modify the original list 'a'. Why not? How memory Management works? Isn't 's' a reference to the same memory cell of 'a[i]'?

I read on the oracle documentation that enhanced for loops can't be used to remove elements from the original array, it makes sense. Is it the same for modifying values?

Thanks in advance

String is immutable.. http://stackoverflow.com/a/8798424/112500 — Shashi, Jan 03 '13 at 10:43
`s` is a variable that contains a reference to the element of the container. — Roman C, Jan 03 '13 at 11:19

Oliver Charlesworth · Accepted Answer · 2013-01-03T11:04:15.303

11

Isn't 's' a reference to the same memory cell of 'a[i]'?

Originally, yes. But then in.readLine produces a reference to a new String object, which you then use to overwrite s. But only s is overwritten, not the underlying string, nor the reference in the array.

edited Jan 03 '13 at 11:04

answered Jan 03 '13 at 10:43

Oliver Charlesworth

267,707
33
569
680

1

@Oliver, Do you mean that the reference s starts refering to some other string object rather than a[i] when we so s = new String ("X")? – Abhinav Sep 19 '15 at 17:09

Henry · Answer 2 · 2013-01-03T11:28:06.253

5

s is a local variable that points to the String instance. It is not associated with a[i], they just happen to have the same value initially.

edited Jan 03 '13 at 11:28

answered Jan 03 '13 at 10:43

Henry

42,982
7
68
84

score 3 · Answer 3 · answered Jan 03 '13 at 10:50

3

You can only write

for (int i = 0; i < a.length; i++) {
  out.println("String: " + a[i]);
  a[i] = in.readLine("New String? ");
}

You can't use for-each loops to modify the original collection or array.

answered Jan 03 '13 at 10:50

Peter Lawrey

525,659
79
751
1,130

You made clear the last point that was still blurry. Thanks a lot! – jnardiello Jan 03 '13 at 11:06
1

If you want to remove() or change entries for a List, you have to use an Iterator explicitly. – Peter Lawrey Jan 03 '13 at 11:13

Averroes · Answer 4 · 2013-01-03T11:07:04.737

1

Think in s like an address to an object. The thing here is that s is pointing out to a certain value of the array when using the for loop. When you reassing s inside the loop is just happen that s points out to another value but the original value of the array is not modified as you are only changing the address s is pointing to.

String[] a = {"dog", "cat", "turtle"};

for (String s : a) {
  //s --> "dog"
  out.println("String: " + s);
  s = in.readLine("New String? ");
  //s --> the new string the user inputs
}

edited Jan 03 '13 at 11:07

answered Jan 03 '13 at 10:53

Averroes

4,168
6
50
63

1

Thanks a lot for the detailed answer. What i was missing is the fact that the readLine is returning a new address. – jnardiello Jan 03 '13 at 11:04
If your question is answered feel free to upvote and accept an answer ;) – Averroes Jan 03 '13 at 11:11

score 1 · Answer 5 · answered Jan 03 '13 at 11:27

1

For every iteration String s initially references to corresponding String object in String a[]. But it is then referenced to another String object that is returned by in.readLine().

answered Jan 03 '13 at 11:27

Xavier DSouza

2,861
7
29
40

Java Enhanced For Loop - Editing Original Array Values

5 Answers5

Linked