Change the reference in a Java List

s is a String variable, and assigning to it doesn't affect the list.

In order to change the list, call list.set(1,"QQQ");, which would put a new String as the second element of the list.

Let's dig a bit into how the enhanced for loop works on the background.

for (String s : list) { .. }

is actually translated to:

for (Iterator<String> iter = list.iterator(); iter.hasNext();) {
    String s = iter.next();
    ..
    s = "QQQ";
}

As you can see, passing a new reference to the s variable, just changes it for the scope of the loop, which doesn't have any effect on the list entry.

If you need to modify the current element that's being handled by the iterator, working with ListIterator makes it very easy:

for (ListIterator<String> iter = list.listIterator(); iter.hasNext();) {
    String s = iter.next();
    ..
    iter.set("QQQ");
}

Note that before iter.set(...), we need to call iter.next().

You could make a temporary list.

private static List<String> list = new ArrayList<String>();

public static void main(String[] args) {
    list.add("AAA");
    list.add("BBB");
    list.add("CCC");

    List<String> templist = new ArrayList<String>();   

    for(String s : list){
       if(s.equals("BBB")) s = "QQQ";

       templist.add(s);
    }
    list = templist;

    System.out.println(list);
}