Java foreach loop: for (Integer i : list) { ... }

Question

When I use JDK5 like below

ArrayList<Integer> list = new ArrayList<Integer>();  
     for (Integer i : list) { 

      //cannot check if already reached last item

   }

on the other hand if I just use an Iterator

ArrayList<Integer> list = new ArrayList<Integer>();
  for (Iterator i = list.iterator(); i.hasNext();) {

          //i can check whether this is last item
          if(i.hasNextItem()){
          }

    }

How can I check if I've already reached last item with for (Integer i : list) {

Solution 1

One way to do that is to use a counter:

ArrayList<Integer> list = new ArrayList<Integer>();
...
int size = list.size();
for (Integer i : list) { 
    ...
    if (--size == 0) {
        // Last item.
        ...
    }
}

Edit

Anyway, as Tom Hawtin said, it is sometimes better to use the "old" syntax when you need to get the current index information, by using a for loop or the iterator, as everything you win when using the Java5 syntax will be lost in the loop itself...

for (int i = 0; i < list.size(); i++) {
    ...

    if (i == (list.size() - 1)) {
        // Last item...
    }
}

or

for (Iterator it = list.iterator(); it.hasNext(); ) {
    ...

    if (!it.hasNext()) {
        // Last item...
    }
}

Solution 2

Sometimes it's just better to use an iterator.

(Allegedly, "85%" of the requests for an index in the posh for loop is for implementing a String join method (which you can easily do without).)

Solution 3

The API does not support that directly. You can use the for(int i..) loop and count the elements or use subLists(0, size - 1) and handle the last element explicitly:

  if(x.isEmpty()) return;
  int last = x.size() - 1;
  for(Integer i : x.subList(0, last)) out.println(i);
  out.println("last " + x.get(last));

This is only useful if it does not introduce redundancy. It performs better than the counting version (after the subList overhead is amortized). (Just in case you cared after the boxing anyway).

Solution 4

Another way, you can use a pass-through object to capture the last value and then do something with it:

List<Integer> list = new ArrayList<Integer>();
Integer lastValue = null;
for (Integer i : list) {
    // do stuff
    lastValue = i;
}
// do stuff with last value

View more solutions

Comments

[Romain Linsolas]

I think he wants to do specific treatment inside the loop when he reaches the last item, not getting the last integer of the list. For that, you can also do list.get(list.size() - 1);

[Brian Agnew]

Shouldn't that be !it.hasNext() to determine the last entry ?

[Brett]

And i == list.size()-1 in the other example (iterators are generally easier to get right than indexes, although the syntax is a bit long-winded).

[Brett]

Nice, but you should add a check that the list has at least one element (or potentially, is not empty).

[phtrivier]

Or which you can do with apache commons StringUtils.join ;)

[Brett]

phtrivier: If you are already dependent upon Apache Commons Lang. I wouldn't introduce a dependency just for that.

[Esko]

I understand, however I'm showing here the general idea of capturing the last object. The question is a bit vague in that sense that he doesn't really say what he wants to do, just how he wants to do it.

[Alnitak]

+1 for the first example - using a decrementing counter is very efficient

[Thomas Jung]

@Tom - There's still a good old join hack that works with for expressions: String join = "x"; StringBuilder buf = new StringBuilder(); for(Integer i : x) buf.append(i).append(join); String result = buf.substring(0, buf.length() - join.length());

[Brett]

Thomas: I prefer: StringBuilder buff = new StringBuilder(); String sep = ""; for (Integer i : x) { buff.append(sep).append(i); sep = "x"; } String str = buff.toString();. No hacky substring.

[Adriaan Koster]

I also love that trick of changing the seperator after the first iteration. Simple and effective.

[Kevin Bourrillion]

FYI, String.join() seems a likely candidate to be in JDK 7.