Another common task in loop iteration, particularly over collections, is inloop modification. This is all done through the iterator, as directly modifying the collection creates all sorts of nasty loop problems. Instead, using methods such as remove( ) from java.util.Iterator allows modifications in a loop-safe way. However, since the iterator is hidden in for/in, this is another limitation of for/in.
This is another one of those "You can't" recipes. To see exactly what it is you can't do, take a look at the following code:
public void removeListItems(PrintStream out, String[] args)
throws IOException {
List<String> wordList = new LinkedList<String>( );
// Assign some words
for (int i=0; i<args.length; i++) {
wordList.add("word " + (i+1) + ": '" + args[i] + "'");
}
// Remove all words with "1" in them. Impossible with for/in
for (Iterator i = wordList.iterator( ); i.hasNext( ); ) {
String word = (String)i.next( );
if (word.indexOf("1") != -1) {
i.remove( );
}
}
// You can print the words using for/in
for (String word : wordList) {
out.println(word);
}
}
In particular, notice the second for loop, which uses remove( ) to yank any words in the List with "1" in the text. This depends on the usage of Iterator, which isn't available in for/in, so you're out of luck here.
...anything else that involves knowing where you are in the list? Nope it's not possible. Here are just a few examples of other things you can't do with for/in:
Iterate backward through the elements of an array or List.
Use a single loop counter to access similarly numbered or positioned elements in two distinct arrays or collections.
Iterate through the elements of a List using calls to get( ) rather than calls to its iterator.
Watch out for this last oneit is a legitimate performance concern. For the java.util.ArrayList class, for example, looping with the size( ) and get( ) methods is measurably faster than using the list's Iterator. In many cases, the performance difference is negligible and irrelevant. But, when writing an inner loop or other performance-critical code, you might prefer to use a for loop with the get( ) method instead of a for/in loop.