Java 1.5 Tiger A Developers Notebook [Electronic resources] نسخه متنی

7.6 Determining List Position and Variable Value

for/in, more than anything else, is



about convenience. However, with
that convenience comes a degree of lost flexibility. One such example is
the inability to determine the position in a list that the for/in construct
resides at. As your code executes within the for/in loop, there is no way
to access the position in the list. Additionally, the list variable itself isn't
accessible, making its access equally impossible.

7.6.1 How do I do that?

In short, you don't. Sometimes it's just as important to realize what you
can't do as it is to learn what you can.

A common iteration technique is to use the loop variable, especially if it's
numerical, in the loop body itself:

List<String> wordList = new LinkedList<String>( );
for (int i=0; i<args.length; i++) {
wordList.add("word " + (i+1) + ": '" + args[i] + "'");
}

This is perfectly legitimate, and really useful if you're performing some
sort of count. However, it's impossible to access the iterator in a for/in
loop, as that's kept internal (and not even generated until compilation
takes place). In this case, you're out of luck. You can use for/in to display
the results of a situation like this, but not to make the assignment
itself:

public void determineListPosition(PrintStream out, String[] args)
throws IOException {
List<String> wordList = new LinkedList<String>( );
for (int i=0; i<args.length; i++) {
wordList.add("word " + (i+1) + ": '" + args[i] + "'");
}
for (String word : wordList) {
out.println(word);
}
}

This is hardly a severe limitation, but it's one you should be aware of.

Another common usage of lists is String concatenation,
and that illustrates
another of the for/in limitations. It's common in String concatenation
to add separators between all but the last of a set of words, such
as when printing a list. This separator is often a comma or perhaps a
space:

StringBuffer longList = new StringBuffer( );
for (int i = 0, len=wordList.size( ); i < len; i++) {
if (i < (len-1)) {
longList.append(wordList.get(i))
.append(", ");
} else {
longList.append(wordList.get(i));
}
}
out.println(longList);

Here, all but the last word in the list has a comma appended to it, while
the last one is appended without a comma. This makes for a nice list output.
However, this rather simple task is impossible with for/in, because
the variable that is used to do all the work, i, is inaccessible in a for/in
loop. Again, a fairly minor inconvenience, but it's certainly not time to
retire our old friend for just yet.