Saturday, 7 July 2012

Item 46: Prefer for-each loops to traditional for loops

Prior to release 1.5, this was the preferred idiom for iterating over a collection:

// No longer the preferred idiom to iterate over a collection!
for (Iterator i = c.iterator(); i.hasNext(); ) {
doSomething((Element); // (No generics before 1.5)

This was the preferred idiom for iterating over an array:

// No longer the preferred idiom to iterate over an array!
for (int i = 0; i < a.length; i++) {

These idioms are better than while loops (Item 45), but they aren’t perfect. The iterator and the index variables are both just clutter. Furthermore, they represent opportunities for error. The iterator and the index variable occur three times in each loop, which gives you two chances to get them wrong. If you do, there is no guarantee that the compiler will catch the problem.

The for-each loop, introduced in release 1.5, gets rid of the clutter and the opportunity for error by hiding the iterator or index variable completely. The resulting idiom applies equally to collections and arrays:

// The preferred idiom for iterating over collections and arrays
for (Element e : elements) {

When you see the colon (:), read it as “in.” Thus, the loop above reads as “for each element e in elements.” Note that there is no performance penalty for using the for-each loop, even for arrays. In fact, it may offer a slight performance advantage over an ordinary for loop in some circumstances, as it computes the limit of the array index only once. While you can do this by hand (Item 45), programmers don’t always do so.

The advantages of the for-each loop over the traditional for loop are even greater when it comes to nested iteration over multiple collections.

// Can you spot the bug?
Collection<Suit> suits = Arrays.asList(Suit.values());
Collection<Rank> ranks = Arrays.asList(Rank.values());
List<Card> deck = new ArrayList<Card>();
for (Iterator<Suit> i = suits.iterator(); i.hasNext(); )
for (Iterator<Rank> j = ranks.iterator(); j.hasNext(); )
deck.add(new Card(,;

Don’t feel bad if you didn’t spot the bug. Many expert programmers have made this mistake at one time or another. The problem is that the next method is called too many times on the iterator for the outer collection (suits). It should be called from the outer loop, so that it is called once per suit, but instead it is called from the inner loop, so it is called once per card. After you run out of suits, the loop throws a NoSuchElementException.

If instead you use a nested for-each loop, the problem simply disappears. The resulting code is as succinct as you could wish for:

// Preferred idiom for nested iteration on collections and arrays
for (Suit suit : suits)
for (Rank rank : ranks)
deck.add(new Card(suit, rank));

Not only does the for-each loop let you iterate over collections and arrays, it lets you iterate over any object that implements the Iterable interface.

In summary, the for-each loop provides compelling advantages over the traditional for loop in clarity and bug prevention, with no performance penalty. You should use it wherever you can. Unfortunately, there are three common situations where you can’t use a for-each loop:

1. Filtering—If you need to traverse a collection and remove selected elements, then you need to use an explicit iterator so that you can call its remove method.

Transforming—If you need to traverse a list or array and replace some or all of the values of its elements, then you need the list iterator or array index in order to set the value of an element.

3. Parallel iteration—If you need to traverse multiple collections in parallel, then you need explicit control over the iterator or index variable, so that all iterators or index variables can be advanced in lockstep.

If you find yourself in any of these situations, use an ordinary for loop, be wary of the traps mentioned in this item, and know that you’re doing the best you can.

Reference: Effective Java 2nd Edition by Joshua Bloch