Is There a Performance Difference Between Using a For-Each Loop and an Iterator for Collection Traversal in Java?

For Each Loop vs. Iterator: Efficiency in Collection Traversal

Introduction

When traversing a collection in Java, the choice arises between using a for-each loop and an iterator. This article explores the efficiency differences between these two approaches.

For-Each Loop

Introduced in Java 5, the for-each loop (also known as the enhanced for loop) is a concise and easy-to-read syntax for iterating over collections. It utilizes the iterator internally:

List<Integer> a = new ArrayList<>();
for (Integer integer : a) {
  integer.toString();
}

Iterator

An iterator provides a standardized interface for iterating over collections. It allows custom control over the traversal process.

List<Integer> a = new ArrayList<>();
for (Iterator<Integer> iterator = a.iterator(); iterator.hasNext();) {
   Integer integer = iterator.next();
   integer.toString();
}

Efficiency Comparison

For simple read-only access to a collection, there is virtually no performance difference between the for-each loop and the iterator approach. The for-each loop internally uses the iterator mechanism.

However, if using the traditional "c-style" loop with get(i):

for (int i = 0; i < list.size(); i++) {
   Object o = list.get(i);
}

The iterator approach (both for-each loop and explicit iterator) can be significantly more efficient for certain data structures. For example, in a linked list, get(i) is an O(n) operation. Using an iterator's O(1) next() operation ensures the loop runs in O(n) time, whereas the get(i) loop would run in O(n²) time.

Bytecode Comparison

To confirm that the for-each loop uses the iterator mechanism, we can compare the generated bytecode for both approaches:

For-Each Loop

List<Integer> a = new ArrayList<>();
for (Integer integer : a) {
  integer.toString();
}

ALOAD 1
INVOKEINTERFACE java/util/List.iterator()Ljava/util/Iterator;
ASTORE 3
...

Iterator

List<Integer> a = new ArrayList<>();
for (Iterator<Integer> iterator = a.iterator(); iterator.hasNext();) {
  Integer integer = iterator.next();
  integer.toString();
}

ALOAD 1
INVOKEINTERFACE java/util/List.iterator()Ljava/util/Iterator;
ASTORE 2
...

As you can observe, the generated bytecode is identical, indicating no performance penalty in using either approach.

Conclusion

In summary, when traversing a collection for read-only access, the difference between a for-each loop and an iterator is minimal. The iterator only offers advantages when working with specific data structures or requiring custom iteration behavior. For simplicity and readability, the for-each loop is generally the preferred choice.

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