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 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 a = new ArrayList();
for (Iterator 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 
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 a = new ArrayList();
for (Integer integer : a) {
  integer.toString();
}
ALOAD 1
INVOKEINTERFACE java/util/List.iterator()Ljava/util/Iterator;
ASTORE 3
...
Iterator
List a = new ArrayList();
for (Iterator 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.