Java – How ‘for each’ Loop Works in Detail

foreachjavasyntactic-sugar

Consider:

List<String> someList = new ArrayList<>();
// add "monkey", "donkey", "skeleton key" to someList

for (String item : someList) {
    System.out.println(item);
}

What would the equivalent for loop look like without using the for each syntax?

People new to Java commonly encounter issues when trying to modify the original data using the new style foreach loop. Use Why doesn't assigning to the iteration variable in a foreach loop change the underlying data? to close duplicates about that common problem. Note that other languages with analogous constructs generally have the same issue; for example, see Why doesn't modifying the iteration variable affect subsequent iterations? for the same issue in Python.

Best Answer

for (Iterator<String> i = someIterable.iterator(); i.hasNext();) {
    String item = i.next();
    System.out.println(item);
}

Note that if you need to use i.remove(); in your loop, or access the actual iterator in some way, you cannot use the for ( : ) idiom, since the actual iterator is merely inferred.

As was noted by Denis Bueno, this code works for any object that implements the Iterable interface.

If the right-hand side of the for (:) idiom is an array rather than an Iterable object, the internal code uses an int index counter and checks against array.length instead. See the Java Language Specification.

for (int i = 0; i < someArray.length; i++) {
    String item = someArray[i];
    System.out.println(item);
}
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