ForEachTraversesRecursiveStructure

DRAFT

Correction

Here is what's right.

A for-each loop requires an array or an Iterable.

Iterable list = //...
for (Object element : list) {
  //...
}

While the built-in java.util.LinkedList class implements Iterable, and thus can be traversed directly with a for-each loop, a recursive data structure that is not a subtype of Iterable can’t directly be traversed this way.

Symptoms

How do you know your students might have this misconception?

Here, the student believes that the for-each loop will traverse all nodes making up a list:

class Node {
  int value;
  Node tail;
  //...
}

Node head = ...;
for (Node node : head) {
  sum += node.value;
}

The following example is similar, but the student believes that the for-each loop will process each value (payload) in the nodes making up a list:

class Node {
  int value;
  Node tail;
  //...
}

Node head = ...;
for (int value : head) {
  sum += value;
}