Iteration

Now that you know several different ways to store a collection of data, the next question is: how do you visit every element inside one? This process is called iteration, and Java offers a few different tools for it.

The Enhanced For-Loop

The enhanced for-loop (sometimes called a "for-each" loop) was briefly introduced in the Control Flow section, but now that you have real collections to work with, it's worth revisiting. It automatically walks through every element of an array or collection, without needing to manually track an index.

ArrayList<Integer> scoredPieces = new ArrayList<Integer>();
scoredPieces.add(1);
scoredPieces.add(2);
scoredPieces.add(3);

for (int piece : scoredPieces) {
  System.out.println(piece);
}

This works for arrays, ArrayList, LinkedList, HashSet, and just about every other collection type in Java.

The Iterator Interface

The enhanced for-loop is convenient, but it has one major limitation: you can't safely add or remove elements from a collection while iterating over it with a for-each loop. Doing so throws a ConcurrentModificationException.

Show code

ArrayList<Integer> scoredPieces = new ArrayList<Integer>();
scoredPieces.add(1);
scoredPieces.add(2);
scoredPieces.add(3);

for (int piece : scoredPieces) {
  if (piece == 2) {
    scoredPieces.remove(Integer.valueOf(2)); // Throws ConcurrentModificationException
  }
}

To safely remove elements while iterating, Java provides the Iterator interface, which gives you direct, manual control over the traversal.

Iterator Formula

Iterator<[type]> [iterator name] = [collection name].iterator();

Show code

import java.util.Iterator;

ArrayList<Integer> scoredPieces = new ArrayList<Integer>();
scoredPieces.add(1);
scoredPieces.add(2);
scoredPieces.add(3);

Iterator<Integer> iterator = scoredPieces.iterator();

while (iterator.hasNext()) {
  int piece = iterator.next();
  if (piece == 2) {
    iterator.remove(); // Safely removes the current element
  }
}

Method	Description
hasNext()	Returns true if there are more elements to visit
next()	Returns the next element and advances the iterator
remove()	Safely removes the last element returned by next()

In fact, the enhanced for-loop is really just a shorthand for exactly this pattern, Java automatically creates an Iterator behind the scenes and calls hasNext() and next() for you. Understanding Iterator helps explain why the for-each loop has the limitations it does, and gives you an escape hatch for the cases where those limitations get in the way.