Coding in the small with Google Collections: Iterables.concat()

Part 7 in a Series.

Iterables.concat() combines multiple iterables (such as ArrayList and HashSet) so you can go through multiple collections' elements in a single pass:

Before:

  public boolean orderContains(Product product) {
    List&lt;LineItem&gt; allLineItems = new ArrayList&lt;LineItem&gt;();
    allLineItems.addAll(getPurchasedItems());
    allLineItems.addAll(getFreeItems());

    for (LineItem lineItem : allLineItems) {
      if (lineItem.getProduct() == product) {
        return true;
      }
    }

    return false;
  }`</pre>

### After:
<pre class="prettyprint">`  public boolean orderContains(Product product) {
    for (LineItem lineItem : Iterables.concat(getPurchasedItems(), getFreeItems())) {
      if (lineItem.getProduct() == product) {
        return true;
      }
    }

    return false;
  }

If ever you only have the Iterator and not the Iterable, the equivalent method is Iterators.concat. As a special treat, both concat methods are optimized so that they don't need a private collection.

Part 8