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The java.util.Collections interface's documentation [API 2006] warns about the consequences of failing to synchronize on an accessible collection object when iterating over its view:

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This noncompliant code example creates a HashMap object and two view objects: a synchronized view of an empty HashMap encapsulated by the mapView field and a set view of the map's keys encapsulated by the setView field. This example synchronizes on setView [Tutorials 2008].

private final Map<Integer, String> mapView =
    Collections.synchronizedMap(new HashMap<Integer, String>());
private final Set<Integer> setView = mapView.keySet();

public Map<Integer, String> getMap() {
  return mapView;
}

public void doSomething() {
  synchronized (setView) {  // Incorrectly synchronizes on setView
    for (Integer k : setView) {
      // ...
    }
  }
}

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