android/guava/src/com/google/common/collect/Maps.java
static boolean containsKeyImpl(Map<?, ?> map, @NullableDecl Object key) {
    return Iterators.contains(keyIterator(map.entrySet().iterator()), key);
  }
static boolean containsKeyImpl(Map<?, ?> map, @NullableDecl Object key) {
    return Iterators.contains(keyIterator(map.entrySet().iterator()), key);
  }
An admittedly inefficient implementation of {@link Map#containsKey}.