@Test public void testForEachKey() { final DoubleArrayList keys = new DoubleArrayList(); OpenDoubleObjectHashMap<TestClass> map = new OpenDoubleObjectHashMap<TestClass>(); map.put((double) 11, anotherItem); map.put((double) 12, anotherItem2); map.put((double) 13, anotherItem3); map.put((double) 14, anotherItem4); map.removeKey((double) 13); map.forEachKey( new DoubleProcedure() { @Override public boolean apply(double element) { keys.add(element); return true; } }); double[] keysArray = keys.toArray(new double[keys.size()]); Arrays.sort(keysArray); assertArrayEquals(new double[] {11, 12, 14}, keysArray, (double) 0.000001); }