Encountered an IllegalStateException while traversing with Flat3Map.entrySet()

//代码示例
    public void testEntrySet() {
        final Flat3Map<Integer, String> map = new Flat3Map<>();
        map.put(1, "A");
        map.put(2, "B");
        map.put(3, "C");
        Iterator<Map.Entry<Integer, String>> it = map.entrySet().iterator();        Map.Entry<Integer, String> mapEntry1 = it.next();
        Map.Entry<Integer, String> mapEntry2 = it.next();
        Map.Entry<Integer, String> mapEntry3 = it.next();
        it.remove();        assertEquals(2, map.size());
    }