HashMap遍历和notice

Iterator<Map.Entry<String, Integer>> entryIterator = map.entrySet().iterator();
        while (entryIterator.hasNext()) {
            Map.Entry<String, Integer> next = entryIterator.next();
            System.out.println("key=" + next.getKey() + " value=" + next.getValue());
        }


Iterator<String> iterator = map.keySet().iterator();
        while (iterator.hasNext()){
            String key = iterator.next();
            System.out.println("key=" + key + " value=" + map.get(key));

        }

map.forEach((key,value)->{
    System.out.println("key=" + key + " value=" + value);
});

所以 HashMap 只能在单线程中使用，并且尽量的预设容量，尽可能的减少扩容。