public static void main(String[] args) {
        HashMap hashMap = new HashMap();
        hashMap.put("aa", 123);//添加"aa"
        hashMap.put("bb", 345);
        hashMap.put("cc", 89);
        hashMap.put("aa", 890);//修改"aa"
        System.out.println(hashMap);
        HashMap hashMap1 = new HashMap();
        hashMap1.put("AA", 123);
        hashMap1.put("BB", 456);
        hashMap.putAll(hashMap1);//添加另一个map中全部的元素
        System.out.println(hashMap);
        //遍历所有的key集
        Set set = hashMap.keySet();
        Iterator iterator = set.iterator();
        while (iterator.hasNext()){
            System.out.println(iterator.next());
        }
        //遍历所有的value集
        Collection values = hashMap.values();
        Iterator iterator1 = values.iterator();
        while (iterator1.hasNext()){
            System.out.println(iterator1.next());
        }
        //遍历所有key-value
        Set entrySet = hashMap.entrySet();
        Iterator iterator2 = entrySet.iterator();
        while (iterator2.hasNext()){
            Object obj = iterator2.next();
            Map.Entry entry = (Map.Entry) obj;
            System.out.println(entry.getKey()+"="+entry.getValue());
        }
        Object value = hashMap.remove("AA");//不存在返回null
        System.out.println(value);
        System.out.println(hashMap);
        System.out.println(hashMap.get("BB"));//不存在返回null
        boolean containsKey = hashMap.containsKey("BB");//判断map中是否存在Key
        boolean containsValue = hashMap.containsValue(456);//判断map中是否存在value
        System.out.println(containsKey);
        System.out.println(containsValue);
        hashMap.clear();//将map清空，不是变为null
        System.out.println(hashMap.size());
    }