【HashMap】根据value获取key

阳光穿透心脏的1/2处 2022-12-10 14:35 351阅读 0赞

**Collections values = map.values();** `map.values()`返回HashMap的value视图集合

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80MzIzMjk1NQ_size_16_color_FFFFFF_t_70_pic_center]

### 获取该value对应的所有key ###

**keySet**

private static List<String> getToKeyI(Map<String, String> map, String value) { 
        List<String> keyList = new ArrayList<>();
        for (String key : map.keySet()) { 
            if (map.get(key).equals(value)) { 
                keyList.add(key);
            }
        }
        return keyList;
    }

**Entry**

Map.Entry<Object, Object> entry = iterator.next();

entry获取到的就是`key=value`，存入了Set中

> 1=a  
> 2=a

public static Object getToKeyII(Map<String, String> map, String value) { 
        //通过entrySet()方法把 map中的每个键值对变成对应成 Set集合中的一个 ket-value 对象
        Set set = map.entrySet();
        Iterator<Map.Entry<Object, Object>> iterator = set.iterator();
        ArrayList<Object> list = new ArrayList();
        while (iterator.hasNext()) { 
            //Map.Entry是一种类型，指向map中的一个键值对组成的对象
            Map.Entry<Object, Object> entry = iterator.next();
            if (entry.getValue().equals(value)) { 
                list.add(entry.getKey());
            }
        }
        return list;
    }

### 重复value只去一个key ###

private static String getToAllKey(Map<String, String> map, String value) { 
        String key = null;
        for (String getKey : map.keySet()) { 
            if (map.get(getKey).equals(value)) { 
                key = getKey;
            }
        }
        return key;
    }

## 完整代码 ##

public class ValueToKey { 
        /** * @param value * @return */
        private static List<String> getToKeyI(Map<String, String> map, String value) { 
            List<String> keyList = new ArrayList<>();
            for (String key : map.keySet()) { 
                if (map.get(key).equals(value)) { 
                    keyList.add(key);
                }
            }
            return keyList;
        }
    
        public static Object getToKeyII(Map<String, String> map, String value) { 
            //通过entrySet()方法把 map中的每个键值对变成对应成 Set集合中的一个 ket-value 对象
            Set set = map.entrySet();
            Iterator<Map.Entry<Object, Object>> iterator = set.iterator();
            ArrayList<Object> list = new ArrayList();
            while (iterator.hasNext()) { 
                //Map.Entry是一种类型，指向map中的一个键值对组成的对象
                Map.Entry<Object, Object> entry = iterator.next();
                if (entry.getValue().equals(value)) { 
                    list.add(entry.getKey());
                }
            }
            return list;
        }
    
        private static String getToAllKey(Map<String, String> map, String value) { 
            String key = null;
            for (String getKey : map.keySet()) { 
                if (map.get(getKey).equals(value)) { 
                    key = getKey;
                }
            }
            return key;
        }
    
    
        public static void main(String[] args) { 
            Map<String, String> map = new HashMap<>();
            map.put("1", "a");
            map.put("2", "a");
            map.put("3", "b");
            map.put("4", "b");
            map.put("5", "c");
    
            //获取 value = "a"的key
            System.out.println("【keySet】根据value获取所有key(相同value的key均会获取) " + getToKeyI(map, "a"));
            System.out.println("【Entry】" + getToKeyII(map, "b"));
    
            Collection<String> values = map.values();
            Set<String> setRes = new HashSet<>();
            for (String value : values) { 
                setRes.add(getToAllKey(map, value));
            }
            System.out.println("根据value获取所有key集合(相同value的key只存一个) " + setRes);
        }
    }

[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80MzIzMjk1NQ_size_16_color_FFFFFF_t_70_pic_center]: /images/20221123/8701bc4ef7774fc58015476d0ac11556.png