Get key from a HashMap using the value

Question

I want to get the key of a HashMap using the value   hashmap   new HashMap lt String  Object gt      haspmap put  one   100   haspmap put  two   200     Which means i want a function that will take the value 100 and will return the string one   It seems that there are a lot of questions here asking the same thing but they don t work for me   Maybe because i am new with java   How to do it

User · Answer

public class Class1   private String extref  MY    public String getExtref         return extref     public String setExtref String extref        return this extref   extref     public static void main String   args         Class1 obj new Class1        String value obj setExtref  AFF        int returnedValue getMethod value            System out println returnedValue              param value     return     private static int getMethod String value          HashMap lt Integer  String gt  hashmap1   new HashMap lt Integer  String gt             hashmap1 put 1  MY            hashmap1 put 2  AFF             if  hashmap1 containsValue value                         for  Map Entry lt Integer String gt  e   hashmap1 entrySet                      Integer key   e getKey                    Object value2   e getValue                    if   value2 toString    equalsIgnoreCase value                                         return key                                                       return 0

User · Answer

if you what to obtain  ONE  by giving in 100 then   initialize hash map by  hashmap   new HashMap lt Object String gt      haspmap put 100  one     and retrieve value by hashMap get 100   hope that helps

User · Answer

You mixed the keys and the values   Hashmap  lt Integer String gt  hashmap   new HashMap lt Integer  String gt      hashmap put 100   one    hashmap put 200   two      Afterwards a  hashmap get 100     will give you  one

User · Answer

You have it reversed  The 100 should be the first parameter  it s the key  and the  one  should be the second parameter  it s the value    Read the javadoc for HashMap and that might help you  HashMap  To get the value  use hashmap get 100

User · Answer

If you need only that  simply use put 100   one    Note that the key is the first argument  and the value is the 2nd  If you need to be able to get by both the key and the value  use BiMap  from guava

User · Answer

The put method in HashMap is defined like  this   Object  put Object key  Object value     key is the first parameter  so in your put   one  is the key  You can t easily look up by value in a HashMap  if you really want to do that  it would be a linear search done by calling  entrySet    like this   for  Map Entry lt Object  Object gt  e   hashmap entrySet          Object key   e getKey        Object value   e getValue        However  that s O n  and kind of defeats the purpose of using a HashMap unless you only need to do it rarely  If you really want to be able to look up by key or value frequently  core Java doesn t have anything for you  but something like BiMap from the Google Collections is what you want

User · Answer

If you are not bound to use Hashmap  I would advise to use pair lt  T T    The individual elements can be accessed by first and second calls   Have a look at this http   www cplusplus com reference utility pair   I used it here   http   codeforces com contest 507 submission 9531943

User · Answer

We can get KEY from VALUE  Below is a sample code      public class Main     public static void main String   args        Map map   new HashMap        map put  key 1   one        map put  key 2   two        map put  key 3   three        map put  key 4   four        System out println getKeyFromValue map  four           public static Object getKeyFromValue Map hm  Object value        for  Object o   hm keySet            if  hm get o  equals value             return o                    return null         I hope this will help everyone

[java] Get key from a HashMap using the value

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