CODEWITHSUNDEEP
javalisthashmap
Max
Max

Reputation: 165

Get Value from Key, Value is a List

how i can get the Key from my Value?

My HashMap:

public static final Map<String, List<String>> Server = new HashMap<>();

my attempt:

  public static Object getKeyFromValue(String value) {
        for (Object o : Server.keySet()) {
             if (Server.get(o).equals(value)) {
                 return o;
             }
         }
         return null;
   }

It dosent work, because the Value is a List.

Upvotes: 1

Views: 8416

Answers (3)

Mahfuz
Mahfuz

Reputation: 401

Just changing from equals to contains works. and all remains same

public static Object getKeyFromValue(String value) {
    for (Object o : Server.keySet()) {
        if (Server.get(o).contains(value)) {
             return o;
         }
     }
     return null;
}

Upvotes: 0

Paul Boddington
Paul Boddington

Reputation: 37645

When you iterate over a Map, if you need both the key and the value it's better to iterate over the entrySet rather than the keySet.

public static String getKeyFromValue(String value) {
    for (Map.Entry<String, List<String>> e : Server.entrySet()) {
        if (e.getValue().contains(value)) {
            return e.getKey();
        }
    }
    return null;
}

This should work, but there are 3 things I don't like about it (apart from Server beginning with a capital letter).

  1. contains for many List implementations (including ArrayList and LinkedList) is slow, because it is a linear search. It may better to use HashSet instead.
  2. If the value occurs in more than one list in the map, the returned key could be any of multiple answers. It may be better for the name of the method to indicate this (e.g. getAnyKeyForValue).
  3. It may be preferable to return an Optional<String> rather than using null to mean that the value was not found.

A Java 8 solution, taking all of these points into consideration and taking advantage of parallelism would be

public static Optional<String> getAnyKeyForValue(String value) {
    return Server.entrySet()
                 .parallelStream()
                 .filter(e->e.getValue().contains(value))
                 .map(Map.Entry::getKey)
                 .findAny();
}

Upvotes: 0

Luiggi Mendoza
Luiggi Mendoza

Reputation: 85781

Use List#contains:

if (Server.get(o).contains(value)) {
    //...
}

Upvotes: 1

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