Get key object out of a HashMap in Java

Question

I would like to retrieve the original object of a key in a HashMap in Java, what is the best way to do it?

For example

HashMap<Integer, Integer> map = new HashMap<Integer, Integer>();
Integer keyObj = new Integer(10);
Integer valueObj = new Integer(100);

// And add maybe 1 million other key value pairs here
//... later in the code, if I want to retrieve the valueObj, given the value of a key to be 10
Integer retrievedValueObj = map.get(10);

//is there a way to retrieve the original keyObj object with value 10 from map?

Basically, the user can query any value of key here just for the key object, 10 is just an example. Some comment say, "you already have the x object, why do you want to get it?" Well, this is the same as saying "you already have the value object, why do you want to get it?" That is the purpose for the HashMap data structure, store and retrieval.

Retrieving a value object is easy but it seems no many people know how to retrieve the key object. It seems like many people don't get why I want to achieve the object of 10 and ask why? why not just value 10. This is just a greatly simplified model.

Well, let me give a little bit context. The keyObj is data in another data structure and I need the exact reference of this original key object. Say, there is a linked list of all the key values, and if I want to remove a particular node in the linked list.

I am not only interested in the value "10", but also the memory location, i.e. the reference in Java of that "10" object. There could be many "10"'s in memory. But that exact object is what I want to retrieve.

The iterator approach answer below give an O(n) approach. But what I am looking for is an O(1) retrieval of the key OBJECT given the key value.

One way I can think of is to store the key object in value as well, like

class KeyAndValue {
     public Integer key;
     public Integer value;
     public KeyAndValue(Integer key, Integer value) {
         this.key = key;
         this.value = value;
     }
}

map<Integer, keyAndValueL> map = new map<Integer, keyAndValueL>();
Integer x = new Integer(10);
map.add(x, new KeyAndValue(x, 100));

//then I can retrieve the reference of x, given value of key 10
Integer newKeyObj = map.get(10).key;

but this approach uses more memory and looks like a hack to me. I am wondering if there is a more elegant way in Java.

It really doesn't make any sense. Why would you want to get the `key` from the map, when you already know it?! — Rahul, Nov 12 '13 at 05:08
Do you mean get all keys for a given value? Check out http://stackoverflow.com/questions/1383797/java-hashmap-how-to-get-key-from-value — musical_coder, Nov 12 '13 at 05:09
@musical_coder No, keys are unique in a HashMap. I try to just use standard Java library rather than 3rd party libraries... — Alfred Zhong, Nov 12 '13 at 06:38
@Trying but set can only tell you if map.keySet().contains(10), it can't get the key object out — Alfred Zhong, Nov 12 '13 at 06:40
@R.J It is very easy to understand. I updated the post, key objects are stored in another data structure as well, so I need the REFERENCE. Imagine the users retrieve the key object are not the same user putting them in. Even they are the same users, it take another O(1) data structure to keep track of what put in the HashMap. I am trying just to use the HashMap itself, you understand? — Alfred Zhong, Nov 12 '13 at 06:44
This is a perfectly valid question. You might have a key that is *equal* to the one used by the map, but it's not *identical*. Unfortunately the `Map` interface provides no way of obtaining the key reference in O(1). Unless you implement your own map or modify e.g. `HashMap` (subclassing won't help, since the method you need is package-private), I think the simplest way would be to store the key reference in the value, as you said. — Cinnam, Oct 15 '15 at 21:37

score 1 · Answer 1 · answered Feb 12 '20 at 14:49

A similar aproach but more generic is to store the "key + value" as an Entry instead of encapsule in another class. Example:

    Map<Integer, Entry<Integer, Integer>> map = new HashMap<Integer, Entry<Integer, Integer>>();
    Integer x = new Integer(10);
    map.put(x, new AbstractMap.SimpleEntry<Integer, Integer>(x, 100));

    //then I can retrieve the reference of x, given value of key 10
    Entry<Integer, Integer> keyObj = map.get(10);

score 0 · Answer 2 · answered Nov 12 '13 at 05:19

try this

        HashMap<Integer, Integer> map = new HashMap<Integer, Integer>();
        Integer keyObj = new Integer(10);
        Integer valueObj = new Integer(100);
        map.put(keyObj, valueObj);
        Set<Integer> keys = map.keySet();
        Iterator<Integer> iterator = keys.iterator();
        while(iterator.hasNext()){
            Integer x = iterator.next();
            if(map.get(x) == 100)
                System.out.println("key is "+ x);
        }

This is O(n) algorithm, is there a O(1) approach? – Alfred Zhong Nov 12 '13 at 06:06 — Alfred Zhong, Nov 12 '13 at 06:06

score 0 · Answer 3 · answered Jul 09 '18 at 17:07

0

You could store a key + value object "as the value" as you mention in your question.

answered Jul 09 '18 at 17:07

rogerdpack

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score 0 · Answer 4 · answered Dec 01 '20 at 03:28

What you are implementing is a variant of the flyweight pattern.

This is easiest implemented using a Map of every managed object to itself:

Map<T, T> cache = new HashMap<>();

And for every object you encounter:

T obj; // comes from somewhere
obj = cache.computeIfAbsent(obj, v -> obj); // reuse, or add to cache if not found

This has O(1) time complexity and uses only one extra object reference for each object so managed.

Get key object out of a HashMap in Java

4 Answers4

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