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POGet key object out of a HashMap in Java
 

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    <p>I would like to retrieve the original object of a key in a HashMap in Java, what is the best way to do it?</p> <p>For example</p> <pre><code>HashMap&lt;Integer, Integer&gt; map = new HashMap&lt;Integer, Integer&gt;(); 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? </code></pre> <p>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.</p> <p>retrieving a value object is easy but it seems no many people know how to retrieve the key object, so this question is definitely useful and I don't get those vote down the question b/c they don't know how to do it. That is exact why we need learn this from some one who know how to do it.</p> <p>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.</p> <p>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. </p> <p>I am very surprised to see some people don't like the question and think it's trivial for a value "10". I do have a reason to ask. 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. </p> <p>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. </p> <p>One way I can think of is to store the key object in value as well, like</p> <pre><code>class KeyAndValue { public Integer key; public Integer value; public KeyAndValue(Integer key, Integer value) { this.key = key; this.value = value; } } map&lt;Integer, keyAndValueL&gt; map = new map&lt;Integer, keyAndValueL&gt;(); 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; </code></pre> <p>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.</p> <p>Thanks a lot!</p>
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    1. COIt really doesn't make any sense. Why would you want to get the `key` from the map, when you already know it?!
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    2. CODo you mean get all keys for a given value? Check out http://stackoverflow.com/questions/1383797/java-hashmap-how-to-get-key-from-value
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    3. COmay be you need `map.keySet()`
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      1. USTrying
 

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