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    <p>If you choose to use the <a href="http://commons.apache.org/collections/" rel="noreferrer">Commons Collections library</a> instead of the standard Java Collections API, you can achieve this with ease.</p> <p>The <a href="http://commons.apache.org/proper/commons-collections/apidocs/org/apache/commons/collections4/BidiMap.html" rel="noreferrer">BidiMap</a> interface in the Collections library is a bi-directional map, allowing you to map a key to a value (like normal maps), and also to map a value to a key, thus allowing you to perform lookups in both directions. Obtaining a key for a value is supported by the <a href="http://commons.apache.org/proper/commons-collections/apidocs/org/apache/commons/collections4/BidiMap.html#getKey%28java.lang.Object%29" rel="noreferrer">getKey() method</a>.</p> <p>There is a caveat though, bidi maps cannot have multiple values mapped to keys, and hence unless your data set has 1:1 mappings between keys and values, you cannot use bidimaps.</p> <p><strong>Update</strong></p> <p>If you want to rely on the Java Collections API, you will have to ensure the 1:1 relationship between keys and values at the time of inserting the value into the map. This is easier said than done.</p> <p>Once you can ensure that, use the <a href="https://docs.oracle.com/javase/7/docs/api/java/util/Map.html#entrySet()" rel="noreferrer">entrySet() method</a> to obtain the set of entries (mappings) in the Map. Once you have obtained the set whose type is <a href="https://docs.oracle.com/javase/7/docs/api/java/util/Map.Entry.html" rel="noreferrer">Map.Entry</a>, iterate through the entries, comparing the <a href="https://docs.oracle.com/javase/7/docs/api/java/util/Map.Entry.html#getValue()" rel="noreferrer">stored value</a> against the expected, and obtain the <a href="https://docs.oracle.com/javase/7/docs/api/java/util/Map.Entry.html#getKey()" rel="noreferrer">corresponding key</a>.</p> <p><strong>Update #2</strong></p> <p>Support for bidi maps with generics can be found in <a href="https://github.com/google/guava" rel="noreferrer">Google Guava</a> and the refactored <a href="http://sourceforge.net/projects/collections/" rel="noreferrer">Commons-Collections</a> libraries (the latter is not an Apache project). Thanks to Esko for pointing out the missing generic support in Apache Commons Collections. Using collections with generics makes more maintainable code.</p>
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    1. POJava Hashmap: How to get key from value?
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    1. CO...and if you like Generics and all that modern stuff, Google Collections has BiMap where you can get key matching specified value by calling biMap.inverse().get(value);
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    2. COYes, Apache Commons Collections doesn't support generics. However, there is Google Collections as you've pointed out (which I don't use yet - no 1.0 release yet), and there is the refactored Commons-Collections with support for Generics. You'll find this as a Sourceforge project @ http://sourceforge.net/projects/collections/
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    3. COThe Google Collections are **not** a refactored version of Commons-Collections.
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