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POAccesing hidden getEntry(Object key) in HashMap
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copied!<p>I have similar problem to one discussed <a href="https://stackoverflow.com/questions/1873330/why-is-getentryobject-key-not-exposed-on-hashmap">here</a>, but with stronger practical usage.</p> <p>For example, I have a <code>Map<String, Integer></code>, and I have some function, which is given a key and in case the mapped integer value is negative, puts <code>NULL</code> to the map:</p> <pre><code>Map<String, Integer> map = new HashMap<String, Integer>(); public void nullifyIfNegative(String key) { Integer value = map.get(key); if (value != null && value.intValue() < 0) { map.put(key, null); } } </code></pre> <p>I this case, the lookup (and hence, <code>hashCode</code> calculation for the key) is done twice: one for lookup and one for replacement. It would be nice to have another method (which is already in <code>HashMap</code>) and allows to make this more effective:</p> <pre><code>public void nullifyIfNegative(String key) { Map.Entry<String, Integer> entry = map.getEntry(key); if (entry != null && entry.getValue().intValue() < 0) { entry.setValue(null); } } </code></pre> <p>The same concerns cases, when you want to manipulate immutable objects, which can be map values:</p> <ul> <li><code>Map<String, String></code>: I want to append something to the string value.</li> <li><code>Map<String, int[]></code>: I want to insert a number into the array.</li> </ul> <p>So the case is quite common. Solutions, which might work, but not for me:</p> <ul> <li>Reflection. Is good, but I cannot sacrifice performance just for this nice feature.</li> <li>Use <a href="http://commons.apache.org/collections/api-release/org/apache/commons/collections/map/AbstractHashedMap.html#getEntry%28java.lang.Object%29" rel="nofollow noreferrer"><code>org.apache.commons.collections.map.AbstractHashedMap</code></a> (it has at least <code>protected getEntry()</code> method), but unfortunately, commons-collections do not support generics.</li> <li>Use <a href="http://collections.sourceforge.net/api/org/apache/commons/collections/map/AbstractHashedMap.html#getEntry%28java.lang.Object%29" rel="nofollow noreferrer">generic commons-collections</a>, but this library (AFAIK) is out-of-date (not in sync with latest library version from Apache), and (what is critical) is not available in central maven repository.</li> <li>Use value wrappers, which means "making values mutable" (e.g. use mutable integers [e.g. <code>org.apache.commons.lang.mutable.MutableInt</code>], or collections instead of arrays). This solutions leads to memory loss, which I would like to avoid.</li> <li>Try to extend <code>java.util.HashMap</code> with custom class implementation (which should be in <code>java.util</code> package) and put it to <a href="http://java.sun.com/j2se/1.5.0/docs/guide/standards/index.html" rel="nofollow noreferrer">endorsed folder</a> (as <code>java.lang.ClassLoader</code> will refuse to load it in <code>Class<?> defineClass(String name, byte[] b, int off, int len)</code>, see sources), but I don't want to patch JDK and it seems like the list of packages that can be endorsed, does not include <code>java.util</code>.</li> </ul> <p>The similar question is already raised on <a href="http://bugs.sun.com/view_bug.do?bug_id=6552529" rel="nofollow noreferrer">sun.com bugtracker</a>, but I would like to know, what is the opinion of the community and what can be the way out taking in mind the maximum memory & performance effectiveness.</p> <p>If you agree, this is nice and beneficiary functionality, please, vote this bug!</p>

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