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e21cbe82a9
Where possible update Java documentation links to Java 11. Additionally update some other links to use HTTPS.
48 lines
1.6 KiB
XML
48 lines
1.6 KiB
XML
<!DOCTYPE qhelp PUBLIC
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"-//Semmle//qhelp//EN"
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"qhelp.dtd">
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<qhelp>
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<overview>
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<p>Java's Collections Framework provides several different ways of iterating
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the contents of a map. You can retrieve the set of keys, the collection of values, or the
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set of "entries" (which are, in effect, key/value pairs).</p>
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<p>The choice of iterator can affect performance. For example, it is considered bad practice to iterate
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the key set of a map if the body of the loop performs a map lookup on each retrieved key
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anyway.</p>
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</overview>
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<recommendation>
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<p>Evaluate the requirements of the loop body. If it does not actually need the key
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apart from looking it up in the map, iterate the map's values (obtained by a call to
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<code>values</code>) instead. If the loop genuinely needs both key and value for
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each mapping in the map, iterate the entry set (obtained by a call to
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<code>entrySet</code>) and retrieve the key and value from each entry. This saves a
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more expensive map lookup each time.</p>
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</recommendation>
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<example>
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<p>In the following example, the first version of the method <code>findId</code> iterates
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the map <code>people</code> using the key set. This is inefficient because the body of the loop
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needs to access the value for each key. In contrast, the second version iterates the map
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using the entry set because the loop body needs both the key and the value for each mapping.</p>
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<sample src="InefficientKeySetIterator.java" />
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</example>
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<references>
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<li>
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Java API Specification:
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<a href="https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/util/Map.html#entrySet()">Map.entrySet()</a>.
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</li>
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</references>
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</qhelp>
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