IBM User Guide for Java V7 on Windows > Reference > Command-line options > System property command-line options

-Djdk.map.althashing.threshold

This system property controls the use of an enhanced hashing algorithm for hashed maps.

-Djdk.map.althashing.threshold=value

This alternative hashing algorithm is used for string keys when a hashed data structure has a capacity larger than value.

A value of 1 ensures that this algorithm is always used, regardless of the hashed map capacity. A value of -1 prevents the use of this algorithm, which is the default value.

The hashed map structures affected by this threshold are: java.util.HashMap, java.util.Hashtable, java.util.LinkedHashMap, java.util.WeakHashMap, and java.util.concurrent.ConcurrentHashMap.

The capacity of a hashed map is related to the number of entries in the map, multiplied by the load factor. Because the capacity of a hashed map is rounded up to the next power of two, setting the threshold to intermediate values has no affect on behavior. For example, threshold values of 600, 700, and 1000 have the same effect. However, values of 1023 and 1024 cause a difference in behavior. For a more detailed description of the capacity and load factor, see http://docs.oracle.com/javase/7/docs/api/java/util/HashMap.html.

When entries are removed from a hashed map the capacity does not shrink. Therefore, if the map ever exceeds the threshold to use alternative hashing for Strings, the map always uses alternative hashing for Strings. This behavior does not change, even if entries are later removed or the map is emptied using clear().

The enhanced hashing algorithm is available from Java™ 7 SR2

Parent: System property command-line options

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