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h2database
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7ca15e40
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7ca15e40
authored
6 年前
作者:
Noel Grandin
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add nice explanation of why ValueHashMap is there
originally from Evgenij Ryazanov
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41bfa75e
隐藏空白字符变更
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ValueHashMap.java
h2/src/main/org/h2/util/ValueHashMap.java
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h2/src/main/org/h2/util/ValueHashMap.java
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7ca15e40
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@@ -12,6 +12,21 @@ import org.h2.value.ValueNull;
/**
* This hash map supports keys of type Value.
* <p>
* ValueHashMap is a very simple implementation without allocation of additional
* objects for entries. It's very fast with good distribution of hashes, but if
* hashes have a lot of collisions this implementation tends to be very slow.
* <p>
* HashMap in archaic versions of Java have some overhead for allocation of
* entries, but slightly better behaviour with limited number of collisions,
* because collisions have no impact on non-colliding entires. HashMap in modern
* versions of Java also have the same overhead, but it builds a trees of keys
* with colliding hashes, that's why even if the all keys have exactly the same
* hash code it still offers a good performance similar to TreeMap. So
* ValueHashMap is faster in typical cases, but may behave really bad in some
* cases. HashMap is slower in typical cases, but its performance does not
* degrade too much even in the worst possible case (if keys are comparable,
* like our Value class).
*
* @param <V> the value type
*/
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