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8186958: Need method to create pre-sized HashMap

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Reviewed-by: chegar, naoto, joehw, lancea, wetmore, smarks
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XenoAmess 2022-04-19 00:03:56 +00:00 • committed by Stuart Marks
parent 41fc078323
commit 87faa85c59
30 changed files with 208 additions and 73 deletions
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38
src/java.base/share/classes/java/util/HashMap.java
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@ -1,5 +1,5 @@
/*
* Copyright (c) 1997, 2021, Oracle and/or its affiliates. All rights reserved.
* Copyright (c) 1997, 2022, Oracle and/or its affiliates. All rights reserved.
* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
*
* This code is free software; you can redistribute it and/or modify it
@ -433,6 +433,10 @@ public class HashMap<K,V> extends AbstractMap<K,V>
* Constructs an empty {@code HashMap} with the specified initial
* capacity and load factor.
*
* @apiNote
* To create a {@code HashMap} with an initial capacity that accommodates
* an expected number of mappings, use {@link #newHashMap(int) newHashMap}.
*
* @param initialCapacity the initial capacity
* @param loadFactor the load factor
* @throws IllegalArgumentException if the initial capacity is negative
@ -455,6 +459,10 @@ public class HashMap<K,V> extends AbstractMap<K,V>
* Constructs an empty {@code HashMap} with the specified initial
* capacity and the default load factor (0.75).
*
* @apiNote
* To create a {@code HashMap} with an initial capacity that accommodates
* an expected number of mappings, use {@link #newHashMap(int) newHashMap}.
*
* @param initialCapacity the initial capacity.
* @throws IllegalArgumentException if the initial capacity is negative.
*/
@ -2545,4 +2553,32 @@ public class HashMap<K,V> extends AbstractMap<K,V>
}
}
/**
* Calculate initial capacity for HashMap based classes, from expected size and default load factor (0.75).
*
* @param numMappings the expected number of mappings
* @return initial capacity for HashMap based classes.
* @since 19
*/
static int calculateHashMapCapacity(int numMappings) {
return (int) Math.ceil(numMappings / (double) DEFAULT_LOAD_FACTOR);
}
/**
* Creates a new, empty HashMap suitable for the expected number of mappings.
* The returned map uses the default load factor of 0.75, and its initial capacity is
* generally large enough so that the expected number of mappings can be added
* without resizing the map.
*
* @param numMappings the expected number of mappings
* @param <K> the type of keys maintained by this map
* @param <V> the type of mapped values
* @return the newly created map
* @throws IllegalArgumentException if numMappings is negative
* @since 19
*/
public static <K, V> HashMap<K, V> newHashMap(int numMappings) {
return new HashMap<>(calculateHashMapCapacity(numMappings));
}
}