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8046362: IdentityHashMap.hash comments should be clarified

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Reviewed-by: aph, martin, rriggs
This commit is contained in:
Stuart Marks 2020-02-13 10:00:33 -08:00
parent 30707f8007
commit 5b4d80d106
1 changed files with 11 additions and 9 deletions
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20
src/java.base/share/classes/java/util/IdentityHashMap.java
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@ -1,5 +1,5 @@
/* /*
* Copyright (c) 2000, 2019, Oracle and/or its affiliates. All rights reserved. * Copyright (c) 2000, 2020, Oracle and/or its affiliates. All rights reserved.
* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
* *
* This code is free software; you can redistribute it and/or modify it * This code is free software; you can redistribute it and/or modify it
@ -115,17 +115,19 @@ import jdk.internal.access.SharedSecrets;
* exception for its correctness: <i>fail-fast iterators should be used only * exception for its correctness: <i>fail-fast iterators should be used only
* to detect bugs.</i> * to detect bugs.</i>
* *
* <p>Implementation note: This is a simple <i>linear-probe</i> hash table,
* as described for example in texts by Sedgewick and Knuth. The array
* alternates holding keys and values. (This has better locality for large
* tables than does using separate arrays.) For many JRE implementations
* and operation mixes, this class will yield better performance than
* {@link HashMap} (which uses <i>chaining</i> rather than linear-probing).
*
* <p>This class is a member of the * <p>This class is a member of the
* <a href="{@docRoot}/java.base/java/util/package-summary.html#CollectionsFramework"> * <a href="{@docRoot}/java.base/java/util/package-summary.html#CollectionsFramework">
* Java Collections Framework</a>. * Java Collections Framework</a>.
* *
* @implNote
* <p>This is a simple <i>linear-probe</i> hash table,
* as described for example in texts by Sedgewick and Knuth. The array
* contains alternating keys and values, with keys at even indexes and values
* at odd indexes. (This arrangement has better locality for large
* tables than does using separate arrays.) For many Java implementations
* and operation mixes, this class will yield better performance than
* {@link HashMap}, which uses <i>chaining</i> rather than linear-probing.
*
* @see System#identityHashCode(Object) * @see System#identityHashCode(Object)
* @see Object#hashCode() * @see Object#hashCode()
* @see Collection * @see Collection
@ -293,7 +295,7 @@ public class IdentityHashMap<K,V>
*/ */
private static int hash(Object x, int length) { private static int hash(Object x, int length) {
int h = System.identityHashCode(x); int h = System.identityHashCode(x);
// Multiply by -127, and left-shift to use least bit as part of hash // Multiply by -254 to use the hash LSB and to ensure index is even
return ((h << 1) - (h << 8)) & (length - 1); return ((h << 1) - (h << 8)) & (length - 1);
} }