本文基于jdk1.8進行分析
關于hashmap的簡介，可以參考這篇文章http://www.ythuaji.com.cn/article/174037.html。
首先看一下hashmap的一些靜態常量。第一個是default_initial_capacity，默認初始大小，16。從注釋中可以了解到，大小必須為2的指數。這里的16，采用的1左移4位實現。而“aka”，是as known as的縮寫。
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/** * the default initial capacity - must be a power of two. **/static final int default_initial_capacity = 1 << 4; // aka 16 |
接下來是最大容量，當通過任何一個構造函數的參數隱式指明時使用該值。必須是2的指數，且小于等于1<<30，即2的30次方。
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/** * the maximum capacity, used if a higher value is implicitly specified * by either of the constructors with arguments. * must be a power of two <= 1<<30. **/static final int maximum_capacity = 1 << 30; |
接下來是負載因子，默認值為0.75f。
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/** * the load factor used when none specified in constructor. **/static final float default_load_factor = 0.75f; |
接下來是和紅黑樹相關的幾個常量。在jdk1.8中，如果哈希表中的鏈表太長，就會轉化為一個紅黑樹。
treeify_threshold，表示要轉為紅黑樹的最小元素個數，即8。把紅黑樹轉化為鏈表的門限個數是6. min_treeify_capacity為64，表示把鏈表轉化為紅黑樹的最小元素個數。否則，如果太多節點在一個鏈表中時，哈希表會擴容，而不會轉化為紅黑樹。
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/** * the bin count threshold for using a tree rather than list for a * bin. bins are converted to trees when adding an element to a * bin with at least this many nodes. the value must be greater * than 2 and should be at least 8 to mesh with assumptions in * tree removal about conversion back to plain bins upon * shrinkage. **/static final int treeify_threshold = 8;/** * the bin count threshold for untreeifying a (split) bin during a * resize operation. should be less than treeify_threshold, and at * most 6 to mesh with shrinkage detection under removal. **/static final int untreeify_threshold = 6;/** * the smallest table capacity for which bins may be treeified. * (otherwise the table is resized if too many nodes in a bin.) * should be at least 4 * treeify_threshold to avoid conflicts * between resizing and treeification thresholds. **/static final int min_treeify_capacity = 64; |
接下來是table，它是保存hashmap的最主要的數據結構，如下圖。從注釋中也可以了解到，table的大小一定是2的指數。
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/** * the table, initialized on first use, and resized as * necessary. when allocated, length is always a power of two. * (we also tolerate length zero in some operations to allow * bootstrapping mechanics that are currently not needed.) **/transient node<k,v>[] table; |
接下來是entryset，如下圖。它保存緩存的映射關系集合。注意，keyset()和values()使用的是父類abstractmap的屬性。
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/** * holds cached entryset(). note that abstractmap fields are used * for keyset() and values(). **/transient set<map.entry<k,v>> entryset; |
最后是一些其他的屬性，包括hashmap中元素個數size，修改次數modcount，下一次進行resize的門限個數，以及負載因子loadfactor，如下圖。需要注意的是，loadfactor是final的，也就是說，它一旦被賦值，就不能再修改了。
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/** * the number of key-value mappings contained in this map. **/transient int size;/** * the number of times this hashmap has been structurally modified * structural modifications are those that change the number of mappings in * the hashmap or otherwise modify its internal structure (e.g., * rehash). this field is used to make iterators on collection-views of * the hashmap fail-fast. (see concurrentmodificationexception). **/transient int modcount;/** * the next size value at which to resize (capacity * load factor). * @serial **/// (the javadoc description is true upon serialization.// additionally, if the table array has not been allocated, this// field holds the initial array capacity, or zero signifying// default_initial_capacity.)int threshold;/** * the load factor for the hash table. * * @serial **/final float loadfactor; |
this is the end.
總結
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原文鏈接：https://blog.csdn.net/li_canhui/article/details/85088659
