[Java] HashMap 源码简要分析

 特性

* 允许null作为key/value。

* 不保证按照插入的顺序输出。使用hash构造的映射一般来讲是无序的。

* 非线程安全。

* 内部原理与Hashtable类似。

 

源码简要分析

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public class HashMap<K,V> {      static final int DEFAULT_INITIAL_CAPACITY = 16 ; // 默认初始容量是16。（必须是2的次方）      static final int MAXIMUM_CAPACITY = 1 << 30 ; // 即2的30次方      static final float DEFAULT_LOAD_FACTOR = 0.75f; // 默认装载因子        Entry[] table;  // Entry表      int size; // Entry[]实际存储的Entry个数      int threshold; // reash的阈值，=capacity * load factor      final float loadFactor;        // 构造函数      public HashMap(int initialCapacity, float loadFactor) {      // 找到一个比initialCapacity大的最小的2的次方数      int capacity = 1;      while (capacity < initialCapacity)             capacity <<= 1;      }        this.loadFactor = loadFactor;      threshold = (int)(capacity * loadFactor);      table = new Entry[capacity];        // addEntry()     void addEntry(int hash, K key, V value, int bucketIndex) {         Entry<K,V> e = table[bucketIndex];         table[bucketIndex] = new Entry<>(hash, key, value, e);         if (size++ >= threshold)             resize(2 * table.length);     }        // put()：添加元素      public V put(K key, V value) {           int hash = hash(key.hashCode());     // key的hash值           int i = indexFor(hash,table.length);     // 槽位             // 寻找是否已经有key存在，如果已经存在，使用新值覆盖旧值，返回旧值           for (Entry<K,V> e = table[i]; e != null; e = e.next) {             Object k;             if (e.hash == hash && ((k = e.key) == key || key.equals(k))) {                 V oldValue = e.value;                 e.value = value;                 return oldValue;             }           }             // 添加Entry           addEntry(hash,key,value,i);           return null;      }        // resize()：重新哈希     void resize(int newCapacity) {         Entry[] oldTable = table;         int oldCapacity = oldTable.length;         Entry[] newTable = new Entry[newCapacity];         transfer(newTable);         table = newTable;         threshold = (int)(newCapacity * loadFactor);     }       /**      * Transfers all entries from current table to newTable.      */     void transfer(Entry[] newTable) {         Entry[] src = table;         int newCapacity = newTable.length;         for (int j = 0; j < src.length; j++) {             Entry<K,V> e = src[j];             if (e != null) {                 src[j] = null;                     do {                     Entry<K,V> next = e.next;                     int i = indexFor(e.hash, newCapacity);                     e.next = newTable[i];                     newTable[i] = e;                     e = next;                 } while (e != null);             }         }     }   }

　　

遍历方式

* 低效遍历：按照Key进行遍历，则每次都需要按Key查找槽位（不同的Key有重复查找），且可能需要遍历槽位上所在Entry链表（不同的Key有重复遍历）。

* 高效遍历：HashMap的entrySet()返回自定义的EntryIterator，是先按照槽位遍历一次，再遍历一次槽位上Entry链表。

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Map<String, String[]> paraMap = new HashMap<String, String[]>(); for( Map.Entry<String, String[]> entry : paraMap.entrySet() ) {     String appFieldDefId = entry.getKey();     String[] values = entry.getValue(); }