Java源码角度分析HashMap用法

Map<Integer,String> maps=new HashMap<Integer,String>();   
maps.put(1, "a");   
maps.put(2, "b");

Map<int,double> maps=new HashMap<int,double>();

public class HashMap<K,V>  
  extends AbstractMap<K,V>  
  implements Map<K,V>, Cloneable, Serializable 

Map<String,String> rm=new HashMap<String,String>(2)

Map<String, Object> map = Maps.newHashMapWithExpectedSize(7);

public HashMap() {   
    this.loadFactor = DEFAULT_LOAD_FACTOR;   
    threshold = (int)(DEFAULT_INITIAL_CAPACITY * DEFAULT_LOAD_FACTOR);   
    table = new Entry[DEFAULT_INITIAL_CAPACITY];   
    init();   
  } 

/** 
   * Constructs an empty <tt>HashMap</tt> with the specified initial 
   * capacity and the default load factor (0.75). 
   * 
   * @param initialCapacity the initial capacity. 
   * @throws IllegalArgumentException if the initial capacity is negative. 
   */ 
  public HashMap(int initialCapacity) { 
    this(initialCapacity, DEFAULT_LOAD_FACTOR); 
  }

DEFAULT_LOAD_FACTOR  //默认加载因子，如果不制定的话是0.75  
DEFAULT_INITIAL_CAPACITY //默认初始化容量，默认是16  
threshold //阈（yu）值 根据加载因子和初始化容量计算得出 ，<span style="color: rgb(54, 46, 43); font-family: "microsoft yahei";">threshold表示当HashMap的size大于threshold时会执行resize操作。

public V put(K key, V value) {  
    if (key == null) //键为空的情况，HashMap和HashTable的一个区别  
      return putForNullKey(value);  
    int hash = hash(key.hashCode()); //根据键的hashCode算出hash值  
    int i = indexFor(hash, table.length); //根据hash值算出究竟该放入哪个数组下标中  
    for (Entry<K,V> e = table[i]; e != null; e = e.next) {//整个for循环实现了如果存在K那么就替换V  
      Object k;  
      if (e.hash == hash && ((k = e.key) == key || key.equals(k))) {  
        V oldValue = e.value;  
        e.value = value;  
        e.recordAccess(this);  
        return oldValue;  
      }  
    }  
  
    modCount++;//计数器  
    addEntry(hash, key, value, i); //添加到数组中  
    return null;  
  }

addEntry(hash, key, value, i);

void addEntry(int hash, K key, V value, int bucketIndex) {   
Entry<K,V> e = table[bucketIndex];   
    table[bucketIndex] = new Entry<K,V>(hash, key, value, e);   
    if (size++ >= threshold)   
     <span style="color:#ff0000;"><strong> resize(2 * table.length);
}

void resize(int newCapacity) {   
    Entry[] oldTable = table;   
    int oldCapacity = oldTable.length;   
    if (oldCapacity == MAXIMUM_CAPACITY) {   
      threshold = Integer.MAX_VALUE;   
      return;   
    }   
  
    Entry[] newTable = new Entry[newCapacity]; <span style="color: rgb(51, 51, 51); font-family: Arial;">new 一个新的数组，</span> 
    <strong> <span style="color:#ff0000;">transfer(newTable);</span> </strong> //将就数组转移到新的数组中 
    table = newTable;   
    threshold = (int)(newCapacity * loadFactor);  //重新计算容量 
  }

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 = <strong><span style="color:#ff0000;">indexFor(e.hash, newCapacity);  //根据hash值个容量重新计算下标</span></strong> 
          e.next = newTable[i];   
          newTable[i] = e;   
          e = next;   
        } while (e != null);   
      }   
    }   
  } 

Map<Integer,String> maps=new HashMap<Integer,String>(1000);