Java HashMap 的使用

蔚落 2021-09-21 08:00 296阅读 0赞

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title: HashMap 的使用  
date: 2018-12-04 11:09:02  
categories:

*  java

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# `HashMap` 的使用 #

在开发中经常使用到 `HashMap`，分析一下 `HashMap` 带参的构造方法

/** * Constructs an empty <tt>HashMap</tt> with the specified initial * capacity and load factor. * * @param initialCapacity the initial capacity * @param loadFactor the load factor * @throws IllegalArgumentException if the initial capacity is negative * or the load factor is nonpositive */
    public HashMap(int initialCapacity, float loadFactor) { 
       if (initialCapacity < 0)
           throw new IllegalArgumentException("Illegal initial capacity: " +
                                               initialCapacity);
       if (initialCapacity > MAXIMUM_CAPACITY)
           initialCapacity = MAXIMUM_CAPACITY;
       if (loadFactor <= 0 || Float.isNaN(loadFactor))
           throw new IllegalArgumentException("Illegal load factor: " +
                                               loadFactor);
       this.loadFactor = loadFactor;
       this.threshold = tableSizeFor(initialCapacity);
    }
    
    /** * 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);
    }
    
    /** * Constructs an empty <tt>HashMap</tt> with the default initial capacity * (16) and the default load factor (0.75). */
    public HashMap() { 
       this.loadFactor = DEFAULT_LOAD_FACTOR; // all other fields defaulted
    }

一个参数的构造方法，传入的参数是初始大小，最终调用两个参数的构造方法，传入初始大小和负载系数。默认的负载系数 `DEFAULT_LOAD_FACTOR` 是 `0.75f`.调用`tableSizeFor(int)`传入初始大小值算出`threshold`，具体的 `tableSizeFor(int)` 方法,`threshold`实际值会在第一次`put(k,v)`重新计算。

/** * Returns a power of two size for the given target capacity. */
    static final int tableSizeFor(int cap) { 
        int n = cap - 1;
        n |= n >>> 1;
        n |= n >>> 2;
        n |= n >>> 4;
        n |= n >>> 8;
        n |= n >>> 16;
        return (n < 0) ? 1 : (n >= MAXIMUM_CAPACITY) ? MAXIMUM_CAPACITY : n + 1;
    }

写一个测试 `tableSizeFor` 方法算出的结果

public static void main(String[] args) { 
      for(int i = 0; i < 20; i++) { 
        System.out.println(i+":"+tableSizeFor(i));
      }
    }
    static final int MAXIMUM_CAPACITY = 1 << 30;
    
    static final int tableSizeFor(int cap) { 
      int n = cap - 1;
      n |= n >>> 1;
      n |= n >>> 2;
      n |= n >>> 4;
      n |= n >>> 8;
      n |= n >>> 16;
      return (n < 0) ? 1 : (n >= MAXIMUM_CAPACITY) ? MAXIMUM_CAPACITY : n + 1;
    }

分析输出，这里输出的规律是，最小的2的幂

0:1
    1:1
    2:2
    3:4
    4:4
    5:8
    6:8
    7:8
    8:8
    9:16
    10:16
    11:16
    12:16
    13:16
    14:16
    15:16
    16:16
    17:32
    18:32
    19:32

`put(k,v)`如果是第一次调用会直接调用`resize()`计算`threshold`分配`table`大小，方法在最后会判断`size`大小是否大于 `threshold`,如果插入元素大于`threshold`则调用`resize()`,然后重新计算 `threshold`，调整`table`的大小.阅读源码发现`threshold`计算方式是`table`大小乘负载系数 `DEFAULT_LOAD_FACTOR`.也就是说如果`new HashMap(8)`，在第一次`put(k,v)`时会计算出`threshold`为6(`负载系数0.75`)，在第6个参数`put(k,v)`时，就会重新扩容。扩容的算法使用左移`<<`算出结果等同`*2`。如果想防止二次扩容可以使用`实际参数数量 / 负载系数(默认0.75) + 1`.

/** * Implements Map.put and related methods * * @param hash hash for key * @param key the key * @param value the value to put * @param onlyIfAbsent if true, don't change existing value * @param evict if false, the table is in creation mode. * @return previous value, or null if none */
    final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
                    boolean evict) { 
        Node<K,V>[] tab; Node<K,V> p; int n, i;
        if ((tab = table) == null || (n = tab.length) == 0)
            n = (tab = resize()).length;
        if ((p = tab[i = (n - 1) & hash]) == null)
            tab[i] = newNode(hash, key, value, null);
        else { 
            Node<K,V> e; K k;
            if (p.hash == hash &&
                ((k = p.key) == key || (key != null && key.equals(k))))
                e = p;
            else if (p instanceof TreeNode)
                e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
            else { 
                for (int binCount = 0; ; ++binCount) { 
                    if ((e = p.next) == null) { 
                        p.next = newNode(hash, key, value, null);
                        if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
                            treeifyBin(tab, hash);
                        break;
                    }
                    if (e.hash == hash &&
                        ((k = e.key) == key || (key != null && key.equals(k))))
                        break;
                    p = e;
                }
            }
            if (e != null) {  // existing mapping for key
                V oldValue = e.value;
                if (!onlyIfAbsent || oldValue == null)
                    e.value = value;
                afterNodeAccess(e);
                return oldValue;
            }
        }
        ++modCount;
        if (++size > threshold)
            resize();
        afterNodeInsertion(evict);
        return null;
    }
    
    final Node<K,V>[] resize() { 
        Node<K,V>[] oldTab = table;
        int oldCap = (oldTab == null) ? 0 : oldTab.length;
        int oldThr = threshold;
        int newCap, newThr = 0;
        if (oldCap > 0) { 
            if (oldCap >= MAXIMUM_CAPACITY) { 
                threshold = Integer.MAX_VALUE;
                return oldTab;
            }
            else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY &&
                      oldCap >= DEFAULT_INITIAL_CAPACITY)
                newThr = oldThr << 1; // double threshold
        }
        else if (oldThr > 0) // initial capacity was placed in threshold
            newCap = oldThr;
        else {                // zero initial threshold signifies using defaults
            newCap = DEFAULT_INITIAL_CAPACITY;
            newThr = (int)(DEFAULT_LOAD_FACTOR * DEFAULT_INITIAL_CAPACITY);
        }
        if (newThr == 0) { 
            float ft = (float)newCap * loadFactor;
            newThr = (newCap < MAXIMUM_CAPACITY && ft < (float)MAXIMUM_CAPACITY ?
                      (int)ft : Integer.MAX_VALUE);
        }
        threshold = newThr;
        @SuppressWarnings({ "rawtypes","unchecked"})
            Node<K,V>[] newTab = (Node<K,V>[])new Node[newCap];
        table = newTab;
        if (oldTab != null) { 
            for (int j = 0; j < oldCap; ++j) { 
                Node<K,V> e;
                if ((e = oldTab[j]) != null) { 
                    oldTab[j] = null;
                    if (e.next == null)
                        newTab[e.hash & (newCap - 1)] = e;
                    else if (e instanceof TreeNode)
                        ((TreeNode<K,V>)e).split(this, newTab, j, oldCap);
                    else {  // preserve order
                        Node<K,V> loHead = null, loTail = null;
                        Node<K,V> hiHead = null, hiTail = null;
                        Node<K,V> next;
                        do { 
                            next = e.next;
                            if ((e.hash & oldCap) == 0) { 
                                if (loTail == null)
                                    loHead = e;
                                else
                                    loTail.next = e;
                                loTail = e;
                            }
                            else { 
                                if (hiTail == null)
                                    hiHead = e;
                                else
                                    hiTail.next = e;
                                hiTail = e;
                            }
                        } while ((e = next) != null);
                        if (loTail != null) { 
                            loTail.next = null;
                            newTab[j] = loHead;
                        }
                        if (hiTail != null) { 
                            hiTail.next = null;
                            newTab[j + oldCap] = hiHead;
                        }
                    }
                }
            }
        }
        return newTab;
    }

# 总结 #

如果想防止二次扩容可以使用此算法: `实际参数数量 / 负载系数(默认0.75) + 1`.