一、ArrayList構(gòu)造函數(shù)

ArrayList的構(gòu)造函數(shù)有三種

private static final long serialVersionUID = 8683452581122892189L;

    private static final int DEFAULT_CAPACITY = 10;

    private static final Object[] EMPTY_ELEMENTDATA = {};

    private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};

    transient Object[] elementData; // non-private to simplify nested class access

     private int size;

  /**
     * Constructs an empty list with the specified initial capacity.
     *
     * @param  initialCapacity  the initial capacity of the list
     * @throws IllegalArgumentException if the specified initial capacity
     *         is negative
     */
     public ArrayList(int initialCapacity) {
         if (initialCapacity > 0) {//固定值大于0
             this.elementData = new Object[initialCapacity];//創(chuàng)建固定空間的數(shù)組
         } else if (initialCapacity == 0) {
              this.elementData = EMPTY_ELEMENTDATA;//創(chuàng)建一個空數(shù)組
         } else {
             throw new IllegalArgumentException("Illegal Capacity: "+
                                               initialCapacity);//
         }
     }

  /**
     * Constructs an empty list with an initial capacity of ten.
     */
    public ArrayList() {
        this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
    }

  /**
     * 構(gòu)造一個包含指定元素的列表集合茫舶，按集合的返回順序迭代器械巡。
     *
     * @param c the collection whose elements are to be placed into this list
     * @throws NullPointerException if the specified collection is null
     */
    public ArrayList(Collection<? extends E> c) {
        elementData = c.toArray();
        if ((size = elementData.length) != 0) {
            // c.toArray might (incorrectly) not return Object[] (see 6260652)
            if (elementData.getClass() != Object[].class)
                elementData = Arrays.copyOf(elementData, size, Object[].class);
        } else {
            // replace with empty array.
            this.elementData = EMPTY_ELEMENTDATA;
        }
    }

二、查看add函數(shù)饶氏，探索擴容原理

以無參構(gòu)造器為例

  /**
     * Appends the specified element to the end of this list.
     * 將指定的元素追加到此列表的末尾
     * @param e element to be appended to this list
     * @return <tt>true</tt> (as specified by {@link Collection#add})
     */
    public boolean add(E e) {
        //檢查容量讥耗，調(diào)用ensureCapacityInternal方法
        ensureCapacityInternal(size + 1);  // Increments modCount!!
        elementData[size++] = e;
        return true;
    }

private static int calculateCapacity(Object[] elementData, int minCapacity) {
        if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
            return Math.max(DEFAULT_CAPACITY, minCapacity);
        }
        return minCapacity;
    }
當(dāng) 要 add 進第1個元素時，minCapacity為1疹启，在Math.max()方法比較后古程，minCapacity 為10。
private void ensureCapacityInternal(int minCapacity) {
        ensureExplicitCapacity(calculateCapacity(elementData, minCapacity));
    }

    private void ensureExplicitCapacity(int minCapacity) {
        modCount++;

        // overflow-conscious code 
        if (minCapacity - elementData.length > 0)
           //調(diào)用grow函數(shù)擴容
            grow(minCapacity);
    }

三喊崖、grow函數(shù)擴容

  /**
     * Increases the capacity to ensure that it can hold at least the
     * number of elements specified by the minimum capacity argument.
     *
     * @param minCapacity the desired minimum capacity
     */
    private void grow(int minCapacity) {
        // oldCapacity為舊容量挣磨，newCapacity為新容量
        int oldCapacity = elementData.length;
        //將oldCapacity 右移一位，其效果相當(dāng)于oldCapacity /2
        //新容量的大小為舊容量的1.5倍
        int newCapacity = oldCapacity + (oldCapacity >> 1);
        //然后檢查新容量是否大于最小需要容量荤懂，若還是小于最小需要容量茁裙，那么就把最小需要容量當(dāng)作數(shù)組的新容量，
        if (newCapacity - minCapacity < 0)
            newCapacity = minCapacity;
        if (newCapacity - MAX_ARRAY_SIZE > 0)
            newCapacity = hugeCapacity(minCapacity);
        // minCapacity is usually close to size, so this is a win:
        //復(fù)制數(shù)組中的元素
        elementData = Arrays.copyOf(elementData, newCapacity);
    }

  private static int hugeCapacity(int minCapacity) {
          if (minCapacity < 0) // overflow
              throw new OutOfMemoryError();
          return (minCapacity > MAX_ARRAY_SIZE) ?
              Integer.MAX_VALUE :
             MAX_ARRAY_SIZE;
     }