java ArrayList

1. java集合框架的結構圖

圖上的ArrayList機實現了List接口，但是查看源碼如下

public class ArrayList<E> extends AbstractList<E>
        implements List<E>, RandomAccess, Cloneable, java.io.Serializable
{

ArrayList繼承了AbstractList，實現了List<E>, RandomAccess, Cloneable, java.io.Serializable接口

2.ArrayList初始化容量爲10

/**
     * Default initial capacity.
     */
    private static final int DEFAULT_CAPACITY = 10;

3. ArrayList底層爲數組實現

/**
     * The array buffer into which the elements of the ArrayList are stored.
     * The capacity of the ArrayList is the length of this array buffer. Any
     * empty ArrayList with elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA
     * will be expanded to DEFAULT_CAPACITY when the first element is added.
     */
    transient Object[] elementData; // non-private to simplify nested class access

4. size屬性值爲當前數組存放數據個數

/**
     * The size of the ArrayList (the number of elements it contains).
     *
     * @serial
     */
    private int size;

5.ArrayList得構造方法：

  a.無參構造函數，直接引用一個未得到數組初始化得數組

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

  b.指定容量的有參構造函數，initialCapacity爲初始化容量，源碼可知：

     如果初始話容量大於0，直接進行初始化。

     如果等於0，則直接進行空數組得引用。

    如果小於0，則直接異常

public ArrayList(int initialCapacity) {
        if (initialCapacity > 0) {
            this.elementData = new Object[initialCapacity];
        } else if (initialCapacity == 0) {
            this.elementData = EMPTY_ELEMENTDATA;
        } else {
            throw new IllegalArgumentException("Illegal Capacity: "+
                                               initialCapacity);
        }
    }

c.參數爲集合，將集合轉化爲數組，如果長度爲0，則直接引用空數組。如果不爲0則構造一個包含指定Collention 元素的列表

    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;
        }
    }

6. trimToSize方法，將ArrayList轉化爲實際長度得數組：

    public void trimToSize() {
        modCount++;
        if (size < elementData.length) {
            elementData = (size == 0)
              ? EMPTY_ELEMENTDATA
              : Arrays.copyOf(elementData, size);
        }
    }

例如：初始化一個ArrayList，初始容量爲10，但是隻向集合中添加一個值，這時數組得長度爲10，但是size爲1，此方法就是讓數組length和size保持一致

7.add方法：

首先進行容量校驗，然後再當前數組實際長度的下一位添加數據

    public boolean add(E e) {
        ensureCapacityInternal(size + 1);  // Increments modCount!!
        elementData[size++] = e;
        return true;
    }

此方法是判斷數組容量，參數爲 : 實際大小+1,

    private void ensureCapacityInternal(int minCapacity) {
        ensureExplicitCapacity(calculateCapacity(elementData, minCapacity));
    }

如果當前數組爲空數組，則比較 size+1 和 默認大小10 ，取大得值，返回得值將傳給下面得方法

    private static int calculateCapacity(Object[] elementData, int minCapacity) {
        if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
            return Math.max(DEFAULT_CAPACITY, minCapacity);
        }
        return minCapacity;
    }

如果獲取到得大小大於當前數組得長度，則需要根據獲得到的數組大小進行擴容，擴容方法如下

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

        // overflow-conscious code
        if (minCapacity - elementData.length > 0)
            grow(minCapacity);
    }

首先獲取當前數組得長度，然後計算出新數組得長度爲：old+old*0.5,然後進行數組拷貝，指向新數組

    private void grow(int minCapacity) {
        // overflow-conscious code
        int oldCapacity = elementData.length;
        int newCapacity = oldCapacity + (oldCapacity >> 1);
        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:
        elementData = Arrays.copyOf(elementData, newCapacity);
    }

8. 刪除元素：

a.按照位置刪除元素

首先利用rangeCheck方法判斷index與size得大小，如果index大於size，那麼直接報錯。

然後獲取指定下標得值，計算移動得是第幾個元素，然後進行數組拷貝，將需要刪除得元素放在最後，賦值爲nu'l'l

    public E remove(int index) {
        rangeCheck(index);

        modCount++;
        E oldValue = elementData(index);

        int numMoved = size - index - 1;
        if (numMoved > 0)
            System.arraycopy(elementData, index+1, elementData, index,
                             numMoved);
        elementData[--size] = null; // clear to let GC do its work

        return oldValue;
    }

b.按照值刪除元素

首先判斷要移除得元素是否爲空，如果爲空，則尋找第一個爲空的元素，刪除。不爲空也同樣是進行查找後進行刪除

    public boolean remove(Object o) {
        if (o == null) {
            for (int index = 0; index < size; index++)
                if (elementData[index] == null) {
                    fastRemove(index);
                    return true;
                }
        } else {
            for (int index = 0; index < size; index++)
                if (o.equals(elementData[index])) {
                    fastRemove(index);
                    return true;
                }
        }
        return false;
    }

    private void fastRemove(int index) {
        modCount++;
        int numMoved = size - index - 1;
        if (numMoved > 0)
            System.arraycopy(elementData, index+1, elementData, index,
                             numMoved);
        elementData[--size] = null; // clear to let GC do its work
    }

9. 清空ArrayList

    循環置空

    public void clear() {
        modCount++;

        // clear to let GC do its work
        for (int i = 0; i < size; i++)
            elementData[i] = null;

        size = 0;
    }