深入理解-ArrayList源码解读

ArrayList

ArrayList继承自AbstractList，实现了List, RandomAccess, Cloneable, java.io.Serializable接口。ArrayList内部是一个动态数组，与Java中的数组相比，它的容量能动态增长。

ArrayList内部的属性

    // 序列化id
    private static final long serialVersionUID = 8683452581122892189L;
    //ArrayList的初始容量大小
    private static final int DEFAULT_CAPACITY = 10;
    //空对象数组
    private static final Object[] EMPTY_ELEMENTDATA = {};
    //空对象数组，如果使用默认构造函数创建，则默认对象内容默认是该值  
    private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};
    //存放当前数据，不参与序列化
    transient Object[] elementData;
    //list大小
    private int size;
    //list最大长度
    private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;

ArrayList构造方法

    //默认构造方法，默认容量是10，elementData初始化为DEFAULTCAPACITY_EMPTY_ELEMENTDATA
    public ArrayList() {
        this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
    }
    //指定容量的构造方法，传入参数小于0抛出异常
    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);
      }
    }
    /**
    *传入参数为Collection对象，先调用toArray()方法将Collection对象转换为Object[]
    *更新size的值，同时判断size的大小，如果是size等于0，直接将空对象EMPTY_ELEMENTDATA的地址赋给elementData
    *如果size的值大于0，则执行Arrays.copy方法，把collection对象的内容（可以理解为深拷贝）copy到elementData中，代码注释toArray可能不是返回Object[]，所以进行深拷贝
    **/
    public ArrayList(Collection 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;
        }
    }

ArrayList主要方法

    // 添加元素之前先检查容量，若容量不足则调用grow()方法，然后将元素添加到队尾，返回true
    public boolean add(E e) {
        ensureCapacityInternal(size + 1);  // Increments modCount!!
        elementData[size++] = e;
        return true;
    }
    /**
    *首先检查index是否在size范围之内，确保数组已使用长度（size）加1之后足够存下一个数据
    *然后将index之后的元素全部向后挪一位，再进行赋值
    **/
    public void add(int index, E element) {
        rangeCheckForAdd(index);
        ensureCapacityInternal(size + 1);  // Increments modCount!!
        System.arraycopy(elementData, index, elementData, index + 1, size - index);
        elementData[index] = element;
        size++;
    }
    /**
    *首先判断index是否超出范围，然后返回指定元素值
    **/
    public E get(int index) {
        rangeCheck(index);
        return elementData(index);
    }
    /**
    *首先判断index是否超出范围，然后将指定索引的数组元素赋值
    **/
    public E set(int index, E element) {
        rangeCheck(index);

        E oldValue = elementData(index);
        elementData[index] = element;
        return oldValue;
    }
    /**
    *首先判断index是否超出范围，numMoved 计算移除元素后需要移动的元素个数
    *通过System.arraycopy方法将后面的元素前移一位，并将最后一为赋值为null，最后返回移除元素的值
    */
    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;
    }

    /**
    *需要遍历整个list去匹配移除元素的值
    */
    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;
    }

    /**
    *需要遍历整个list去匹配元素值，然后返回第一个匹配元素的索引值，否则返回-1
    */
    public int indexOf(Object o) {
        if (o == null) {
            for (int i = 0; i < size; i++)
                if (elementData[i]==null)
                    return i;
        } else {
            for (int i = 0; i < size; i++)
                if (o.equals(elementData[i]))
                    return i;
        }
        return -1;
    }
    //minCapacity 最小即为初始容量大小10
    private void ensureCapacityInternal(int minCapacity) {
        if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
            minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);
        }
        ensureExplicitCapacity(minCapacity);
    }

    //需求长度大于现在长度，则扩充数组长度
    private void ensureExplicitCapacity(int minCapacity) {
        modCount++;
        // overflow-conscious code
        if (minCapacity - elementData.length > 0)
            grow(minCapacity);
    }
    /**
    *首先将oldCapacity 右移一位缩小1/2，newCapacity 扩大为oldCapacity 的1.5倍
    *如果newCapacity不能满足需求，那就直接扩大为minCapacity大小
    *如果newCapacity超过list最大容量，则返回Interger类型最大值
    *最后更新elementData 
    **/
    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);
    }