Java 集合系列（二）ArrayList详解

ArrayList

ArrayList 是通过一个数组来实现的，因此它是在连续的存储位置存放对象的引用，只不过它比 Array 更智能，能够根据集合长度进行自动扩容。

假设让我们来实现一个简单的能够自动扩容的数组，我们最容易想到的点就是：

1. add()的时候需要判断当前数组size+1是否等于此时定义的数组大小；
2. 若小于直接添加即可；否则，需要先扩容再进行添加。

实际上，ArrayList的内部实现原理也是这样子，我们可以来研究分析一下ArrayList的源码

add(E e) 源码分析

/**
   * Appends the specified element to the end of this list.
   *
   * @param e element to be appended to this list
   * @return <tt>true</tt> (as specified by {@link Collection#add})
   */
  public boolean add(E e) {
    ensureCapacityInternal(size + 1);  // 进行扩容校验
    elementData[size++] = e;      // 将值添加到数组后面，并将 size+1
    return true;
  }

  /**
   * 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

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

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

  /**
   * Shared empty array instance used for default sized empty instances. We
   * distinguish this from EMPTY_ELEMENTDATA to know how much to inflate when
   * first element is added.
   */
  private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};

  // 返回最大的 index
  private static int calculateCapacity(Object[] elementData, int minCapacity) {
    if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {  // 与空数组实例对比
      return Math.max(DEFAULT_CAPACITY, minCapacity);
    }
    return minCapacity;
  }

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

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

扩容调用方法，实际也就是数组复制的过程

/**
   * 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) {
    // 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);
  }

add(int index, E element) 源码分析

/**
   * Inserts the specified element at the specified position in this
   * list. Shifts the element currently at that position (if any) and
   * any subsequent elements to the right (adds one to their indices).
   *
   * @param index index at which the specified element is to be inserted
   * @param element element to be inserted
   * @throws IndexOutOfBoundsException {@inheritDoc}
   */
  public void add(int index, E element) {
    rangeCheckForAdd(index);  // 校验index是否超过当前定义的数组大小范围，超过则抛出 IndexOutOfBoundsException

    ensureCapacityInternal(size + 1); // Increments modCount!!
    System.arraycopy(elementData, index, elementData, index + 1,
             size - index);   // 复制，向后移动
    elementData[index] = element;
    size++;
  }

  /**
   * A version of rangeCheck used by add and addAll.
   */
  private void rangeCheckForAdd(int index) {
    if (index > size || index < 0)
      throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
  }

从上面的源码分析可知，扩容和随机插入元素的消耗比较大，因此在实际开发中，应尽量指定ArrayList大小，减少在随机插入操作。

优缺点

优点

1. 封装了一个动态再分配的对象数组
2. 使用索引进行随机访问效率高

缺陷

1. 在数组中增删一个元素，所有元素都要往后往前移动，效率低下

知识脑图

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