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概述

从类的继承图上我们可知道，ArrayList实现了List接口。

• 同时List是顺序容器，即元素存放的数据与放进去的顺序相同，允许放入null元素，

• ArrayList底层基于数组实现。

• 每个ArrayList都有一个容量(capacity)，表示底层数组的实际大小，容器内存储元素的个数不能多于当前容量。

• 当向容器中添加元素时，如果容量不足，容器自动扩容。

• ArrayList<E>，可以看到是泛型类型， Java泛型只是编译器提供的语法糖，数组是一个Object数组，可以容纳任何类型的对象。
  

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方法的执行效率

• size(), isEmpty(), get(), set()方法均能在常数时间内完成
• add()方法的时间开销跟插入位置有关
• addAll()方法的时间开销跟添加元素的个数成正比。
• 其余方法大都是线性时间。

为追求效率，ArrayList没有实现同步(synchronized)，如果需要多个线程并发访问，用户可以手动同步，也可使用Vector替代

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源码剖析

底层数据结构 -数组

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构造函数

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

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

    /**
     * Constructs a list containing the elements of the specified
     * collection, in the order they are returned by the collection's
     * iterator.
     *
     * @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) {
        Object[] a = c.toArray();
        if ((size = a.length) != 0) {
            if (c.getClass() == ArrayList.class) {
                elementData = a;
            } else {
                elementData = Arrays.copyOf(a, size, Object[].class);
            }
        } else {
            // replace with empty array.
            elementData = EMPTY_ELEMENTDATA;
        }
    }

演示如下：

        /**
         * 初始化的时候指定容量
         */
        List list = new ArrayList<>(1);
        list.add(1);
        list.add(2);
        System.out.println(list.size());

        /**
         * 默认构造函数 ，数组大小为0
         */
        list = new ArrayList();
        list.add("artisan");
        list.add("review");
        list.add("java");
        System.out.println(list.size());



        /**
         * 使用集合初始化一个ArrayList
         */
        list = new ArrayList(Arrays.asList("I" , "Love" ,"Code"));
        System.out.println(list.size());

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自动扩容机制

• 每当向数组中添加元素时，都需要检查添加后元素的个数是否会超出当前数组的长度，如果超出，数组将会进行扩容，以满足添加数据的需求。
  

• 数组进行扩容时，会将老数组中的元素重新拷贝一份到新的数组中，每次数组容量的增长大约是其原容量的1.5倍。

  这种操作的代价是很高的，因此在实际使用时，我们应该尽量避免数组容量的扩张。当我们可预知要保存的元素的多少时，要在构造ArrayList实例时，就指定其容量，以避免数组扩容的发生。

  或者根据实际需求，通过调用ensureCapacity方法来手动增加ArrayList实例的容量。

• ArrayList#ensureCapacity(int minCapacity)暴漏了public方法可以允许程序猿手工扩容增加ArrayList实例的容量，以减少递增式再分配的数量。

我们来看下效率对比

 /**
         * 扩容对比
         */

        long begin = System.currentTimeMillis();
        // 初始化1亿的数据量
        final int number = 100000000 ;
        Object o = new Object();
        ArrayList list1 = new ArrayList<String>();
        for (int i = 0; i < number; i++) {
            list1.add(o);
        }
        System.out.println("依赖ArrayList的自动扩容机制，添加数据耗时:" +(System.currentTimeMillis() - begin));


        begin = System.currentTimeMillis();
        ArrayList list2 = new ArrayList<String>();
        // 手工扩容
        list2.ensureCapacity(number);
        for (int i = 0; i < number; i++) {
            list2.add(o);
        }
        System.out.println("手工ensureCapacity扩容后，添加数据耗时:" + (System.currentTimeMillis() - begin));

原因是因为，第一段如果没有一次性扩到想要的最大容量的话，它就会在添加元素的过程中，一点一点的进行扩容，要知道对数组扩容是要进行数组拷贝的，这就会浪费大量的时间。如果已经预知容器可能会装多少元素，最好显示的调用ensureCapacity这个方法一次性扩容到位。

过程图如下：

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set()

底层是一个数组, 那ArrayList的set()方法也就是直接对数组的指定位置赋值

    /**
     * Replaces the element at the specified position in this list with
     * the specified element.
     *
     * @param index index of the element to replace
     * @param element element to be stored at the specified position
     * @return the element previously at the specified position
     * @throws IndexOutOfBoundsException {@inheritDoc}
     */
    public E set(int index, E element) {
        rangeCheck(index);

        E oldValue = elementData(index);
        elementData[index] = element;
        return oldValue;
    }

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get

get()方法也很简单，需要注意的是由于底层数组是Object[]，得到元素后需要进行类型转换。

  /**
     * Returns the element at the specified position in this list.
     *
     * @param  index index of the element to return
     * @return the element at the specified position in this list
     * @throws IndexOutOfBoundsException {@inheritDoc}
     */
    public E get(int index) {
        rangeCheck(index);

        return elementData(index);
    }

  @SuppressWarnings("unchecked")
    E elementData(int index) {
        return (E) elementData[index];
    }

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add()/addAll()

这两个方法都是向容器中添加新元素，这可能会导致capacity不足，因此在添加元素之前，都需要进行剩余空间检查，如果需要则自动扩容。扩容操作最终是通过grow()方法完成的

/**
     * 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);  // Increments modCount!!
        elementData[size++] = e;
        return true;
    }

    /**
     * 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);

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

• add(E e) 在末尾添加
• add(int index, E e)需要先对元素进行移动，然后完成插入操作，也就意味着该方法有着线性的时间复杂度。

• addAll()方法能够一次添加多个元素，根据位置不同也有两个把本

  一个是在末尾添加的addAll(Collection<? extends E> c)方法，

  一个是从指定位置开始插入的addAll(int index, Collection<? extends E> c)方法。

  跟add()方法类似，在插入之前也需要进行空间检查，如果需要则自动扩容；如果从指定位置插入，也会存在移动元素的情况。]

  addAll()的时间复杂度不仅跟插入元素的多少有关，也跟插入的位置相关。

 /**
     * Appends all of the elements in the specified collection to the end of
     * this list, in the order that they are returned by the
     * specified collection's Iterator.  The behavior of this operation is
     * undefined if the specified collection is modified while the operation
     * is in progress.  (This implies that the behavior of this call is
     * undefined if the specified collection is this list, and this
     * list is nonempty.)
     *
     * @param c collection containing elements to be added to this list
     * @return <tt>true</tt> if this list changed as a result of the call
     * @throws NullPointerException if the specified collection is null
     */
    public boolean addAll(Collection<? extends E> c) {
        Object[] a = c.toArray();
        int numNew = a.length;
        ensureCapacityInternal(size + numNew);  // Increments modCount
        System.arraycopy(a, 0, elementData, size, numNew);
        size += numNew;
        return numNew != 0;
    }

    /**
     * Inserts all of the elements in the specified collection into this
     * list, starting at the specified position.  Shifts the element
     * currently at that position (if any) and any subsequent elements to
     * the right (increases their indices).  The new elements will appear
     * in the list in the order that they are returned by the
     * specified collection's iterator.
     *
     * @param index index at which to insert the first element from the
     *              specified collection
     * @param c collection containing elements to be added to this list
     * @return <tt>true</tt> if this list changed as a result of the call
     * @throws IndexOutOfBoundsException {@inheritDoc}
     * @throws NullPointerException if the specified collection is null
     */
    public boolean addAll(int index, Collection<? extends E> c) {
        rangeCheckForAdd(index);

        Object[] a = c.toArray();
        int numNew = a.length;
        ensureCapacityInternal(size + numNew);  // Increments modCount

        int numMoved = size - index;
        if (numMoved > 0)
            System.arraycopy(elementData, index, elementData, index + numNew,
                             numMoved);

        System.arraycopy(a, 0, elementData, index, numNew);
        size += numNew;
        return numNew != 0;
    }

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remove()

remove()方法也有两个方法

• 一个是remove(int index)删除指定位置的元素
• 一个是remove(Object o)删除第一个满足o.equals(elementData[index])的元素

 /**
     * Removes the element at the specified position in this list.
     * Shifts any subsequent elements to the left (subtracts one from their
     * indices).
     *
     * @param index the index of the element to be removed
     * @return the element that was removed from the list
     * @throws IndexOutOfBoundsException {@inheritDoc}
     */
    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;
    }

删除操作是add()操作的逆过程，需要将删除点之后的元素向前移动一个位置。需要注意的是为了让GC起作用，必须显式的为最后一个位置赋null值。

上面代码中如果不手动赋null值，除非对应的位置被其他元素覆盖，否则原来的对象就一直不会被回收。

    /**
     * Removes the first occurrence of the specified element from this list,
     * if it is present.  If the list does not contain the element, it is
     * unchanged.  More formally, removes the element with the lowest index
     * <tt>i</tt> such that
     * <tt>(o==null&nbsp;?&nbsp;get(i)==null&nbsp;:&nbsp;o.equals(get(i)))</tt>
     * (if such an element exists).  Returns <tt>true</tt> if this list
     * contained the specified element (or equivalently, if this list
     * changed as a result of the call).
     *
     * @param o element to be removed from this list, if present
     * @return <tt>true</tt> if this list contained the specified element
     */
    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;
    }

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trimToSize()

将底层数组的容量调整为当前列表保存的实际元素的大小

  /**
     * Trims the capacity of this <tt>ArrayList</tt> instance to be the
     * list's current size.  An application can use this operation to minimize
     * the storage of an <tt>ArrayList</tt> instance.
     */
    public void trimToSize() {
        modCount++;
        if (size < elementData.length) {
            elementData = (size == 0)
              ? EMPTY_ELEMENTDATA
              : Arrays.copyOf(elementData, size);
        }
    }

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indexOf(), lastIndexOf()

获取元素的第一次出现的index

   /**
     * Returns the index of the first occurrence of the specified element
     * in this list, or -1 if this list does not contain the element.
     * More formally, returns the lowest index <tt>i</tt> such that
     * <tt>(o==null&nbsp;?&nbsp;get(i)==null&nbsp;:&nbsp;o.equals(get(i)))</tt>,
     * or -1 if there is no such index.
     */
    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;
    }

获取元素的最后一次出现的index

    /**
     * Returns the index of the last occurrence of the specified element
     * in this list, or -1 if this list does not contain the element.
     * More formally, returns the highest index <tt>i</tt> such that
     * <tt>(o==null&nbsp;?&nbsp;get(i)==null&nbsp;:&nbsp;o.equals(get(i)))</tt>,
     * or -1 if there is no such index.
     */
    public int lastIndexOf(Object o) {
        if (o == null) {
            for (int i = size-1; i >= 0; i--)
                if (elementData[i]==null)
                    return i;
        } else {
            for (int i = size-1; i >= 0; i--)
                if (o.equals(elementData[i]))
                    return i;
        }
        return -1;
    }

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Fail-Fast

ArrayList同样采用了快速失败的机制，通过记录modCount参数来实现。在面对并发的修改时，迭代器很快就会完全失败，而不是冒着在将来某个不确定时间发生任意不确定行为的风险。

具体参考前段时间写的一篇博文如下：

Java - Java集合中的快速失败Fail Fast 机制