集合之ArrayList

阅读更多

public class ArrayList extends AbstractList
        implements List, RandomAccess, Cloneable, java.io.Serializable
{
    private static final long serialVersionUID = 8683452581122892189L;

    /**
     * 默认容量为10
     */
    private static final int DEFAULT_CAPACITY = 10;

    /**
     * 空的数组
     */
    private static final Object[] EMPTY_ELEMENTDATA = {};

    /**
     * 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 = {};

    /**
     * 用于存储数据的数组，创建ArrayList的时候被初始化
     */
    transient Object[] elementData; // non-private to simplify nested class access

    /**
     *元素的数量（不是elementData的长度）
     */
    private int size;

    /**
     *指定容量的构造方法
     ** 	如果确定列表长度，可以使用此方法来创建列表，省去了扩容的消耗
     ** 	长度小于10的情况，也可以使用此方法来创建列表 节省空间	
     */
    public ArrayList(int initialCapacity) {
    	/**
    	 * initialCapacity大于0时，创建initialCapacity长度的数组
    	 */
        if (initialCapacity > 0) {
            this.elementData = new Object[initialCapacity];
        }
        //initialCapacity为0时，就使用空数组EMPTY_ELEMENTDATA
        else if (initialCapacity == 0) {
            this.elementData = EMPTY_ELEMENTDATA;
        }
        //initialCapacity小于0时抛出异常
        else {
            throw new IllegalArgumentException("Illegal Capacity: "+
                                               initialCapacity);
        }
    }

    /**
     * 默认构造器：创建一个空数组
     */
    public ArrayList() {
        this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
    }

    /**
     * 初始化一个ArrayList，包含c集合的所有元素
     */
    public ArrayList(Collection c) {
    	//调用Collection的toArray返回一个数组
        elementData = c.toArray();
        
        if ((size = elementData.length) != 0) {
        	//toArray返回的可能不是Object[]类型
        	/**以下情况返回的是String类型
        	 * String[] ss = new String[10];
		 	 * List l = Arrays.asList(ss);
		 	 * System.out.println(l.toArray());
        	 */
            if (elementData.getClass() != Object[].class)
                elementData = Arrays.copyOf(elementData, size, Object[].class);
        } else {
            // 如果size为0，elementData为空数组
            this.elementData = EMPTY_ELEMENTDATA;
        }
    }

    /**
     * 清除elementData中空余的空间（elementData的长度可能大于size）
     */
    public void trimToSize() {
        modCount++;
        /**
         * 如果size小于elementData的长度，就清除空闲空间
         */
        if (size < elementData.length) {
            elementData = (size == 0)
              ? EMPTY_ELEMENTDATA
            //重新生成一个size长度的数组，并将elementData中的元素拷贝进来
              : Arrays.copyOf(elementData, size);
        }
    }

    /**
     * 确保容量大小（共外部调用）
     */
    public void ensureCapacity(int minCapacity) {
        int minExpand = (elementData != DEFAULTCAPACITY_EMPTY_ELEMENTDATA)
            /**
             * 如果elementData != DEFAULTCAPACITY_EMPTY_ELEMENTDATA说明最小容量为minCapacity
             */
            ? 0
            /**
             * 如果elementData= DEFAULTCAPACITY_EMPTY_ELEMENTDATA需要扩展，默认初始化长度为10
             */
            : DEFAULT_CAPACITY;

        if (minCapacity > minExpand) {
            ensureExplicitCapacity(minCapacity);
        }
    }

    /**
     * 确保容量大小（内部调用）
     */
    private void ensureCapacityInternal(int minCapacity) {
        if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
        	//最小容量为DEFAULT_CAPACITY 10
            minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);
        }

        ensureExplicitCapacity(minCapacity);
    }

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

        // minCapacity大于elementData.length就扩展
        if (minCapacity - elementData.length > 0)
            grow(minCapacity);
    }

    /**
     * The maximum size of array to allocate.
     * Some VMs reserve some header words in an array.
     * Attempts to allocate larger arrays may result in
     * OutOfMemoryError: Requested array size exceeds VM limit
     */
    private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;

    /**
     * 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) {
        // 获得当前数组长度
        int oldCapacity = elementData.length;
        //新的数组长度=当前数组长度+0.5倍
        int newCapacity = oldCapacity + (oldCapacity >> 1);
        //如果新的数组长度小于最小容量minCapacity，则新的数组长度为minCapacity
        if (newCapacity - minCapacity < 0)
            newCapacity = minCapacity;
        /**
         * 新的数组长度大于MAX_ARRAY_SIZE（Integer.MAX_VALUE - 8），调用hugeCapacity
         * 	要么抛出OOM，要么扩展到Integer.MAX_VALUE长度
         */
        if (newCapacity - MAX_ARRAY_SIZE > 0)
            newCapacity = hugeCapacity(minCapacity);
        //创建一个newCapacity长度的新数组，拷贝之前的数据
        elementData = Arrays.copyOf(elementData, newCapacity);
    }

    private static int hugeCapacity(int minCapacity) {
    	/**
    	 * 如果当前数组长度已经达到Integer.MAX_VALUE，此时再继续添加数据就会得到负值
    	 * 	Integer.MAX_VALUE+1 <0
    	 */
        if (minCapacity < 0) // overflow
            throw new OutOfMemoryError();
        return (minCapacity > MAX_ARRAY_SIZE) ?
            Integer.MAX_VALUE ://最多扩展到Integer.MAX_VALUE长度
            MAX_ARRAY_SIZE;
    }

    /**
     *返回当前列表内元素的个数
     */
    public int size() {
        return size;
    }

    /**
     * 返回当前列表内是否有元素
     */
    public boolean isEmpty() {
        return size == 0;
    }

    /**
     * 判断o是否在列表里
     */
    public boolean contains(Object o) {
        return indexOf(o) >= 0;
    }

    /**
     * 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 i such that
     * (o==null ? get(i)==null : o.equals(get(i))),
     * 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;
    }

    /**
     * 返回o在列表中的位置
     */
    public int lastIndexOf(Object o) {
    	/**
    	 * o为null时，遍历数组（最大长度为size）通过==来判断
    	 */
        if (o == null) {
            for (int i = size-1; i >= 0; i--)
                if (elementData[i]==null)
                    return i;
        } else {
        	/**
        	 * o不为null时，遍历数组（最大长度为size）通过equals来判断
        	 * 	如果某个对象重写equals方法，两个不同的对象可能相等
        	 */
            for (int i = size-1; i >= 0; i--)
                if (o.equals(elementData[i]))
                    return i;
        }
        return -1;
    }

    /**
     * 拷贝一份列表，只是拷贝了元素的引用
     */
    public Object clone() {
        try {
            ArrayList v = (ArrayList) super.clone();
            v.elementData = Arrays.copyOf(elementData, size);
            v.modCount = 0;
            return v;
        } catch (CloneNotSupportedException e) {
            // this shouldn't happen, since we are Cloneable
            throw new InternalError(e);
        }
    }


    public Object[] toArray() {
        return Arrays.copyOf(elementData, size);
    }

    @SuppressWarnings("unchecked")
    public  T[] toArray(T[] a) {
        if (a.length < size)
            // a的长度小于列表元素个数时，就只创建一个size长度的新数组
            return (T[]) Arrays.copyOf(elementData, size, a.getClass());
        System.arraycopy(elementData, 0, a, 0, size);
        if (a.length > size)
            a[size] = null;//将第size值设为null
        return a;
    }

    // Positional Access Operations

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

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

        return elementData(index);
    }

    public E set(int index, E element) {
        rangeCheck(index);

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

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

    public void add(int index, E element) {
        rangeCheckForAdd(index);

        ensureCapacityInternal(size + 1);  // Increments modCount!!
        //将从index开始的元素都往后移一位
        System.arraycopy(elementData, index, elementData, index + 1,
                         size - index);
        elementData[index] = element;
        size++;
    }

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

        modCount++;
        E oldValue = elementData(index);
        /**
         * 计算需要移动的元素个数
         */
        int numMoved = size - index - 1;
        if (numMoved > 0)
            System.arraycopy(elementData, index+1, elementData, index,
                             numMoved);
        //列表最后一个元素设为null（不是数组最后一位设为null）
        elementData[--size] = null; // clear to let GC do its work

        return oldValue;
    }

    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) {
    	//不需要检查边界
    	
        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
    }

    /**
     * Removes all of the elements from this list.  The list will
     * be empty after this call returns.
     */
    public void clear() {
        modCount++;

        //只是将元素全部置为null，elementData数组长度没变
        for (int i = 0; i < size; i++)
            elementData[i] = null;

        size = 0;
    }

    /**
     * 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 true if this list changed as a result of the call
     * @throws NullPointerException if the specified collection is null
     */
    public boolean addAll(Collection 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 true 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 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;
    }

    /**
     * 实现原理：将toIndex之后的元素往前移动到fromIndex之后
     */
    protected void removeRange(int fromIndex, int toIndex) {
        modCount++;
        int numMoved = size - toIndex;
        System.arraycopy(elementData, toIndex, elementData, fromIndex,
                         numMoved);

        // clear to let GC do its work
        int newSize = size - (toIndex-fromIndex);
        for (int i = newSize; i < size; i++) {
            elementData[i] = null;
        }
        size = newSize;
    }

    /**
     * Checks if the given index is in range.  If not, throws an appropriate
     * runtime exception.  This method does *not* check if the index is
     * negative: It is always used immediately prior to an array access,
     * which throws an ArrayIndexOutOfBoundsException if index is negative.
     */
    private void rangeCheck(int index) {
        if (index >= size)
            throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
    }

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

    /**
     * Constructs an IndexOutOfBoundsException detail message.
     * Of the many possible refactorings of the error handling code,
     * this "outlining" performs best with both server and client VMs.
     */
    private String outOfBoundsMsg(int index) {
        return "Index: "+index+", Size: "+size;
    }

    /**
     * Removes from this list all of its elements that are contained in the
     * specified collection.
     *
     * @param c collection containing elements to be removed from this list
     * @return {@code true} if this list changed as a result of the call
     * @throws ClassCastException if the class of an element of this list
     *         is incompatible with the specified collection
     * (optional)
     * @throws NullPointerException if this list contains a null element and the
     *         specified collection does not permit null elements
     * (optional),
     *         or if the specified collection is null
     * @see Collection#contains(Object)
     */
    public boolean removeAll(Collection c) {
        Objects.requireNonNull(c);
        return batchRemove(c, false);
    }

    
    /**
     * 保留c中的元素，其他的元素都删除
     * @param c
     * @return
     */
    public boolean retainAll(Collection c) {
        Objects.requireNonNull(c);
        return batchRemove(c, true);
    }

    /**
     * 批量删除
     * @param c
     * @param complement true表示删除c以外的元素 false表示删除c中所含的元素
     * @return
     */
    private boolean batchRemove(Collection c, boolean complement) {
        final Object[] elementData = this.elementData;
        int r = 0, w = 0;
        boolean modified = false;
        try {
            for (; r < size; r++)
                if (c.contains(elementData[r]) == complement)
                    elementData[w++] = elementData[r];
        } finally {
            // Preserve behavioral compatibility with AbstractCollection,
            // even if c.contains() throws.
            if (r != size) {
                System.arraycopy(elementData, r,
                                 elementData, w,
                                 size - r);
                w += size - r;
            }
            if (w != size) {
                // clear to let GC do its work
                for (int i = w; i < size; i++)
                    elementData[i] = null;
                modCount += size - w;
                size = w;
                modified = true;
            }
        }
        return modified;
    }
}

 

相关的面试题：

     

1、ArrayList的大小是如何自动增加的？你能分享一下你的代码吗？

    arrayList每次新增元素的时候回确保容量是否够用，如果不够用会扩大容量，重新创建一个更大容量的数组，并将原有元素拷贝进去，具体代码如下

    private void grow(int minCapacity) {
        // 获得当前数组长度
        int oldCapacity = elementData.length;
        //新的数组长度=当前数组长度+0.5倍
        int newCapacity = oldCapacity + (oldCapacity >> 1);
        //如果新的数组长度小于最小容量minCapacity，则新的数组长度为minCapacity
        if (newCapacity - minCapacity < 0)
            newCapacity = minCapacity;
        /**
         * 新的数组长度大于MAX_ARRAY_SIZE（Integer.MAX_VALUE - 8），调用hugeCapacity
         * 	要么抛出OOM，要么扩展到Integer.MAX_VALUE长度
         */
        if (newCapacity - MAX_ARRAY_SIZE > 0)
            newCapacity = hugeCapacity(minCapacity);
        //创建一个newCapacity长度的新数组，拷贝之前的数据
        elementData = Arrays.copyOf(elementData, newCapacity);
    }

 

2、什么情况下你会使用ArrayList？什么时候你会选择LinkedList？

这又是一个大多数面试者都会困惑的问题。多数情况下，当你遇到访问元素比插入或者是删除元素更加频繁的时候，你应该使用ArrayList。另外一方面，当你在某个特别的索引中，插入或者是删除元素更加频繁，或者你压根就不需要访问元素的时候，你会选择LinkedList。这里的主要原因是，在ArrayList中访问元素的最糟糕的时间复杂度是”1″，而在LinkedList中可能就是”n”了。在ArrayList中增加或者删除某个元素，通常会调用System.arraycopy方法，这是一种极为消耗资源的操作，因此，在频繁的插入或者是删除元素的情况下，LinkedList的性能会更加好一点。

3、当传递ArrayList到某个方法中，或者某个方法返回ArrayList，什么时候要考虑安全隐患？如何修复安全违规这个问题呢？

当array被当做参数传递到某个方法中，如果array在没有被复制的情况下直接被分配给了成员变量，那么就可能发生这种情况，即当原始的数组被调用的方法改变的时候，传递到这个方法中的数组也会改变。下面的这段代码展示的就是安全违规以及如何修复这个问题。

ArrayList被直接赋给成员变量——安全隐患：

修复这个安全隐患：

4、如何复制某个ArrayList到另一个ArrayList中去？写出你的代码？

下面就是把某个ArrayList复制到另一个ArrayList中去的几种技术：

1. 使用clone()方法，比如ArrayList newArray = oldArray.clone();
2. 使用ArrayList构造方法，比如：ArrayList myObject = new ArrayList(myTempObject);
3. 使用Collection的copy方法。

注意1和2是浅拷贝(shallow copy)。

5、在索引中ArrayList的增加或者删除某个对象的运行过程？效率很低吗？解释一下为什么？

在ArrayList中增加或者是删除元素，要调用System.arraycopy这种效率很低的操作，如果遇到了需要频繁插入或者是删除的时候，你可以选择其他的Java集合，比如LinkedList。看一下下面的代码：

在ArrayList的某个索引i处添加元素：

    public void add(int index, E element) {
        rangeCheckForAdd(index);

        ensureCapacityInternal(size + 1);  // Increments modCount!!
        //将从index开始的元素都往后移一位
        System.arraycopy(elementData, index, elementData, index + 1,
                         size - index);
        elementData[index] = element;
        size++;
    }

 删除ArrayList的某个索引i处的元素：

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

        modCount++;
        E oldValue = elementData(index);
        /**
         * 计算需要移动的元素个数
         */
        int numMoved = size - index - 1;
        if (numMoved > 0)
            System.arraycopy(elementData, index+1, elementData, index,
                             numMoved);
        //列表最后一个元素设为null（不是数组最后一位设为null）
        elementData[--size] = null; // clear to let GC do its work

        return oldValue;
    }