首页 > 代码库 > ArrayList源码解析(三)

ArrayList源码解析(三)

 1.isEmpty( )

   如果此列表中没有元素,则返回 true

    /**     * Returns <tt>true</tt> if this list contains no elements.*/    public boolean isEmpty() {        return size == 0;    }

  判断ArrayList是否为空,size为0时,即不包含任何成员时为空,返回true。

2.indexOf(Object o) 

 返回此列表中首次出现的指定元素的索引,或如果此列表不包含元素,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;    }

  原理就是从前向后遍历数组,看其中是否有元素与所给的元素相等,有则返回索引。

3.lastIndexOf(Object o)

  返回此列表中最后一次出现的指定元素的索引,或如果此列表不包含索引,则返回 -1。

    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;    }

  此方法原理同IndexOf,只是从后向前遍历

4.contains(Object o)

   /**     * Returns <tt>true</tt> if this list contains the specified element.     * More formally, returns <tt>true</tt> if and only if this list contains     * at least one element <tt>e</tt> such that     * <tt>(o==null&nbsp;?&nbsp;e==null&nbsp;:&nbsp;o.equals(e))</tt>.     * @param o element whose presence in this list is to be tested     * @return <tt>true</tt> if this list contains the specified element     */    public boolean contains(Object o) {        return indexOf(o) >= 0;    }

  contains方法中调用indexOf方法,如果ArrayList中含有此元素,则IndexOf返回的索引值一定大于等于零

5.clone()

  返回此 ArrayList 实例的浅表副本。

    /**     * Returns a shallow copy of this <tt>ArrayList</tt> instance.  (The     * elements themselves are not copied.)     *     * @return a clone of this <tt>ArrayList</tt> instance     */    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);        }    }

  clone方法先调用根类Object的clone方法复制一个ArrayList v,然后将当前ArrayList的elementData复制到一个新数组,并让ArrayList v的elementData指向新数组。

  因为v是新建立的,没有被修改过,modCount赋值为0。

  注意,clone只是对ArrayList的浅复制,成员本身并没有被复制。

6.toArray()

   按适当顺序(从第一个到最后一个元素)返回包含此列表中所有元素的数组

    /**     * Returns an array containing all of the elements in this list     * in proper sequence (from first to last element).     *     * <p>The returned array will be "safe" in that no references to it are     * maintained by this list.  (In other words, this method must allocate     * a new array).  The caller is thus free to modify the returned array.     *     * <p>This method acts as bridge between array-based and collection-based     * APIs.     *     * @return an array containing all of the elements in this list in     *         proper sequence     */    public Object[] toArray() {        return Arrays.copyOf(elementData, size);    }

  toArray()方法返回一个包含ArrayList所有元素的数组。该方法其实分配了一个新的Object数组,将elementData的成员复制到新的Object数组中,并将这个数组返回。

  注意,ArrayList中没有指向toArray返回的数组的引用,调用者可以随意修改

  toArray方法充当着基于Array和基于Collection的APIs的桥梁。

7.toArray(T[] a)     

  按适当顺序(从第一个到最后一个元素)返回包含此列表中所有元素的数组;返回数组的运行时类型是指定数组的运行时类型。

    /**     * Returns an array containing all of the elements in this list in proper     * sequence (from first to last element); the runtime type of the returned     * array is that of the specified array.  If the list fits in the     * specified array, it is returned therein.  Otherwise, a new array is     * allocated with the runtime type of the specified array and the size of     * this list.     *     * <p>If the list fits in the specified array with room to spare     * (i.e., the array has more elements than the list), the element in     * the array immediately following the end of the collection is set to     * <tt>null</tt>.  (This is useful in determining the length of the     * list <i>only</i> if the caller knows that the list does not contain     * any null elements.)     *     * @param a the array into which the elements of the list are to     *          be stored, if it is big enough; otherwise, a new array of the     *          same runtime type is allocated for this purpose.     * @return an array containing the elements of the list     * @throws ArrayStoreException if the runtime type of the specified array     *         is not a supertype of the runtime type of every element in     *         this list     * @throws NullPointerException if the specified array is null     */    @SuppressWarnings("unchecked")    public <T> T[] toArray(T[] a) {        if (a.length < size)            // Make a new array of a‘s runtime type, but my contents:            return (T[]) Arrays.copyOf(elementData, size, a.getClass());        System.arraycopy(elementData, 0, a, 0, size);        if (a.length > size)            a[size] = null;        return a;    }

  这个方法是将ArrayList的所有元素拷贝到数组a中。

  当ArrayList的size大于a的length时,也就是说a太小容纳不下ArrayList的所有元素,则调用Arrays.copyOf进行复制,这时不会再使用a,而是新开一个数组足够大的数组,将元素复制到新数组后将新数组返回。

  如果a足够大可以容纳ArrayList的所有元素时,调用System.arraycopy进行复制,a的前size个位置是从ArrayList复制过来的元素,如果a.size==ArrayList.length,恰好装满,直接返回;如果a.size>ArrayList.length,则a[size]=null,表示结束。

8. elementData(int index)

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

  elementData方法返回其内部数组elementData的index位置上的元素。

9.rangeCheck(int index)、  rangeCheckForAdd(int index)

    /**     * 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));    }

  rangeCheck方法其实就是检测index是否越界,ArrayList只有size个元素,index>=size时抛出越界异常。

  rangeCheckForAdd()方法也是检测index是否越界,条件是index<0 || index >size。

  值得注意的一点是rangeCheck和rangeCheckForAdd方法检测越界的提条件的差别

  rangeCheck供get、set和remove方法调用,这些都是在已经存在的元素上操作,范围自然是0到size-1

  rangeCheckForAdd方法由add方法调用,添加元素的位置可以是0到size-1,表示在元素中间插入,也可以是size,表示在所有元素之后插入。但是在大于size的位置插入时不可以,因为插入的元素和原有元素之间就留下了空档

10.get(int index)

    返回此列表中指定位置上的元素。

    /**     * 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);    }

11.set(int index, E element)         

   用指定的元素替代此列表中指定位置上的元素。

    /**     * 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;    }

12.remvoe(int index)

   移除此列表中指定位置上的元素

    /**     * 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;    }

  值得注意的是,当移除某个元素之后,它之后的所有元素都要向前移动一位,计算方式为 numMoved=size-index-1;

  当index为size-1时,没有需要移动的元素;

  numMoved>0时,调用System.arraycopy()方法移动元素,而且elementData[--size]=null,这里还将size减了1

13.fastRemove(int index)

    /*     * Private remove method that skips bounds checking and does not     * return the value removed.     */    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    }

  fastRemove方法是私有的,仅供remove(Object o)方法调用

  fastRemove方法和remove(int index)方法的不同之处只在于fastRemove不进行边界检查,不返回值;因为fastRemove不需要这些功能。

14.remove(Object o)

  移除此列表中首次出现的指定元素(如果存在)。

    /**     * 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;    }

  方法移除ArrayList中出现的第一个元素o,采用遍历的方式获得要删除元素的索引index然后调用fastRemove(int index)将其移除

  注意对元素o的判等方式,为null时使用==,否则使用equals方法。

15.removeRange(int fromIndex, int toIndex)         

   移除列表中索引在 fromIndex(包括)和 toIndex(不包括)之间的所有元素。

    /**     * Removes from this list all of the elements whose index is between     * {@code fromIndex}, inclusive, and {@code toIndex}, exclusive.     * Shifts any succeeding elements to the left (reduces their index).     * This call shortens the list by {@code (toIndex - fromIndex)} elements.     * (If {@code toIndex==fromIndex}, this operation has no effect.)     *     * @throws IndexOutOfBoundsException if {@code fromIndex} or     *         {@code toIndex} is out of range     *         ({@code fromIndex < 0 ||     *          fromIndex >= size() ||     *          toIndex > size() ||     *          toIndex < fromIndex})     */    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;    }

  removeRange(int fromIndex, int toIndex)方法是移除list中fromIndex到toIndex之间的元素,注意包括fromIndex,但是不包括toIndex

  还要注意,fromIndex的合法取值为0到size-1,toIndex的合法取值为0到size,而且toIndex≥fromIndex

  当fromIndex==toIndex时, 此函数不做任何操作

  当移除n个元素之后,这之后的所有元素都要向前移动n位,计算方式为 numMoved=size-toIndex;

  当toIndex为size时,没有需要移动的元素;

  numMoved>0时,调用System.arraycopy()方法移动元素。

  List的size变为newSize = size - (toIndex-fromIndex),并且elementData从newSize到size-1都要填充为null,最后size=newSize。

16.outOfBoundsMsg(int 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;    }

  此方法只是用来辅助在发生异常时输出越界信息的。

17.clear()

  移除此列表中的所有元素。

    /**     * Removes all of the elements from this list.  The list will     * be empty after this call returns.     */    public void clear() {        modCount++;        // clear to let GC do its work        for (int i = 0; i < size; i++)            elementData[i] = null;        size = 0;    }

  clear方法比较简单,将所有的元素赋值为null即可。

18.removeAll(Collection<?> c)、retainAll(Collection<?> c)

  前者移除ArrayList中那些也包含在指定 collection 中的所有元素(可选操作)。此调用返回后,collection 中将不包含任何与指定 collection 相同的元素。

  后者仅保留ArrayList中那些也包含在指定 collection 的元素(可选操作)。换句话说,移除此 collection 中未包含在指定 collection 中的所有元素。

    public boolean removeAll(Collection<?> c) {        Objects.requireNonNull(c);        return batchRemove(c, false);    }    public boolean retainAll(Collection<?> c) {        Objects.requireNonNull(c);        return batchRemove(c, true);    }

这两个方法都是调用batchRemove实现

  batchRemove(c,false);

  batchRemove(c,true);

不同的地方只有第二个参数一个为false,一个为true

19.batchRemove(Collection<?> c, boolean complement)

  实现移除或保留ArrayList中那些也包含在指定 collection 中的所有元素。

    private boolean batchRemove(Collection<?> c, boolean complement) {        final Object[] elementData = http://www.mamicode.com/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;    }
  这个方法的重点在下面几句
            for (; r < size; r++)                if (c.contains(elementData[r]) == complement)                    elementData[w++] = elementData[r];

  当complement为true时,elementData中所有包含在c中的元素被保留了下来;

  当complement为false时,elementData中所有未包含在c中的元素被保留了下来。

 

ArrayList源码解析(三)