数据结构复习之Vector

/**
     * The number of times this list has been <i>structurally modified</i>.
     * Structural modifications are those that change the size of the
     * list, or otherwise perturb it in such a fashion that iterations in
     * progress may yield incorrect results.
     *
     * <p>This field is used by the iterator and list iterator implementation
     * returned by the {@code iterator} and {@code listIterator} methods.
     * If the value of this field changes unexpectedly, the iterator (or list
     * iterator) will throw a {@code ConcurrentModificationException} in
     * response to the {@code next}, {@code remove}, {@code previous},
     * {@code set} or {@code add} operations.  This provides
     * <i>fail-fast</i> behavior, rather than non-deterministic behavior in
     * the face of concurrent modification during iteration.
     *
     * <p><b>Use of this field by subclasses is optional.</b> If a subclass
     * wishes to provide fail-fast iterators (and list iterators), then it
     * merely has to increment this field in its {@code add(int, E)} and
     * {@code remove(int)} methods (and any other methods that it overrides
     * that result in structural modifications to the list).  A single call to
     * {@code add(int, E)} or {@code remove(int)} must add no more than
     * one to this field, or the iterators (and list iterators) will throw
     * bogus {@code ConcurrentModificationExceptions}.  If an implementation
     * does not wish to provide fail-fast iterators, this field may be
     * ignored.
     */
    protected transient int modCount = 0;

迭代器要用到modCount属性

/**
     * The array buffer into which the components of the vector are
     * stored. The capacity of the vector is the length of this array buffer,
     * and is at least large enough to contain all the vector's elements.
     *
     * <p>Any array elements following the last element in the Vector are null.
     *
     * @serial
     */
    protected Object[] elementData;

Vector的元素就存储在这个Object数组里，因而Vector的容量就是这个数组的长度，其值至少要大于Vector的要存储的元素的数量，该数组最后一个元素之后的元素都是null。

/**
     * The number of valid components in this {@code Vector} object.
     * Components {@code elementData[0]} through
     * {@code elementData[elementCount-1]} are the actual items.
     *
     * @serial
     */
    protected int elementCount;

这个值代表Vector内有效数据的数量，而且起始元素的下标为0。

 /**
     * The amount by which the capacity of the vector is automatically
     * incremented when its size becomes greater than its capacity.  If
     * the capacity increment is less than or equal to zero, the capacity
     * of the vector is doubled each time it needs to grow.
     *
     * @serial
     */
    protected int capacityIncrement;

有了这个值后，当Vector的大小超过了它的容量的时，Vector的容量才可以实现自动增长。当capacityIncrement小于等于0时，vector的容量就会在需要增长时直接翻倍。（该值可以在初始化时和初始容量一块指定，不指定时默认为0，初始容量默认为10）

/**
     * Constructs a vector 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
     *       vector
     * @throws NullPointerException if the specified collection is null
     * @since   1.2
     */
    public Vector(Collection<? extends E> c) {
        elementData = c.toArray();
        elementCount = elementData.length;
        // c.toArray might (incorrectly) not return Object[] (see 6260652)
        if (elementData.getClass() != Object[].class)
            elementData = Arrays.copyOf(elementData, elementCount, Object[].class);
    }

也可以通过给定的集合来初始化一个Vector，通过该集合的迭代器的顺序来赋值。（c.toArray might (incorrectly) not return Object[] (see 6260652)不理解，待查资料）

 /**
     * Copies the components of this vector into the specified array.
     * The item at index {@code k} in this vector is copied into
     * component {@code k} of {@code anArray}.
     *
     * @param  anArray the array into which the components get copied
     * @throws NullPointerException if the given array is null
     * @throws IndexOutOfBoundsException if the specified array is not
     *         large enough to hold all the components of this vector
     * @throws ArrayStoreException if a component of this vector is not of
     *         a runtime type that can be stored in the specified array
     * @see #toArray(Object[])
     */
    public synchronized void copyInto(Object[] anArray) {
        System.arraycopy(elementData, 0, anArray, 0, elementCount);
    }

将vector的内容复制给另一指定数组。

/**
     * Trims the capacity of this vector to be the vector's current
     * size. If the capacity of this vector is larger than its current
     * size, then the capacity is changed to equal the size by replacing
     * its internal data array, kept in the field {@code elementData},
     * with a smaller one. An application can use this operation to
     * minimize the storage of a vector.
     */
    public synchronized void trimToSize() {
        modCount++;
        int oldCapacity = elementData.length;
        if (elementCount < oldCapacity) {
            elementData = Arrays.copyOf(elementData, elementCount);
        }
    }

如果vector的容量大于实际存储的元素数量，那么就将vector的容量调整为实际存储的元素数量，通过对内部数组重新赋值来实现，可以通过该操作最小化vector所占的内存。

/**
     * Increases the capacity of this vector, if necessary, to ensure
     * that it can hold at least the number of components specified by
     * the minimum capacity argument.
     *
     * <p>If the current capacity of this vector is less than
     * {@code minCapacity}, then its capacity is increased by replacing its
     * internal data array, kept in the field {@code elementData}, with a
     * larger one.  The size of the new data array will be the old size plus
     * {@code capacityIncrement}, unless the value of
     * {@code capacityIncrement} is less than or equal to zero, in which case
     * the new capacity will be twice the old capacity; but if this new size
     * is still smaller than {@code minCapacity}, then the new capacity will
     * be {@code minCapacity}.
     *
     * @param minCapacity the desired minimum capacity
     */
    public synchronized void ensureCapacity(int minCapacity) {
        if (minCapacity > 0) {
            modCount++;
            ensureCapacityHelper(minCapacity);
        }
    }
 
    /**
     * This implements the unsynchronized semantics of ensureCapacity.
     * Synchronized methods in this class can internally call this
     * method for ensuring capacity without incurring the cost of an
     * extra synchronization.
     *
     * @see #ensureCapacity(int)
     */
    private void ensureCapacityHelper(int minCapacity) {
        // overflow-conscious code
        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;
 
    private void grow(int minCapacity) {
        // overflow-conscious code
        int oldCapacity = elementData.length;
        int newCapacity = oldCapacity + ((capacityIncrement > 0) ?
                                         capacityIncrement : oldCapacity);
        if (newCapacity - minCapacity < 0)
            newCapacity = minCapacity;
        if (newCapacity - MAX_ARRAY_SIZE > 0)
            newCapacity = hugeCapacity(minCapacity);
        elementData = Arrays.copyOf(elementData, newCapacity);
    }
 
    private static int hugeCapacity(int minCapacity) {
        if (minCapacity < 0) // overflow
            throw new OutOfMemoryError();
        return (minCapacity > MAX_ARRAY_SIZE) ?
            Integer.MAX_VALUE :
            MAX_ARRAY_SIZE;
    }

增加vector的容量，如果有必要，要保证增加后的容量由minCapacity给出的最小容量。如果vector当前的容量小于minCapacity，就通过对内部数组重新赋值一个更大的数组的方式来扩容。当capacityIncrement小于等于0时，新数组的大小是之前的2倍，大于0时新数组的大小等于旧数组的大小与capacityIncrement只和。如果增加后的数组容量仍然小于minCapacity，那么新数组的大小为minCapacity。（ensureCapacityHelper方法不加synchronized是因为ensureCapacity方法加了synchronized，而且ensureCapacityHelper方法是在ensureCapacity内部调用的，因而没必要增加额外的synchronized）

而设置MAX_ARRAY_SIZE是因为，vector内部元素的数量elementCount和容量都是int型因而不能超出int的范围，而减8是因为某些VM里设有头结点，所以要留出这部分的空间。

hugeCapacity方法中判断minCapacity是否小于零则是因为负数减MAX_ARRAY_SIZE也可能大于0，例如

但是这里又为什么返回了Integer.MAX_VALUE呢（待解决）

/**
     * Sets the size of this vector. If the new size is greater than the
     * current size, new {@code null} items are added to the end of
     * the vector. If the new size is less than the current size, all
     * components at index {@code newSize} and greater are discarded.
     *
     * @param  newSize   the new size of this vector
     * @throws ArrayIndexOutOfBoundsException if the new size is negative
     */
    public synchronized void setSize(int newSize) {
        modCount++;
        if (newSize > elementCount) {
            ensureCapacityHelper(newSize);
        } else {
            for (int i = newSize ; i < elementCount ; i++) {
                elementData[i] = null;
            }
        }
        elementCount = newSize;
    }

setSize方法用来修改当前vector的大小，如果newSize大于当前元素数量elementCount那么调用ensureCapacityHelper方法进行扩容（此时没有经过ensureCapacity方法），当newSize小于elementCount时，将数组下标大于等于newSize的元素的值设为null，两种情况最后都要将elementCount大小改为newSize。

/**
     * Returns the current capacity of this vector.
     *
     * @return  the current capacity (the length of its internal
     *          data array, kept in the field {@code elementData}
     *          of this vector)
     */
    public synchronized int capacity() {
        return elementData.length;
    }
 
    /**
     * Returns the number of components in this vector.
     *
     * @return  the number of components in this vector
     */
    public synchronized int size() {
        return elementCount;
    }
 
    /**
     * Tests if this vector has no components.
     *
     * @return  {@code true} if and only if this vector has
     *          no components, that is, its size is zero;
     *          {@code false} otherwise.
     */
    public synchronized boolean isEmpty() {
        return elementCount == 0;
    }

以上三个方法分别返回vector容量、元素数量elementCount以及判断vector是否为空。

/**
     * Returns an enumeration of the components of this vector. The
     * returned {@code Enumeration} object will generate all items in
     * this vector. The first item generated is the item at index {@code 0},
     * then the item at index {@code 1}, and so on.
     *
     * @return  an enumeration of the components of this vector
     * @see     Iterator
     */
    public Enumeration<E> elements() {
        return new Enumeration<E>() {
            int count = 0;
 
            public boolean hasMoreElements() {
                return count < elementCount;
            }
 
            public E nextElement() {
                synchronized (Vector.this) {
                    if (count < elementCount) {
                        return elementData(count++);
                    }
                }
                throw new NoSuchElementException("Vector Enumeration");
            }
        };
    }

该方法返回vector内部元素按下标顺序的Enumeration（类似枚举类型）

/**
     * Returns the index of the first occurrence of the specified element in
     * this vector, searching forwards from {@code index}, or returns -1 if
     * the element is not found.
     * More formally, returns the lowest index {@code i} such that
     * <tt>(i&nbsp;&gt;=&nbsp;index&nbsp;&amp;&amp;&nbsp;(o==null&nbsp;?&nbsp;get(i)==null&nbsp;:&nbsp;o.equals(get(i))))</tt>,
     * or -1 if there is no such index.
     *
     * @param o element to search for
     * @param index index to start searching from
     * @return the index of the first occurrence of the element in
     *         this vector at position {@code index} or later in the vector;
     *         {@code -1} if the element is not found.
     * @throws IndexOutOfBoundsException if the specified index is negative
     * @see     Object#equals(Object)
     */
    public synchronized int indexOf(Object o, int index) {
        if (o == null) {
            for (int i = index ; i < elementCount ; i++)
                if (elementData[i]==null)
                    return i;
        } else {
            for (int i = index ; i < elementCount ; i++)
                if (o.equals(elementData[i]))
                    return i;
        }
        return -1;
    }

indexOf方法从给定index开始查找vector内是否存有Object o（可以为空），如果有则返回第一次出现的下标，如果不存在返回-1。

/**
     * Returns {@code true} if this vector contains the specified element.
     * More formally, returns {@code true} if and only if this vector
     * contains at least one element {@code e} such that
     * <tt>(o==null&nbsp;?&nbsp;e==null&nbsp;:&nbsp;o.equals(e))</tt>.
     *
     * @param o element whose presence in this vector is to be tested
     * @return {@code true} if this vector contains the specified element
     */
    public boolean contains(Object o) {
        return indexOf(o, 0) >= 0;
    }
 
    /**
     * Returns the index of the first occurrence of the specified element
     * in this vector, or -1 if this vector does not contain the element.
     * More formally, returns the lowest index {@code i} 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.
     *
     * @param o element to search for
     * @return the index of the first occurrence of the specified element in
     *         this vector, or -1 if this vector does not contain the element
     */
    public int indexOf(Object o) {
        return indexOf(o, 0);
    }

这两个方法都调用上边的indexOf方法，contains判断是否包含Object o，如果存在，返回true，否则返回false，indexOf也判断是否包含Object o，如果包含则返回第一次出现的下标，不存在返回-1。

/**
     * Returns the index of the last occurrence of the specified element in
     * this vector, searching backwards from {@code index}, or returns -1 if
     * the element is not found.
     * More formally, returns the highest index {@code i} such that
     * <tt>(i&nbsp;&lt;=&nbsp;index&nbsp;&amp;&amp;&nbsp;(o==null&nbsp;?&nbsp;get(i)==null&nbsp;:&nbsp;o.equals(get(i))))</tt>,
     * or -1 if there is no such index.
     *
     * @param o element to search for
     * @param index index to start searching backwards from
     * @return the index of the last occurrence of the element at position
     *         less than or equal to {@code index} in this vector;
     *         -1 if the element is not found.
     * @throws IndexOutOfBoundsException if the specified index is greater
     *         than or equal to the current size of this vector
     */
    public synchronized int lastIndexOf(Object o, int index) {
        if (index >= elementCount)
            throw new IndexOutOfBoundsException(index + " >= "+ elementCount);
 
        if (o == null) {
            for (int i = index; i >= 0; i--)
                if (elementData[i]==null)
                    return i;
        } else {
            for (int i = index; i >= 0; i--)
                if (o.equals(elementData[i]))
                    return i;
        }
        return -1;
    }

lastIndexOf方法类似indexOf方法，区别在于lastIndexOf方法从后向前查找，因而返回的是最后一次出现的下标，不存在仍然返回-1。

/**
     * Returns the index of the last occurrence of the specified element
     * in this vector, or -1 if this vector does not contain the element.
     * More formally, returns the highest index {@code i} 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.
     *
     * @param o element to search for
     * @return the index of the last occurrence of the specified element in
     *         this vector, or -1 if this vector does not contain the element
     */
    public synchronized int lastIndexOf(Object o) {
        return lastIndexOf(o, elementCount-1);
    }

该方法调用lastIndexOf发法，返回Object o最后一次出现的下标，不存在仍然返回-1。

/**
     * Returns the component at the specified index.
     *
     * <p>This method is identical in functionality to the {@link #get(int)}
     * method (which is part of the {@link List} interface).
     *
     * @param      index   an index into this vector
     * @return     the component at the specified index
     * @throws ArrayIndexOutOfBoundsException if the index is out of range
     *         ({@code index < 0 || index >= size()})
     */
    public synchronized E elementAt(int index) {
        if (index >= elementCount) {
            throw new ArrayIndexOutOfBoundsException(index + " >= " + elementCount);
        }
 
        return elementData(index);
    }
 
    /**
     * Returns the first component (the item at index {@code 0}) of
     * this vector.
     *
     * @return     the first component of this vector
     * @throws NoSuchElementException if this vector has no components
     */
    public synchronized E firstElement() {
        if (elementCount == 0) {
            throw new NoSuchElementException();
        }
        return elementData(0);
    }
 
    /**
     * Returns the last component of the vector.
     *
     * @return  the last component of the vector, i.e., the component at index
     *          <code>size()&nbsp;-&nbsp;1</code>.
     * @throws NoSuchElementException if this vector is empty
     */
    public synchronized E lastElement() {
        if (elementCount == 0) {
            throw new NoSuchElementException();
        }
        return elementData(elementCount - 1);
    }

elementAt返回指定下标的元素，firstElement返回第一个元素，lastElement返回最后一个元素。

/**
     * Sets the component at the specified {@code index} of this
     * vector to be the specified object. The previous component at that
     * position is discarded.
     *
     * <p>The index must be a value greater than or equal to {@code 0}
     * and less than the current size of the vector.
     *
     * <p>This method is identical in functionality to the
     * {@link #set(int, Object) set(int, E)}
     * method (which is part of the {@link List} interface). Note that the
     * {@code set} method reverses the order of the parameters, to more closely
     * match array usage.  Note also that the {@code set} method returns the
     * old value that was stored at the specified position.
     *
     * @param      obj     what the component is to be set to
     * @param      index   the specified index
     * @throws ArrayIndexOutOfBoundsException if the index is out of range
     *         ({@code index < 0 || index >= size()})
     */
    public synchronized void setElementAt(E obj, int index) {
        if (index >= elementCount) {
            throw new ArrayIndexOutOfBoundsException(index + " >= " +
                                                     elementCount);
        }
        elementData[index] = obj;
    }

该方法用来修改指定index的值（注意，不能添加index >= elementCount时会抛出异常），该方法与List接口中的set方法功能一样，区别在于set方法参数顺序更接近数组的用法，此外set还会返回旧元素的值。

/**
     * Deletes the component at the specified index. Each component in
     * this vector with an index greater or equal to the specified
     * {@code index} is shifted downward to have an index one
     * smaller than the value it had previously. The size of this vector
     * is decreased by {@code 1}.
     *
     * <p>The index must be a value greater than or equal to {@code 0}
     * and less than the current size of the vector.
     *
     * <p>This method is identical in functionality to the {@link #remove(int)}
     * method (which is part of the {@link List} interface).  Note that the
     * {@code remove} method returns the old value that was stored at the
     * specified position.
     *
     * @param      index   the index of the object to remove
     * @throws ArrayIndexOutOfBoundsException if the index is out of range
     *         ({@code index < 0 || index >= size()})
     */
    public synchronized void removeElementAt(int index) {
        modCount++;
        if (index >= elementCount) {
            throw new ArrayIndexOutOfBoundsException(index + " >= " +
                                                     elementCount);
        }
        else if (index < 0) {
            throw new ArrayIndexOutOfBoundsException(index);
        }
        int j = elementCount - index - 1;
        if (j > 0) {
            System.arraycopy(elementData, index + 1, elementData, index, j);
        }
        elementCount--;
        elementData[elementCount] = null; /* to let gc do its work */
    }

该方法通过将给定index（不包括index）后的所有有效元素前移的方式达到删除下标为index的元素的目的。

/**
     * Inserts the specified object as a component in this vector at the
     * specified {@code index}. Each component in this vector with
     * an index greater or equal to the specified {@code index} is
     * shifted upward to have an index one greater than the value it had
     * previously.
     *
     * <p>The index must be a value greater than or equal to {@code 0}
     * and less than or equal to the current size of the vector. (If the
     * index is equal to the current size of the vector, the new element
     * is appended to the Vector.)
     *
     * <p>This method is identical in functionality to the
     * {@link #add(int, Object) add(int, E)}
     * method (which is part of the {@link List} interface).  Note that the
     * {@code add} method reverses the order of the parameters, to more closely
     * match array usage.
     *
     * @param      obj     the component to insert
     * @param      index   where to insert the new component
     * @throws ArrayIndexOutOfBoundsException if the index is out of range
     *         ({@code index < 0 || index > size()})
     */
    public synchronized void insertElementAt(E obj, int index) {
        modCount++;
        if (index > elementCount) {
            throw new ArrayIndexOutOfBoundsException(index
                                                     + " > " + elementCount);
        }
        ensureCapacityHelper(elementCount + 1);
        System.arraycopy(elementData, index, elementData, index + 1, elementCount - index);
        elementData[index] = obj;
        elementCount++;
    }

该方法将index及其后的所有有效元素后移一位，然后将obj对象赋值给下标为index元素，从而实现插入元素（当index=elementCount效果等同于在最后添加一个元素）