本文基於JDK1.7

Vector與ArrayList

• Vector用同步實現了執行緒安全，ArrayList是執行緒不安全的，但是可以通過Collections.synchronizedList(list)來實現執行緒安全，不要求執行緒安全的場景用ArrayList效率更高
• Vector與ArrayList一樣是一種可以根據需要自動伸縮的列表，它們都是基於陣列實現的，加入了自動擴容機制
• Vector在初始化時除了提供initialCapacity外還可以額外提供capacityIncrement引數，從而在擴容時ArrayList一般會擴充套件到原來的1.5倍，而Vector在capacityIncrement不為零時會擴充套件capacityIncrement個元素
• 對Vector與ArrayList相同，在批量插入元素前，調整列表的容量到合理值，可以避免頻繁的擴容引起的開銷
• Vector是一個比較老的類，為了適配新的List介面對許多原有方法進行了包裝，使得其程式碼較為臃腫，例如：

    public synchronized boolean add(E e) {
        modCount++;
        ensureCapacityHelper(elementCount + 1);
        elementData[elementCount++] = e;
        return true;
    }  
    public
 
 synchronized void addElement(E obj) {
        modCount++;
        ensureCapacityHelper(elementCount + 1);
        elementData[elementCount++] = obj;
    }  

• 在序列化方面，Vector只提供了WriteObject方法，沒有提供ReadObject方法，沒有完全實現定製化
  java中一個類實現了Seriaizable就可以實現序列化，ArrayList中儲存元素的陣列被宣告為transient，即elementData陣列不會被預設的序列化寫入到輸出中。ArrayList實現了ReadObject和WriteObject兩個方法來進行定製的序列化。這樣做的優勢在於elementData陣列中可能存在很大的空間是沒有元素的，如果在序列化過程中這部分也被序列化會耗費大量的資源，因此ArrayList定製序列化，writeObject時只輸出實體元素。在對端恢復ArrayList時，ReadObject先確定陣列大小，然後將元素拷貝到對應陣列中去，避免了新增元素時頻繁的擴容，提高了效率

    private transient Object[] elementData;
    private void writeObject(java.io.ObjectOutputStream s)
        throws java.io.IOException{
        // Write out element count, and any hidden stuff
        int expectedModCount = modCount;
        s.defaultWriteObject();
        // Write out size as capacity for behavioural compatibility with clone()
        s.writeInt(size);
        // Write out all elements in the proper order.
        for (int i=0; i<size; i++) {
            s.writeObject(elementData[i]);
        }
        if (modCount != expectedModCount) {
            throw new ConcurrentModificationException();
        }
    }
    private void readObject(java.io.ObjectInputStream s)
        throws java.io.IOException, ClassNotFoundException {
        elementData = EMPTY_ELEMENTDATA;
        // Read in size, and any hidden stuff
        s.defaultReadObject();
        // Read in capacity
        s.readInt(); // ignored
        if (size > 0) {
            // be like clone(), allocate array based upon size not capacity
            ensureCapacityInternal(size);
            Object[] a = elementData;
            // Read in all elements in the proper order.
            for (int i=0; i<size; i++) {
                a[i] = s.readObject();
            }
        }
    }  

Vector全部原始碼

• Vector的多數方法實現機制與ArrayList是相同的，只是在方法上添加了synchronized約束
• 由於Vector比List接口出現要早，很多方法名比較長如addElement等，為了適配新的介面List，又添加了許多很類似的方法

public class Vector<E> extends AbstractList<E> implements List<E>, RandomAccess, Cloneable, java.io.Serializable {
    protected Object[] elementData;
    protected int elementCount;
    protected int capacityIncrement;
    private static final long serialVersionUID = -2767605614048989439L;
    public Vector(int initialCapacity, int capacityIncrement) {
        super();
        if (initialCapacity < 0)
            throw new IllegalArgumentException("Illegal Capacity: " + initialCapacity);
        this.elementData = new Object[initialCapacity];
        this.capacityIncrement = capacityIncrement;
    }
    public Vector(int initialCapacity) {
        this(initialCapacity, 0);
    }
    public Vector() {
        this(10);
    }
    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);
    }
    public synchronized void copyInto(Object[] anArray) {
        System.arraycopy(elementData, 0, anArray, 0, elementCount);
    }
    public synchronized void trimToSize() {
        modCount++;
        int oldCapacity = elementData.length;
        if (elementCount < oldCapacity) {
            elementData = Arrays.copyOf(elementData, elementCount);
        }
    }
    public synchronized void ensureCapacity(int minCapacity) {
        if (minCapacity > 0) {
            modCount++;
            ensureCapacityHelper(minCapacity);
        }
    }
    private void ensureCapacityHelper(int minCapacity) {
        // overflow-conscious code
        if (minCapacity - elementData.length > 0)
            grow(minCapacity);
    }
    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;
    }
    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;
    }
    public synchronized int capacity() {
        return elementData.length;
    }
    public synchronized int size() {
        return elementCount;
    }
    public synchronized boolean isEmpty() {
        return elementCount == 0;
    }
    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");
            }
        };
    }
    public boolean contains(Object o) {
        return indexOf(o, 0) >= 0;
    }
    public int indexOf(Object o) {
        return indexOf(o, 0);
    }
    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;
    }
    public synchronized int lastIndexOf(Object o) {
        return lastIndexOf(o, elementCount - 1);
    }
    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;
    }
    public synchronized E elementAt(int index) {
        if (index >= elementCount) {
            throw new ArrayIndexOutOfBoundsException(index + " >= " + elementCount);
        }
        return elementData(index);
    }
    public synchronized E firstElement() {
        if (elementCount == 0) {
            throw new NoSuchElementException();
        }
        return elementData(0);
    }
    public synchronized E lastElement() {
        if (elementCount == 0) {
            throw new NoSuchElementException();
        }
        return elementData(elementCount - 1);
    }
    public synchronized void setElementAt(E obj, int index) {
        if (index >= elementCount) {
            throw new ArrayIndexOutOfBoundsException(index + " >= " + elementCount);
        }
        elementData[index] = obj;
    }
    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--;
    }
    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++;
    }
    public synchronized void addElement(E obj) {
        modCount++;
        ensureCapacityHelper(elementCount + 1);
        elementData[elementCount++] = obj;
    }
    public synchronized boolean removeElement(Object obj) {
        modCount++;
        int i = indexOf(obj);
        if (i >= 0) {
            removeElementAt(i);
            return true;
        }
        return false;
    }
    public synchronized void removeAllElements() {
        modCount++;
        // Let gc do its work
        for (int i = 0; i < elementCount; i++)
            elementData[i] = null;
        elementCount = 0;
    }
    public synchronized Object clone() {
        try {
            @SuppressWarnings("unchecked")
            Vector<E> v = (Vector<E>) super.clone();
            v.elementData = Arrays.copyOf(elementData, elementCount);
            v.modCount = 0;
            return v;
        } catch (CloneNotSupportedException e) {
            // this shouldn't happen, since we are Cloneable
            throw new InternalError();
        }
    }
    public synchronized Object[] toArray() {
        return Arrays.copyOf(elementData, elementCount);
    }
    @SuppressWarnings("unchecked")
    public synchronized <T> T[] toArray(T[] a) {
        if (a.length < elementCount)
            return (T[]) Arrays.copyOf(elementData, elementCount, a.getClass());
        System.arraycopy(elementData, 0, a, 0, elementCount);
        if (a.length > elementCount)
            a[elementCount] = null;
        return a;
    }
    // Positional Access Operations
    @SuppressWarnings("unchecked")
    E elementData(int index) {
        return (E) elementData[index];
    }
    public synchronized E get(int index) {
        if (index >= elementCount)
            throw new ArrayIndexOutOfBoundsException(index);
        return elementData(index);
    }
    public synchronized E set(int index, E element) {
        if (index >= elementCount)
            throw new ArrayIndexOutOfBoundsException(index);
        E oldValue = elementData(index);
        elementData[index] = element;
        return oldValue;
    }
    public synchronized boolean add(E e) {
        modCount++;
        ensureCapacityHelper(elementCount + 1);
        elementData[elementCount++] = e;
        return true;
    }
    public boolean remove(Object o) {
        return removeElement(o);
    }
    public void add(int index, E element) {
        insertElementAt(element, index);
    }
    public synchronized E remove(int index) {
        modCount++;
        if (index >= elementCount)
            throw new ArrayIndexOutOfBoundsException(index);
        E oldValue = elementData(index);
        int numMoved = elementCount - index - 1;
        if (numMoved > 0)
            System.arraycopy(elementData, index + 1, elementData, index, numMoved);
        elementData[--elementCount] = null; // Let gc do its work
        return oldValue;
    }
    public void clear() {
        removeAllElements();
    }
    // Bulk Operations
    public synchronized boolean containsAll(Collection<?> c) {
        return super.containsAll(c);
    }
    public synchronized boolean addAll(Collection<? extends E> c) {
        modCount++;
        Object[] a = c.toArray();
        int numNew = a.length;
        ensureCapacityHelper(elementCount + numNew);
        System.arraycopy(a, 0, elementData, elementCount, numNew);
        elementCount += numNew;
        return numNew != 0;
    }
    public synchronized boolean removeAll(Collection<?> c) {
        return super.removeAll(c);
    }
    public synchronized boolean retainAll(Collection<?> c) {
        return super.retainAll(c);
    }
    public synchronized boolean addAll(int index, Collection<? extends E> c) {
        modCount++;
        if (index < 0 || index > elementCount)
            throw new ArrayIndexOutOfBoundsException(index);
        Object[] a = c.toArray();
        int numNew = a.length;
        ensureCapacityHelper(elementCount + numNew);
        int numMoved = elementCount - index;
        if (numMoved > 0)
            System.arraycopy(elementData, index, elementData, index + numNew, numMoved);
        System.arraycopy(a, 0, elementData, index, numNew);
        elementCount += numNew;
        return numNew != 0;
    }
    public synchronized boolean equals(Object o) {
        return super.equals(o);
    }
    public synchronized int hashCode() {
        return super.hashCode();
    }
    public synchronized String toString() {
        return super.toString();
    }
    public synchronized List<E> subList(int fromIndex, int toIndex) {
        return Collections.synchronizedList(super.subList(fromIndex, toIndex), this);
    }
    protected synchronized void removeRange(int fromIndex, int toIndex) {
        modCount++;
        int numMoved = elementCount - toIndex;
        System.arraycopy(elementData, toIndex, elementData, fromIndex, numMoved);
        // Let gc do its work
        int newElementCount = elementCount - (toIndex - fromIndex);
        while (elementCount != newElementCount)
            elementData[--elementCount] = null;
    }
    private void writeObject(java.io.ObjectOutputStream s) throws java.io.IOException {
        final java.io.ObjectOutputStream.PutField fields = s.putFields();
        final Object[] data;
        synchronized (this) {
            fields.put("capacityIncrement", capacityIncrement);
            fields.put("elementCount", elementCount);
            data = elementData.clone();
        }
        fields.put("elementData", data);
        s.writeFields();
    }
    public synchronized ListIterator<E> listIterator(int index) {
        if (index < 0 || index > elementCount)
            throw new IndexOutOfBoundsException("Index: " + index);
        return new ListItr(index);
    }
    public synchronized ListIterator<E> listIterator() {
        return new ListItr(0);
    }
    public synchronized Iterator<E> iterator() {
        return new Itr();
    }
    private class Itr implements Iterator<E> {
        int cursor; // index of next element to return
        int lastRet = -1; // index of last element returned; -1 if no such
        int expectedModCount = modCount;
        public boolean hasNext() {
            // Racy but within spec, since modifications are checked
            // within or after synchronization in next/previous
            return cursor != elementCount;
        }
        public E next() {
            synchronized (Vector.this) {
                checkForComodification();
                int i = cursor;
                if (i >= elementCount)
                    throw new NoSuchElementException();
                cursor = i + 1;
                return elementData(lastRet = i);
            }
        }
        public void remove() {
            if (lastRet == -1)
                throw new IllegalStateException();
            synchronized (Vector.this) {
                checkForComodification();
                Vector.this.remove(lastRet);
                expectedModCount = modCount;
            }
            cursor = lastRet;
            lastRet = -1;
        }
        final void checkForComodification() {
            if (modCount != expectedModCount)
                throw new ConcurrentModificationException();
        }
    }
    final class ListItr extends Itr implements ListIterator<E> {
        ListItr(int index) {
            super();
            cursor = index;
        }
        public boolean hasPrevious() {
            return cursor != 0;
        }
        public int nextIndex() {
            return cursor;
        }
        public int previousIndex() {
            return cursor - 1;
        }
        public E previous() {
            synchronized (Vector.this) {
                checkForComodification();
                int i = cursor - 1;
                if (i < 0)
                    throw new NoSuchElementException();
                cursor = i;
                return elementData(lastRet = i);
            }
        }
        public void set(E e) {
            if (lastRet == -1)
                throw new IllegalStateException();
            synchronized (Vector.this) {
                checkForComodification();
                Vector.this.set(lastRet, e);
            }
        }
        public void add(E e) {
            int i = cursor;
            synchronized (Vector.this) {
                checkForComodification();
                Vector.this.add(i, e);
                expectedModCount = modCount;
            }
            cursor = i + 1;
            lastRet = -1;
        }
    }
}