Vector是List接口的另一实现，有非常长的历史了，从jdk1.0开始就有Vector了，先于ArrayList出现，与ArrayList的最大区别是：Vector 是线程安全的，简单浏览一下Vector:
类定义：
1 public class Vector<E>
2 extends AbstractList<E>
3 implements List<E>, RandomAccess, Cloneable, java.io.Serializable
Vector支持快速随机访问，和arrayList一样。
Vector 初始容量为10，支持自定义每次扩容长度，如果不自定义扩容长度，那么默认扩容后的长度是原来的2倍。
1 /**
2 * Constructs an empty vector with the specified initial capacity and
3 * capacity increment.
4 *
5 * @param initialCapacity the initial capacity of the vector
6 * @param capacityIncrement the amount by which the capacity is
7 * increased when the vector overflows
8 * @throws IllegalArgumentException if the specified initial capacity
9 * is negative
10 */
11 public Vector(int initialCapacity, int capacityIncrement) {
12 super();
13 if (initialCapacity < 0)
14 throw new IllegalArgumentException("Illegal Capacity: "+
15 initialCapacity);
16 this.elementData = new Object[initialCapacity];
17 this.capacityIncrement = capacityIncrement;
18 }
19
20 /**
21 * Constructs an empty vector with the specified initial capacity and
22 * with its capacity increment equal to zero.
23 *
24 * @param initialCapacity the initial capacity of the vector
25 * @throws IllegalArgumentException if the specified initial capacity
26 * is negative
27 */
28 public Vector(int initialCapacity) {
29 this(initialCapacity, 0);
30 }
31
32 /**
33 * Constructs an empty vector so that its internal data array
34 * has size {@code 10} and its standard capacity increment is
35 * zero.
36 */
37 public Vector() {
38 this(10);
39 }
Vector最大的特点是Vector内部基本上都使用了synchronized进行同步或者调用使用了 synchronized同步关键字的方法：
1 /**
2 * Returns {@code true} if this vector contains the specified element.
3 * More formally, returns {@code true} if and only if this vector
4 * contains at least one element {@code e} such that
5 * <tt>(o==null ? e==null : o.equals(e))</tt>.
6 *
7 * @param o element whose presence in this vector is to be tested
8 * @return {@code true} if this vector contains the specified element
9 */
10 public boolean contains(Object o) {
11 return indexOf(o, 0) >= 0;
12 }
13
14 /**
15 * Returns the index of the first occurrence of the specified element
16 * in this vector, or -1 if this vector does not contain the element.
17 * More formally, returns the lowest index {@code i} such that
18 * <tt>(o==null ? get(i)==null : o.equals(get(i)))</tt>,
19 * or -1 if there is no such index.
20 *
21 * @param o element to search for
22 * @return the index of the first occurrence of the specified element in
23 * this vector, or -1 if this vector does not contain the element
24 */
25 public int indexOf(Object o) {
26 return indexOf(o, 0);
27 }
28
29 /**
30 * Returns the index of the first occurrence of the specified element in
31 * this vector, searching forwards from {@code index}, or returns -1 if
32 * the element is not found.
33 * More formally, returns the lowest index {@code i} such that
34 * <tt>(i >= index && (o==null ? get(i)==null : o.equals(get(i))))</tt>,
35 * or -1 if there is no such index.
36 *
37 * @param o element to search for
38 * @param index index to start searching from
39 * @return the index of the first occurrence of the element in
40 * this vector at position {@code index} or later in the vector;
41 * {@code -1} if the element is not found.
42 * @throws IndexOutOfBoundsException if the specified index is negative
43 * @see Object#equals(Object)
44 */
45 public synchronized int indexOf(Object o, int index) {
46 if (o == null) {
47 for (int i = index ; i < elementCount ; i++)
48 if (elementData[i]==null)
49 return i;
50 } else {
51 for (int i = index ; i < elementCount ; i++)
52 if (o.equals(elementData[i]))
53 return i;
54 }
55 return -1;
56 }
57
58 /**
59 * Returns the index of the last occurrence of the specified element
60 * in this vector, or -1 if this vector does not contain the element.
61 * More formally, returns the highest index {@code i} such that
62 * <tt>(o==null ? get(i)==null : o.equals(get(i)))</tt>,
63 * or -1 if there is no such index.
64 *
65 * @param o element to search for
66 * @return the index of the last occurrence of the specified element in
67 * this vector, or -1 if this vector does not contain the element
68 */
69 public synchronized int lastIndexOf(Object o) {
70 return lastIndexOf(o, elementCount-1);
71 }
72
73 /**
74 * Returns the index of the last occurrence of the specified element in
75 * this vector, searching backwards from {@code index}, or returns -1 if
76 * the element is not found.
77 * More formally, returns the highest index {@code i} such that
78 * <tt>(i <= index && (o==null ? get(i)==null : o.equals(get(i))))</tt>,
79 * or -1 if there is no such index.
80 *
81 * @param o element to search for
82 * @param index index to start searching backwards from
83 * @return the index of the last occurrence of the element at position
84 * less than or equal to {@code index} in this vector;
85 * -1 if the element is not found.
86 * @throws IndexOutOfBoundsException if the specified index is greater
87 * than or equal to the current size of this vector
88 */
89 public synchronized int lastIndexOf(Object o, int index) {
90 if (index >= elementCount)
91 throw new IndexOutOfBoundsException(index + " >= "+ elementCount);
92
93 if (o == null) {
94 for (int i = index; i >= 0; i--)
95 if (elementData[i]==null)
96 return i;
97 } else {
98 for (int i = index; i >= 0; i--)
99 if (o.equals(elementData[i]))
100 return i;
101 }
102 return -1;
103 }
Vector 的实现实际上跟ArrayList差别不大，ArrayList可参考 java1.7集合源码阅读：ArrayList 。