java 集合系列目录：

第1部分 List概括

先回顾一下List的框架图

(01) List 是一个接口，它继承于Collection的接口。它代表着有序的队列。

(02) AbstractList 是一个抽象类，它继承于AbstractCollection。AbstractList实现List接口中除size()、get(int location)之外的函数。

(03) AbstractSequentialList 是一个抽象类，它继承于AbstractList。AbstractSequentialList 实现了“链表中，根据index索引值操作链表的全部函数”。

(04) ArrayList, LinkedList, Vector, Stack是List的4个实现类。

ArrayList 是一个数组队列，相当于动态数组。它由数组实现，随机访问效率高，随机插入、随机删除效率低。

LinkedList 是一个双向链表。它也可以被当作堆栈、队列或双端队列进行操作。LinkedList随机访问效率低，但随机插入、随机删除效率低。

Vector 是矢量队列，和ArrayList一样，它也是一个动态数组，由数组实现。但是ArrayList是非线程安全的，而Vector是线程安全的。

Stack 是栈，它继承于Vector。它的特性是：先进后出(FILO, First In Last Out)。

第2部分 List使用场景

下面先概括的说明一下各个List的使用场景，后面再分析原因。

如果涉及到“栈”、“队列”、“链表”等操作，应该考虑用List，具体的选择哪个List，根据下面的标准来取舍。

(01) 对于需要快速插入，删除元素，应该使用LinkedList。

(02) 对于需要快速随机访问元素，应该使用ArrayList。

(03)对于“单线程环境” 或者 “多线程环境，但List仅仅只会被单个线程操作”，此时应该使用非同步的类(如ArrayList)。

对于“多线程环境，且List可能同时被多个线程操作”，此时，应该使用同步的类(如Vector)。

通过下面的测试程序，验证上面的(01)和(02)结论。参考代码如下：

1 /**

2 * 对比ArrayList和LinkedList的插入、随机读取效率、删除的效率3 *4 * @ClassName: List_test_15 *@authorXingle6 * @date 2014-5-29 下午5:25:117 */

8 public classList_test_1 {9 private static int COUNT_ = 100000;10

11 private static LinkedList linkedList = new LinkedList();12 private static ArrayList arraylist = new ArrayList();13 private static Vector vector = new Vector();14 private static Stack stack = new Stack();15

16 public static voidmain(String[] args) {17 //插入

18 insertByPosition(stack);19 insertByPosition(linkedList);20 insertByPosition(arraylist);21 insertByPosition(vector);22

23

24 //读取

25 readByPosition(stack);26 readByPosition(linkedList);27 readByPosition(arraylist);28 readByPosition(vector);29

30

31 //删除

32 deleteByPosition(stack);33 deleteByPosition(linkedList);34 deleteByPosition(arraylist);35 deleteByPosition(vector);36

37

38 }39

40 /**

41 * 从list的指定位置删除COUNT个元素，并统计时间42 *43 *@authorxingle44 * @data 2014-5-29 下午5:33:5545 */

46 private static void deleteByPosition(Listlist) {47 long start =getCurrentTime();48 for (int i = 0; i < COUNT_; i++) {49 list.remove(0);50 }51 long end =getCurrentTime();52 long interval = end -start;53 System.out.println(getListName(list) + " : delete " +COUNT_54 + " delete "+COUNT_+" elements from the 1st position use time：" +interval55 + " ms");56

57 }58

59 /**

60 * 根据position，从list中读取元素，并统计时间61 *62 *@paramlist63 *@authorxingle64 * @data 2014-5-29 下午5:32:5865 */

66 private static void readByPosition(Listlist) {67 long start =getCurrentTime();68 for (int i = 0; i < COUNT_; i++) {69 list.get(i);70 }71 long end =getCurrentTime();72 long interval = end -start;73 System.out.println(getListName(list) + " : read " +COUNT_74 + " elements by position use time：" +interval75 + " ms");76

77 }78

79 /**

80 * 向list的指定位置插入COUNT个元素，并统计时间81 *82 *@paramlist83 *@authorxingle84 * @data 2014-5-29 下午5:32:1685 */

86 private static void insertByPosition(Listlist) {87 long start =getCurrentTime();88 for (int i = 0; i < COUNT_; i++) {89 list.add(0, i);90 }91 long end =getCurrentTime();92 long interval = end -start;93 System.out.println(getListName(list) + " : insert " +COUNT_94 + " elements into the 1st position use time：" +interval95 + " ms");96 }97

98 /**

99 * 获取list名称100 *101 *@return

102 *@authorxingle103 * @data 2014-5-29 下午5:38:02104 */

105 private static String getListName(Listlist) {106 if (list instanceofLinkedList)107 return "LinkedList";108 else if (list instanceofArrayList)109 return "ArrayList";110 else if (list instanceofStack)111 return "Stack";112 else if(list instanceofVector)113 return "Vector";114 else

115 return "List";116 }117

118 /**

119 * 获取当前时间120 *121 *@return

122 *@authorxingle123 * @data 2014-5-29 下午5:35:33124 */

125 private static longgetCurrentTime() {126 returnSystem.currentTimeMillis();127 }128

129 }

执行结果：

Stack : insert 100000 elements into the 1st position use time：1724 ms

LinkedList : insert 100000 elements into the 1st position use time：31 ms

ArrayList : insert 100000 elements into the 1st position use time：1724 ms

Vector : insert 100000 elements into the 1st position use time：1651 ms

Stack : read 100000 elements by position use time：9 ms

LinkedList : read 100000 elements by position use time：8969 ms

ArrayList : read 100000 elements by position use time：10 ms

Vector : read 100000 elements by position use time：10 ms

Stack : delete 100000 delete 100000 elements from the 1st position use time：2674 ms

LinkedList : delete 100000 delete 100000 elements from the 1st position use time：23 ms

ArrayList : delete 100000 delete 100000 elements from the 1st position use time：2757 ms

Vector : delete 100000 delete 100000 elements from the 1st position use time：2087 ms

从中，我们可以发现：

插入10万个元素，LinkedList所花时间最短：31ms。

删除10万个元素，LinkedList所花时间最短：23ms。

遍历10万个元素，LinkedList所花时间最长：8969 ms；而ArrayList、Stack和Vector则相差不多，都只用了几秒。

考虑到Vector是支持同步的，而Stack又是继承于Vector的；因此，得出结论：

(01) 对于需要快速插入，删除元素，应该使用LinkedList。

(02) 对于需要快速随机访问元素，应该使用ArrayList。

(03)对于“单线程环境” 或者 “多线程环境，但List仅仅只会被单个线程操作”，此时应该使用非同步的类。

第3部分 LinkedList和ArrayList性能差异分析

下面我们看看为什么LinkedList中插入元素很快，而ArrayList中插入元素很慢！

LinkedList.java中向指定位置插入元素的代码如下：

/*** Inserts the specified element at the specified position in this

* list. Shifts the element currently at that position (if any) and

* any subsequent elements to the right (adds one to their indices).

*

*@paramindex index at which the specified element is to be inserted

*@paramelement element to be inserted

*@throwsIndexOutOfBoundsException {@inheritDoc}*/

public void add(intindex, E element) {

rangeCheckForAdd(index);

ensureCapacityInternal(size+ 1); //Increments modCount!!

System.arraycopy(elementData, index, elementData, index + 1,

size-index);

elementData[index]=element;

size++;

}private void ensureCapacityInternal(intminCapacity) {if (elementData ==EMPTY_ELEMENTDATA) {

minCapacity=Math.max(DEFAULT_CAPACITY, minCapacity);

}

ensureExplicitCapacity(minCapacity);

}private void ensureExplicitCapacity(intminCapacity) {

modCount++;//overflow-conscious code

if (minCapacity - elementData.length > 0)

grow(minCapacity);

}/*** Increases the capacity to ensure that it can hold at least the

* number of elements specified by the minimum capacity argument.

*

*@paramminCapacity the desired minimum capacity*/

private void grow(intminCapacity) {//overflow-conscious code

int oldCapacity =elementData.length;int newCapacity = oldCapacity + (oldCapacity >> 1);if (newCapacity - minCapacity < 0)

newCapacity=minCapacity;if (newCapacity - MAX_ARRAY_SIZE > 0)

newCapacity=hugeCapacity(minCapacity);//minCapacity is usually close to size, so this is a win:

elementData =Arrays.copyOf(elementData, newCapacity);

}private static int hugeCapacity(intminCapacity) {if (minCapacity < 0) //overflow

throw newOutOfMemoryError();return (minCapacity > MAX_ARRAY_SIZE) ?Integer.MAX_VALUE :

MAX_ARRAY_SIZE;

}public static T[] copyOf(U[] original, int newLength, Class extends T[]>newType) {

T[] copy= ((Object)newType == (Object)Object[].class)? (T[]) newObject[newLength]

: (T[]) Array.newInstance(newType.getComponentType(), newLength);

System.arraycopy(original,0, copy, 0,

Math.min(original.length, newLength));returncopy;

}

ensureCapacity(size+1) 的作用是“确认ArrayList的容量，若容量不够，则增加容量。”

真正耗时的操作是 System.arraycopy(elementData, index, elementData, index + 1, size - index);

Sun JDK包的java/lang/System.java中的arraycopy()声明如下：

public static native void arraycopy(Object src, int srcPos, Object dest, int destPos, int length);

arraycopy()是个JNI函数，它是在JVM中实现的。sunJDK中看不到源码，不过可以在OpenJDK包中看到的源码。网上有对arraycopy()的分析说明，请参考：实际上，System.arraycopy(elementData, index, elementData, index + 1, size - index); 会移动index之后所有元素即可。这就意味着，ArrayList的add(int index, E element)函数，会引起index之后所有元素的改变！

通过上面的分析，我们就能理解为什么LinkedList中插入元素很快，而ArrayList中插入元素很慢。

“删除元素”与“插入元素”的原理类似，这里就不再过多说明。

接下来，我们看看“为什么LinkedList中随机访问很慢，而ArrayList中随机访问很快”。

先看看LinkedList随机访问的代码

/*** Returns the element at the specified position in this list.

*

*@paramindex index of the element to return

*@returnthe element at the specified position in this list

*@throwsIndexOutOfBoundsException {@inheritDoc}*/

public E get(intindex) {

checkElementIndex(index);returnnode(index).item;

}/*** Returns the (non-null) Node at the specified element index.*/Node node(intindex) {//assert isElementIndex(index);

if (index < (size >> 1)) {

Node x =first;for (int i = 0; i < index; i++)

x=x.next;returnx;

}else{

Node x =last;for (int i = size - 1; i > index; i--)

x=x.prev;returnx;

}

}private static class Node{

E item;

Nodenext;

Nodeprev;

Node(Node prev, E element, Nodenext) {this.item =element;this.next =next;this.prev =prev;

}

}

从中，可以看出：通过get(int index)获取LinkedList第index个元素时。先是在双向链表中找到要index位置的元素；找到之后再返回。

双向链表查找index位置的节点时，有一个加速动作：若index < 双向链表长度的1/2，则从前向后查找; 否则，从后向前查找。

下面看看ArrayList随机访问的代码

/*** Returns the element at the specified position in this list.

*

*@paramindex index of the element to return

*@returnthe element at the specified position in this list

*@throwsIndexOutOfBoundsException {@inheritDoc}*/

public E get(intindex) {

rangeCheck(index);returnelementData(index);

}//Positional Access Operations

@SuppressWarnings("unchecked")

E elementData(intindex) {return(E) elementData[index];

}

从中，可以看出：通过get(int index)获取ArrayList第index个元素时。直接返回数组中index位置的元素，而不需要像LinkedList一样进行查找。

第4部分 Vector和ArrayList比较

相同之处

1 它们都是List

它们都继承于AbstractList，并且实现List接口。

ArrayList和Vector的类定义如下：

//ArrayList的定义

public class ArrayList extends AbstractList

implements List, RandomAccess, Cloneable, java.io.Serializable//Vector的定义

public class Vector

extends AbstractList

implements List, RandomAccess, Cloneable, java.io.Serializable

2 它们都实现了RandomAccess和Cloneable接口

实现RandomAccess接口，意味着它们都支持快速随机访问；

实现Cloneable接口，意味着它们能克隆自己。

3 它们都是通过数组实现的，本质上都是动态数组

ArrayList.java中定义数组elementData用于保存元素

/*** The array buffer into which the elements of the ArrayList are stored.

* The capacity of the ArrayList is the length of this array buffer. Any

* empty ArrayList with elementData == EMPTY_ELEMENTDATA will be expanded to

* DEFAULT_CAPACITY when the first element is added.*/

private transient Object[] elementData;

Vector.java中也定义了数组elementData用于保存元素

/*** 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.

*

*

Any array elements following the last element in the Vector are null.

*

*@serial

*/

protected Object[] elementData;

4 它们的默认数组容量是10

若创建ArrayList或Vector时，没指定容量大小；则使用默认容量大小10。

ArrayList的默认构造函数如下：

/*** Constructs an empty list with an initial capacity of ten.*/

publicArrayList() {super();this.elementData =EMPTY_ELEMENTDATA;

}/*** Default initial capacity.*/

private static final int DEFAULT_CAPACITY = 10;

Vector的默认构造函数如下：

/*** Constructs an empty vector so that its internal data array

* has size {@code10} and its standard capacity increment is

* zero.*/

publicVector() {this(10);

}

5 它们都支持Iterator和listIterator遍历

它们都继承于AbstractList，而AbstractList中分别实现了 “iterator()接口返回Iterator迭代器” 和 “listIterator()返回ListIterator迭代器”。

不同之处

1 线程安全性不一样

ArrayList是非线程安全；

而Vector是线程安全的，它的函数都是synchronized的，即都是支持同步的。

ArrayList适用于单线程，Vector适用于多线程。

2 对序列化支持不同

ArrayList支持序列化，而Vector不支持；即ArrayList有实现java.io.Serializable接口，而Vector没有实现该接口。

3 构造函数个数不同

ArrayList有3个构造函数，而Vector有4个构造函数。Vector除了包括和ArrayList类似的3个构造函数之外，另外的一个构造函数可以指定容量增加系数。

ArrayList的构造函数如下：

/*** capacity是ArrayList的默认容量大小。当由于增加数据导致容量不足时，容量会添加上一次容量大小的一半

* Constructs an empty list with the specified initial capacity.

*

*@paraminitialCapacity the initial capacity of the list

*@throwsIllegalArgumentException if the specified initial capacity

* is negative*/

public ArrayList(intinitialCapacity) {super();if (initialCapacity < 0)throw new IllegalArgumentException("Illegal Capacity: "+initialCapacity);this.elementData = newObject[initialCapacity];

}/*** 默认构造函数

* Constructs an empty list with an initial capacity of ten.*/

publicArrayList() {super();this.elementData =EMPTY_ELEMENTDATA;

}/*** Constructs a list containing the elements of the specified

* collection, in the order they are returned by the collection's

* iterator.

*

*@paramc the collection whose elements are to be placed into this list

*@throwsNullPointerException if the specified collection is null*/

public ArrayList(Collection extends E>c) {

elementData=c.toArray();

size=elementData.length;//c.toArray might (incorrectly) not return Object[] (see 6260652)

if (elementData.getClass() != Object[].class)

elementData= Arrays.copyOf(elementData, size, Object[].class);

}

Vector的构造函数如下：

/*** Constructs an empty vector with the specified initial capacity and

* capacity increment.

*

*@paraminitialCapacity the initial capacity of the vector

*@paramcapacityIncrement the amount by which the capacity is

* increased when the vector overflows

*@throwsIllegalArgumentException if the specified initial capacity

* is negative*/

public Vector(int initialCapacity, intcapacityIncrement) {super();if (initialCapacity < 0)throw new IllegalArgumentException("Illegal Capacity: "+initialCapacity);this.elementData = newObject[initialCapacity];this.capacityIncrement =capacityIncrement;

}/*** Constructs an empty vector with the specified initial capacity and

* with its capacity increment equal to zero.

*

*@paraminitialCapacity the initial capacity of the vector

*@throwsIllegalArgumentException if the specified initial capacity

* is negative*/

public Vector(intinitialCapacity) {this(initialCapacity, 0);

}/*** Constructs an empty vector so that its internal data array

* has size {@code10} and its standard capacity increment is

* zero.*/

publicVector() {this(10);

}/*** Constructs a vector containing the elements of the specified

* collection, in the order they are returned by the collection's

* iterator.

*

*@paramc the collection whose elements are to be placed into this

* vector

*@throwsNullPointerException if the specified collection is null

*@since1.2*/

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);

}

4 容量增加方式不同

逐个添加元素时，若ArrayList容量不足时，新的容量扩大1.5倍。

ArrayList中容量增长的主要函数如下：

/*** Increases the capacity to ensure that it can hold at least the

* number of elements specified by the minimum capacity argument.

*

*@paramminCapacity the desired minimum capacity*/

private void grow(intminCapacity) {//overflow-conscious code

int oldCapacity =elementData.length;int newCapacity = oldCapacity + (oldCapacity >> 1);//对容量扩大1.5倍

if (newCapacity - minCapacity < 0)

newCapacity=minCapacity;if (newCapacity - MAX_ARRAY_SIZE > 0)

newCapacity=hugeCapacity(minCapacity);//minCapacity is usually close to size, so this is a win:

elementData =Arrays.copyOf(elementData, newCapacity);

}

Vector中容量增长的主要函数如下：

在进行动态扩容时，Vector的新容量大小为原有容量加上capacityIncrement，如果这个数不大于0，则扩容为原始容量的2倍。

private void grow(intminCapacity) {//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);

}

5 对Enumeration的支持不同。Vector支持通过Enumeration去遍历，而List不支持

Vector中实现Enumeration的代码如下：

/*** Returns an enumeration of the components of this vector. The

* returned {@codeEnumeration} object will generate all items in

* this vector. The first item generated is the item at index {@code0},

* then the item at index {@code1}, and so on.

*

*@returnan enumeration of the components of this vector

*@seeIterator*/

public Enumerationelements() {return new Enumeration() {int count = 0;public booleanhasMoreElements() {return count

}publicE nextElement() {synchronized (Vector.this) {if (count

}

}throw new NoSuchElementException("Vector Enumeration");

}

};

}