全新java初学者实践教程10(java SE5.0版)
9&'HhJm jdk5的集合类
sQ8kLS_q8 diD[/&k#kh }cERCS\t 7>xfQ 上次课我们学过了数组,知道它只是一组数(或是对象),但是有些自己的特性。在[font="Times]java里还有一类东西与数组类似,也是有着特性的一组数[font="Times](或是对象[font="Times]),叫做集合类。
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mE @ 我们上节课讲到了,数组的长度在创建时已经确定了,但是有时候我们事先根本不知道长度是多少啊,比如我们做电子商务网站时,有个购物车程序。你总不能用数组规定,人家只能买[font="Times]5样东西吧。你就是把长度定为[font="Times]10000也不行,万一遇上个特别有钱的呢!呵呵,这只是开玩笑的。我们会使用集合类解决这个问题。
bs?&;R.5 [font="Times] 集合类是放在[font="Times]java.util.*;这个包里。集合类存放的都是对象的引用,而非对象本身,为了说起来方便些,我们称集合中的对象就是指集合中对象的引用([font="Times]reference)。引用的概念大家不会忘了吧,在前边我们讲
数据类型时讲的。
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nk [font="Times] 集合类型主要有[font="Times]3种:[font="Times]set(集)、[font="Times]list(列表)、[font="Times]map(映射[font="Times])和[font="Times]Queue(队列)。[font="Times]//队列为[font="Times]jdk5中的加上的[font="Times]
"eR-(c1 nD#uOep9 [font="Times](1) Set
:zsMkdU 集([font="Times]set)是最简单的一种集合,它的对象不按特定方式排序,只是简单的把对象加入集合中,就像往口袋里放东西。对集中成员的访问和操作是通过集中对象的引用进行的,所以集中不能有重复对象。我们知道数学上的集合也是[font="Times]Set这个,集合里面一定是没有重复的元素的。
]DLs'W;) &eWnS~hJ ([font="Times]2)[font="Times]List
:p.f zL6X 列表([font="Times]List)的主要特征是其对象以线性方式存储,没有特定顺序,只有一个开头和一个结尾,当然,它与根本没有顺序的[font="Times]Set是不同的。它是链表嘛,一条链肯定有顺序这个顺序就不一定了。
1|oE3 @%rj1Gn F3&:KZ!V&m [font="Times] ([font="Times]3)[font="Times]Map
3'6by!N,d [font="Times] 映射([font="Times]Map),这个在[font="Times]java里不是地图的意思,其实地图也是映射哈。它里面的东西是键-值对([font="Times]key-value)出现的,键值对是什么呢?举个例子,比如我们查字典,用部首查字法。目录那个字就是键,这个字的解释就是值。键和值成对出现。这样说可以理解吧。这也是很常用的数据结构哦。
Ed>n/)Sm Nuc2CB)J R|R3Ob.e [font="Times] ([font="Times]4)[font="Times]Queue
\x=!' [font="Times] 在[font="Times]jdk5.0以前,通常的实现方式是使用[font="Times]java.util.List集合来模仿[font="Times]Queue。[font="Times]Queue的概念通过把对象添加(称为[font="Times]enqueuing的操作)到[font="Times]List的尾部(即[font="Times]Queue的后部)并通过从[font="Times]List的头部(即[font="Times]Queue的前部)提取对象而从[font="Times] List中移除(称为[font="Times]dequeuing的操作)来模拟。你需要执行先进先出的动作时可以直接使用[font="Times]Queue接口就可以了。
* a@78&N [font="Times] 这[font="Times]4个东西,有时候功能还不太完善,需要有些子类继承它的特性。[font="Times]Set的子接口有[font="Times]TreeSet,SortedSet,[font="Times]List的有[font="Times]ArrayList等,[font="Times]Map里有[font="Times]HashMap,HashTable等,[font="Times]Queue里面有[font="Times]BlockingQueue等。我们来看看例子吧:
|jyD@Q,4 "rR$2`v" ECEDNib 实践:[font="Times] Set举例
GK-__Y. [font="Times] import java.util.*;
T*8rR" [font="Times]public class SetExample {
b<?A [font="Times] public static void main(String[] args) {
2v9T&xo= [font="Times] Set set = new HashSet(); //HashSet是[font="Times]Set的子接口
T|FF&|Pk [font="Times] set.add("one");
d
,!sZ&v [font="Times] set.add("second");
YsXf+_._ [font="Times] set.add("3rd");
ThmN^N [font="Times] set.add(new Integer(4));
v05B7^1@_ [font="Times] set.add(new Float( 5.0F ));
@-F[3`HeA [font="Times] set.add("second");
"I`g(q#Uo [font="Times] set.add(new Integer(4));
"=ogO/_Q" [font="Times] System.out.println(set);
j;+!BKWy4 [font="Times] }}
4xE [S EFYyr f@ [font="Times]List举例:
"7T9d) [font="Times] import java.util.*;
B-
VhUS [font="Times]public class ListExample {
sb7~sa&- [font="Times] public static void main(String[] args) {
<RMrp@[ [font="Times] List list = new ArrayList();
%+y92'GqG/ [font="Times] list.add("one");
D`G ;kp [font="Times] list.add("second");
mCY+V~^~kz [font="Times] list.add("3rd");
u0vq`5L [font="Times] list.add(new Integer(4));
0R0j7\{ [font="Times] list.add(new Float( 5.0F ));
Wr%E}mX- [font="Times] list.add("second");
)<D(Mb2p| [font="Times] list.add(new Integer(4));
} #[MV+D [font="Times] System.out.println(list);
v$O%U[e< [font="Times] }}
&J\V
!uVo f2sv$#' 6o_t;cpT [font="Times]Map举例
KT)A{i [font="Times]import java.util.Map;
Z|FWQ8gZ4m [font="Times]import java.util.HashMap;
O6LuFT. [font="Times]import java.util.Iterator;
X676*;:!. [font="Times]import java.io.FileReader;
RtrESwtR [font="Times]
2Mc}>UI?eO [font="Times]public class MapExample {
BwN65_5p [font="Times] public static void main(String[] args) throws java.io.FileNotFoundException {
A
'Q
nL [font="Times] Map word_count_map = new HashMap();
p&nIUx" [font="Times] FileReader reader = new FileReader(args[0]);
lbw*T [font="Times] Iterator words = new WordStreamIterator(reader);
YGBVGpE9 [font="Times]
W2F %E [font="Times] while ( words.hasNext() ) {
pZu2[ [font="Times] String word = (String) words.next();
Zd/ACZ[ [font="Times] String word_lowercase = word.toLowerCase();
2d,q?VH$ [font="Times] Integer frequency = (Integer)word_count_map.get(word_lowercase);
rUX1Iu7 [font="Times]if ( frequency == null ) {
$e=pdD~ [font="Times] frequency = new Integer(1);
=R08B)yR [font="Times] } else {
3[RbVT [font="Times] int value = frequency.intValue();
7b,5*]oZ [font="Times] frequency = new Integer(value + 1);}
s
V70a3# [font="Times] word_count_map.put(word_lowercase, frequency);
[@U2a$k+d [font="Times] }
/( /)nYAjk [font="Times] System.out.println(word_count_map);
{iGk~qN [font="Times] }}
sLE@Cm]k -r9G5Z!|n MtYi8"+<e. [font="Times]Queue举例:
`%lgT+~T [font="Times]import java.io.IOException;
RCED
K\*m [font="Times]import java.io.PrintStream;
-5Qsc/s& [font="Times]import java.util.LinkedList;
"o
+" Jd [font="Times]import java.util.Queue;
7l"N%e [font="Times]
5*=a*nD11 [font="Times]public class QueueTester {
%Lec\(-4L [font="Times] public Queue q; //发现了一个奇怪的语法,这个尖括号是泛型声明
fr~e!!$H [font="Times] public QueueTester() {q = new LinkedList();}
~/hyf] *j [font="Times]public void testFIFO(PrintStream out) throws IOException {
u:+wuyu [font="Times] q.add("First");
sK~d{)+T [font="Times] q.add("Second");
zG_e= [font="Times] q.add("Third");
eGrC0[SH [font="Times] Object o;
Is~bA_-
; [font="Times] while ((o = q.poll()) != null) {
f
n9[Li [font="Times] out.println(o);}}
Y[=Gv6Fr [font="Times] public static void main(String[] args) {
*@J [font="Times] QueueTester tester = new QueueTester();
H\tz"<*`` [font="Times] try { tester.testFIFO(System.out);
&j}\ZD [font="Times] } catch (IOException e) {
bP%0T++vo [font="Times] e.printStackTrace(); } }}
;m{*iKL6{ 2| u 'J X4emhB 上述例子和一些相关例子(共6个例子)打包下载[font="Times]
>|jSd2_p 总结:
k~ #F@_ 刚才我们看了上述例子了,对集合类有了一个初步的认识,它们跟数组的区别不只是长度问题,在集合类里面放进去的类型可以是任意的。不像数组,数组里面的类型是一样的。这样的话,对于集合类来说即使好事,也是坏事。因为你不考虑类型可以随意的放,但是你放进去什么就不知道了不容易找。
V#;6<H" 还有啊,什么叫泛型声明啊?我们下次课就告诉你。这可是[font="Times]jdk5的酷炫之处哦。