全新java初学者实践教程10(java SE5.0版)
~0|hobk jdk5的集合类
vM/v}6;_K2 AtDrQ<>y' $lA,{Q 59J9V3na 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。
UAZ&*{MM^ 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。
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\ C,^ [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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G[hU4L Yur)_m [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]
;r"B?] JO |$2N$6\SP [font="Times](1) Set
J
*?_SnZ 集([font="Times]set)是最简单的一种集合,它的对象不按特定方式排序,只是简单的把对象加入集合中,就像往口袋里放东西。对集中成员的访问和操作是通过集中对象的引用进行的,所以集中不能有重复对象。我们知道数学上的集合也是[font="Times]Set这个,集合里面一定是没有重复的元素的。
Vz]=J;`Mz C:MGi7f ([font="Times]2)[font="Times]List
^^l"brPa 列表([font="Times]List)的主要特征是其对象以线性方式存储,没有特定顺序,只有一个开头和一个结尾,当然,它与根本没有顺序的[font="Times]Set是不同的。它是链表嘛,一条链肯定有顺序这个顺序就不一定了。
9G+rxyWMW D:tZiS=0 ycD.:w p\' [font="Times] ([font="Times]3)[font="Times]Map
YCO:bBmp: [font="Times] 映射([font="Times]Map),这个在[font="Times]java里不是地图的意思,其实地图也是映射哈。它里面的东西是键-值对([font="Times]key-value)出现的,键值对是什么呢?举个例子,比如我们查字典,用部首查字法。目录那个字就是键,这个字的解释就是值。键和值成对出现。这样说可以理解吧。这也是很常用的数据结构哦。
W2qQKv w lg#c6#q cV,Dl`1r [font="Times] ([font="Times]4)[font="Times]Queue
Po.BcytM [font="Times] 在[font="Times]jdk5.0以前,通常的实现方式是使用[font="Times]java.util.List集合来模仿[font="Times]Queue。[font="Times]Queue的概念通过把对象添加(称为[font="Times]enqueuing的操作)到[font="Times]List的尾部(即[font="Times]Queue的后部)并通过从[font="Times]List的头部(即[font="Times]Queue的前部)提取对象而从[font="Times] List中移除(称为[font="Times]dequeuing的操作)来模拟。你需要执行先进先出的动作时可以直接使用[font="Times]Queue接口就可以了。
|9]K:A [font="Times] 这[font="Times]4个东西,有时候功能还不太完善,需要有些子类继承它的特性。[font="Times]Set的子接口有[font="Times]TreeSet,SortedSet,[font="Times]List的有[font="Times]ArrayList等,[font="Times]Map里有[font="Times]HashMap,HashTable等,[font="Times]Queue里面有[font="Times]BlockingQueue等。我们来看看例子吧:
Tpx,41(k 98'XSL| PSS/JFZ^ 实践:[font="Times] Set举例
J6D$ i+ [font="Times] import java.util.*;
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|:x"L [font="Times]public class SetExample {
N06O.bji [font="Times] public static void main(String[] args) {
agT[y/gb [font="Times] Set set = new HashSet(); //HashSet是[font="Times]Set的子接口
"IJMvTmj [font="Times] set.add("one");
MWh+h7k' [font="Times] set.add("second");
qXhf?x [font="Times] set.add("3rd");
_C=[bI@ [font="Times] set.add(new Integer(4));
>0#q!H,X [font="Times] set.add(new Float( 5.0F ));
d ovwB`5 [font="Times] set.add("second");
^l&4UnLlc [font="Times] set.add(new Integer(4));
ky$:C,1t [font="Times] System.out.println(set);
^)^|;C\` [font="Times] }}
W r7e_ _kX/LR"L+ [font="Times]List举例:
%uqD\`- [font="Times] import java.util.*;
+\vY; !^ [font="Times]public class ListExample {
BV?N_/DXp [font="Times] public static void main(String[] args) {
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[font="Times] List list = new ArrayList();
{0WIDD [font="Times] list.add("one");
4Xk;Qd [font="Times] list.add("second");
F6]!?@ [font="Times] list.add("3rd");
T)f_W [font="Times] list.add(new Integer(4));
t0d '> [font="Times] list.add(new Float( 5.0F ));
@aN=U= [font="Times] list.add("second");
+{i"G,3 [font="Times] list.add(new Integer(4));
PFgjWp"Y [font="Times] System.out.println(list);
l'".}6S [font="Times] }}
42wC."A b(-t)5^} }.V0SM6 [font="Times]Map举例
>@"3Q` [font="Times]import java.util.Map;
IYg3ve`x [font="Times]import java.util.HashMap;
5>[sCl- [font="Times]import java.util.Iterator;
@^6OV) [font="Times]import java.io.FileReader;
U{uWk3I_b [font="Times]
gdFoTcHgO| [font="Times]public class MapExample {
NG!cEo:2aa [font="Times] public static void main(String[] args) throws java.io.FileNotFoundException {
3nC#$L- [font="Times] Map word_count_map = new HashMap();
s1 ^mk] [font="Times] FileReader reader = new FileReader(args[0]);
! vVjZ [font="Times] Iterator words = new WordStreamIterator(reader);
p2DNbY\] [font="Times]
Mth`s{sATa [font="Times] while ( words.hasNext() ) {
@j2*.ee [font="Times] String word = (String) words.next();
HT=Am [font="Times] String word_lowercase = word.toLowerCase();
jsi#l [font="Times] Integer frequency = (Integer)word_count_map.get(word_lowercase);
c$<O0dI [font="Times]if ( frequency == null ) {
7cP[o+ [font="Times] frequency = new Integer(1);
vJAAAS [font="Times] } else {
G[<[#$( [font="Times] int value = frequency.intValue();
[Ro0eH [font="Times] frequency = new Integer(value + 1);}
/Q>{YsRRB [font="Times] word_count_map.put(word_lowercase, frequency);
uQtk|)T E [font="Times] }
~eOj:H [font="Times] System.out.println(word_count_map);
{e[pSD6 [font="Times] }}
AH87UkNL = *;Xc-_ m#'rI=}! [font="Times]Queue举例:
Q1I_=fT [font="Times]import java.io.IOException;
*5_8\7d [font="Times]import java.io.PrintStream;
y_4krY|Zx [font="Times]import java.util.LinkedList;
#JR ,C
-w [font="Times]import java.util.Queue;
9eN2)a/ [font="Times]
VO;UV$$ [font="Times]public class QueueTester {
| ]!Ky[P [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
B6'%J [font="Times] public QueueTester() {q = new LinkedList();}
&Bz7fKCo [font="Times]public void testFIFO(PrintStream out) throws IOException {
V_A,d8=lt [font="Times] q.add("First");
[gh[F [font="Times] q.add("Second");
LXu"rfp [font="Times] q.add("Third");
%v+fN?%x,d [font="Times] Object o;
.Lr)~ [font="Times] while ((o = q.poll()) != null) {
G<^]0`"+)t [font="Times] out.println(o);}}
)\D40,p [font="Times] public static void main(String[] args) {
e]*=sp!T [font="Times] QueueTester tester = new QueueTester();
_QMHPRELk [font="Times] try { tester.testFIFO(System.out);
r{B,uj" [font="Times] } catch (IOException e) {
0.BUfuuh [font="Times] e.printStackTrace(); } }}
&kjwIg{ kGuk
-P $sL|'ZMbS 上述例子和一些相关例子(共6个例子)打包下载[font="Times]
q>|[JJ*6_N 总结:
&A9A#It 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。
$,QpSK`9i 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。