全新java初学者实践教程10(java SE5.0版)
%~u]|q<{ jdk5的集合类
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Y t(D 9]4Q@% 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。
sPH2KwEv 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。
3SVGx<,2 [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。
l/.{F ;3F 5\ mRH [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]
v=U<exM6% ]G/m,Zv*: [font="Times](1) Set
=RoG?gd{R 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。
eV9U+]C` pv_o4qEN ([font="Times]2)[font="Times]List
3:J>-MO 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。
AGlBvRX7e G@]3EP Hfcpqa [font="Times] ([font="Times]3)[font="Times]Map
Jj4HJ9 [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。
$J[( 3 broLC5hbQU c+Q'4E0| [font="Times] ([font="Times]4)[font="Times]Queue
++cS^ Lo [font="Times] 在[font="Times]jdk5.0以前,通常的实现方式是使用[font="Times]java.util.List集合来模仿[font="Times]Queue。[font="Times]Queue的概念通过把对象添加(称为[font="Times]enqueuing的操作)到[font="Times]List的尾部(即[font="Times]Queue的后部)并通过从[font="Times]List的头部(即[font="Times]Queue的前部)提取对象而从[font="Times] List中移除(称为[font="Times]dequeuing的操作)来模拟。你需要执行先进先出的动作时可以直接使用[font="Times]Queue接口就可以了。
"Ep"$d [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:
c;l!i- XiUq#84Q MmF&jd-= 实践:[font="Times] Set举例
w#A)B<Y/" [font="Times] import java.util.*;
(MwRe?Ih [font="Times]public class SetExample {
,}oAc [font="Times] public static void main(String[] args) {
;Afz`Se1@ [font="Times] Set set = new HashSet(); //HashSet是[font="Times]Set的子接口
p~D}Iyww1_ [font="Times] set.add("one");
djd/QAfSC [font="Times] set.add("second");
)U/jD [font="Times] set.add("3rd");
R9J!}az' [font="Times] set.add(new Integer(4));
ZpTDM1ro [font="Times] set.add(new Float( 5.0F ));
s8h*nZ)v [font="Times] set.add("second");
=sJ?]U [font="Times] set.add(new Integer(4));
R\j~X@vI [font="Times] System.out.println(set);
&K ~k'P~m [font="Times] }}
&g`IRz BWt`l,nF [font="Times]List举例:
Y;i=c6 [font="Times] import java.util.*;
o) )` "^ [font="Times]public class ListExample {
c6h?b[] [font="Times] public static void main(String[] args) {
inut'@=G/ [font="Times] List list = new ArrayList();
!T6R[ [font="Times] list.add("one");
Oa|c ?|+ [font="Times] list.add("second");
|RX#5Q>z [font="Times] list.add("3rd");
X\^nV [font="Times] list.add(new Integer(4));
[doEArwn [font="Times] list.add(new Float( 5.0F ));
s68(jYC7[ [font="Times] list.add("second");
2W AeSUX [font="Times] list.add(new Integer(4));
.-gJS-.c [font="Times] System.out.println(list);
D,#UJPyg [font="Times] }}
|]\zlH"w fY<#KM6X AwM`[`ReE [font="Times]Map举例
`7"="T~ * [font="Times]import java.util.Map;
<BR^Dv07U [font="Times]import java.util.HashMap;
.. `I<2 [font="Times]import java.util.Iterator;
#M-!/E [font="Times]import java.io.FileReader;
SUS=sR/N [font="Times]
fG0 ?"x@> [font="Times]public class MapExample {
1J?v\S$ma` [font="Times] public static void main(String[] args) throws java.io.FileNotFoundException {
5EYGA\ [font="Times] Map word_count_map = new HashMap();
.9~j%]q [font="Times] FileReader reader = new FileReader(args[0]);
,H=k5WA4m [font="Times] Iterator words = new WordStreamIterator(reader);
6VW*8~~Xy [font="Times]
ZW4f " [font="Times] while ( words.hasNext() ) {
e~)[I! n [font="Times] String word = (String) words.next();
'uL4ezTtA [font="Times] String word_lowercase = word.toLowerCase();
(x=$b(I [font="Times] Integer frequency = (Integer)word_count_map.get(word_lowercase);
YWZ;@,W [font="Times]if ( frequency == null ) {
@G5T8qwN [font="Times] frequency = new Integer(1);
VjQ&A#
[font="Times] } else {
H 0l1=y [font="Times] int value = frequency.intValue();
1@]&iZ] [font="Times] frequency = new Integer(value + 1);}
)[rVg/m [font="Times] word_count_map.put(word_lowercase, frequency);
l\<.*6r [font="Times] }
fO<40!%9cQ [font="Times] System.out.println(word_count_map);
gOF^?M11x [font="Times] }}
rN0<y4)! sJ6.3=
c F8pA)!AH [font="Times]Queue举例:
=uP?
?E [font="Times]import java.io.IOException;
(bwD:G9 [font="Times]import java.io.PrintStream;
B[b>T= [font="Times]import java.util.LinkedList;
65EMB% [font="Times]import java.util.Queue;
0 QTI;3 [font="Times]
YT(N][V [font="Times]public class QueueTester {
kx,.)qKk [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
=p5DT [font="Times] public QueueTester() {q = new LinkedList();}
<-VBb[M# [font="Times]public void testFIFO(PrintStream out) throws IOException {
h}Otz " [font="Times] q.add("First");
`/O`%6,f1! [font="Times] q.add("Second");
6tKrR{3#A [font="Times] q.add("Third");
QLqtE;;)JK [font="Times] Object o;
?=1eHnP!R [font="Times] while ((o = q.poll()) != null) {
qb>ULP0 [font="Times] out.println(o);}}
r:*G{m- [font="Times] public static void main(String[] args) {
ON2o^-%= [font="Times] QueueTester tester = new QueueTester();
H|%J" [font="Times] try { tester.testFIFO(System.out);
{npm9w<; [font="Times] } catch (IOException e) {
:=Olp;+_ [font="Times] e.printStackTrace(); } }}
*,\v|]fc B+W7zv oE 'P 上述例子和一些相关例子(共6个例子)打包下载[font="Times]
10SI&O 总结:
?I+L 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。
8dE0y P 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。