全新java初学者实践教程10(java SE5.0版)
y3 R+060\3 jdk5的集合类
U/e$.K3v 1HF=,K+ _dT,%q Wq+6`o 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。
1?6;Oc^ 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。
ng-g\&- [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。
7n-;++a5] K&t+3O [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]
<23oyMR0 =+T$1 [font="Times](1) Set
OIuEC7XM^C 集([font="Times]set)是最简单的一种集合,它的对象不按特定方式排序,只是简单的把对象加入集合中,就像往口袋里放东西。对集中成员的访问和操作是通过集中对象的引用进行的,所以集中不能有重复对象。我们知道数学上的集合也是[font="Times]Set这个,集合里面一定是没有重复的元素的。
h|_E>6d) 4v"9I( ([font="Times]2)[font="Times]List
@w)Vt$+b] 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。
a)+;<GZ~ 'nOc_b0 gg>O:np8 [font="Times] ([font="Times]3)[font="Times]Map
.Y8z3O [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。
?I6us X9$ nMK,g>wp e5!LbsJv [font="Times] ([font="Times]4)[font="Times]Queue
TwlrncK* [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。
jpl"KN?X [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:
%<|w:z$vp |fx*F}1 vjy 59m 实践:[font="Times] Set举例
zFy0SzF [font="Times] import java.util.*;
o1j_5c
PS [font="Times]public class SetExample {
#S57SD [font="Times] public static void main(String[] args) {
,4bqjkX5q [font="Times] Set set = new HashSet(); //HashSet是[font="Times]Set的子接口
Y <'T;@ [font="Times] set.add("one");
|U*wMYC [font="Times] set.add("second");
XwKB+Yj0 [font="Times] set.add("3rd");
jl;%?bx [font="Times] set.add(new Integer(4));
Hshm;\' [font="Times] set.add(new Float( 5.0F ));
}uJH!@j [font="Times] set.add("second");
_S4 3_hW [font="Times] set.add(new Integer(4));
/bE=]nM [font="Times] System.out.println(set);
EkV
LSur [font="Times] }}
lg)jc3 I`?6>Z+%) [font="Times]List举例:
_5F8F4QY` [font="Times] import java.util.*;
/8\gT(@ [font="Times]public class ListExample {
?0>%
a$` [font="Times] public static void main(String[] args) {
3cuVyf<v [font="Times] List list = new ArrayList();
+I$ k_ [font="Times] list.add("one");
1(z&0Y ; [font="Times] list.add("second");
EgAM,\ [font="Times] list.add("3rd");
1l$c*STK [font="Times] list.add(new Integer(4));
9%>GOY [font="Times] list.add(new Float( 5.0F ));
P8lx\DA [font="Times] list.add("second");
Ww9%6 #it [font="Times] list.add(new Integer(4));
q*,g [font="Times] System.out.println(list);
1wX0x.4d [font="Times] }}
KZFnp=i iElE-g@Ws
jpcbW [font="Times]Map举例
:x*|?zII [font="Times]import java.util.Map;
M_wqb'= [font="Times]import java.util.HashMap;
GCn^+`.h1t [font="Times]import java.util.Iterator;
2!Ip!IQ: [font="Times]import java.io.FileReader;
{\!_S+}{ [font="Times]
p>w{.hC@ [font="Times]public class MapExample {
4l0ON>W( [font="Times] public static void main(String[] args) throws java.io.FileNotFoundException {
Xr54/.{&@ [font="Times] Map word_count_map = new HashMap();
%d<uOCf\Q [font="Times] FileReader reader = new FileReader(args[0]);
R5i v]8X4W [font="Times] Iterator words = new WordStreamIterator(reader);
%E\&9, [font="Times]
=YZyH4eI [font="Times] while ( words.hasNext() ) {
.VFa,&5;3 [font="Times] String word = (String) words.next();
8cq H0{ [font="Times] String word_lowercase = word.toLowerCase();
qJY'"_Q{ [font="Times] Integer frequency = (Integer)word_count_map.get(word_lowercase);
Y|eB;Dm1q [font="Times]if ( frequency == null ) {
YzTmXwuA5 [font="Times] frequency = new Integer(1);
JZQT} [font="Times] } else {
Vo@[ [font="Times] int value = frequency.intValue();
mez )G| [font="Times] frequency = new Integer(value + 1);}
OGgP~hd [font="Times] word_count_map.put(word_lowercase, frequency);
;J"b% ~Gn [font="Times] }
7_,)"J2^ [font="Times] System.out.println(word_count_map);
[nQ<pTg~r [font="Times] }}
m'|{AjH
z6 cR0+`& (E'f'g [font="Times]Queue举例:
$tGk,.#j [font="Times]import java.io.IOException;
Z:Hk'|q}I [font="Times]import java.io.PrintStream;
EeW %5/; [font="Times]import java.util.LinkedList;
jD@KG [font="Times]import java.util.Queue;
5ZG-3qj [font="Times]
c< ke)@ [font="Times]public class QueueTester {
cTy;?(E [font="Times] public Queue q; //发现了一个奇怪的语法,这个尖括号是泛型声明
4~<
:Pj [font="Times] public QueueTester() {q = new LinkedList();}
a78&< [font="Times]public void testFIFO(PrintStream out) throws IOException {
?S9!;x< [font="Times] q.add("First");
~~h@(2/Q>x [font="Times] q.add("Second");
?eUhHKS5 [font="Times] q.add("Third");
6qsT/ [font="Times] Object o;
9v_B$F$_T [font="Times] while ((o = q.poll()) != null) {
[[{y?-U [font="Times] out.println(o);}}
s`2Hf&%aZJ [font="Times] public static void main(String[] args) {
>L6V! [font="Times] QueueTester tester = new QueueTester();
1b)^5U ; [font="Times] try { tester.testFIFO(System.out);
YG<7Zv
[font="Times] } catch (IOException e) {
9Ra_[1 [font="Times] e.printStackTrace(); } }}
a0y7a/@c f_}FYeg ocwh*t)<k 上述例子和一些相关例子(共6个例子)打包下载[font="Times]
A;~u"g 'z& 总结:
c hE~UQ 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。
b"-eQb 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。