全新java初学者实践教程10(java SE5.0版)
7hzd. jdk5的集合类
Md*~hb8J VYo2m { Mv$~T|e7 ZRy'lW 上次课我们学过了数组,知道它只是一组数(或是对象),但是有些自己的特性。在[font="Times]java里还有一类东西与数组类似,也是有着特性的一组数[font="Times](或是对象[font="Times]),叫做集合类。
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i6F>f 我们上节课讲到了,数组的长度在创建时已经确定了,但是有时候我们事先根本不知道长度是多少啊,比如我们做电子商务网站时,有个购物车程序。你总不能用数组规定,人家只能买[font="Times]5样东西吧。你就是把长度定为[font="Times]10000也不行,万一遇上个特别有钱的呢!呵呵,这只是开玩笑的。我们会使用集合类解决这个问题。
(nDen5Q| [font="Times] 集合类是放在[font="Times]java.util.*;这个包里。集合类存放的都是对象的引用,而非对象本身,为了说起来方便些,我们称集合中的对象就是指集合中对象的引用([font="Times]reference)。引用的概念大家不会忘了吧,在前边我们讲
数据类型时讲的。
n40&4n 2(i@\dZCb< [font="Times] 集合类型主要有[font="Times]3种:[font="Times]set(集)、[font="Times]list(列表)、[font="Times]map(映射[font="Times])和[font="Times]Queue(队列)。[font="Times]//队列为[font="Times]jdk5中的加上的[font="Times]
=C(((T. (i.7\$4 [font="Times](1) Set
d69synEw>k 集([font="Times]set)是最简单的一种集合,它的对象不按特定方式排序,只是简单的把对象加入集合中,就像往口袋里放东西。对集中成员的访问和操作是通过集中对象的引用进行的,所以集中不能有重复对象。我们知道数学上的集合也是[font="Times]Set这个,集合里面一定是没有重复的元素的。
}m -A #4. Dk^T_7{ ([font="Times]2)[font="Times]List
)*`cJ_t 列表([font="Times]List)的主要特征是其对象以线性方式存储,没有特定顺序,只有一个开头和一个结尾,当然,它与根本没有顺序的[font="Times]Set是不同的。它是链表嘛,一条链肯定有顺序这个顺序就不一定了。
1$S`>M%a bSkr:|A7 =M],5<2; [font="Times] ([font="Times]3)[font="Times]Map
khb/"VYd [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。
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zy 6{ [font="Times] ([font="Times]4)[font="Times]Queue
W @]t [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。
8#ZF<BY [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:
..fbRt 2uzy]faM m85Hx1!p. 实践:[font="Times] Set举例
O!k C [font="Times] import java.util.*;
F T$x#> [font="Times]public class SetExample {
SH5a&OVZhn [font="Times] public static void main(String[] args) {
N
Wf IRL [font="Times] Set set = new HashSet(); //HashSet是[font="Times]Set的子接口
{'UK>S [font="Times] set.add("one");
7m.>2U [font="Times] set.add("second");
>* h3u7t [font="Times] set.add("3rd");
r:U/a=V [font="Times] set.add(new Integer(4));
W}P9I&3 [font="Times] set.add(new Float( 5.0F ));
.YlhK=d4 [font="Times] set.add("second");
uWkn}P [font="Times] set.add(new Integer(4));
l&_PsnU [font="Times] System.out.println(set);
NI85|*h [font="Times] }}
:zQNnq:| sI5S)^'IQ [font="Times]List举例:
62 9g_P) [font="Times] import java.util.*;
` 1vDp. [font="Times]public class ListExample {
"Ky&x$dje [font="Times] public static void main(String[] args) {
<"@5. f1"Y [font="Times] List list = new ArrayList();
&W{<Yf9 [font="Times] list.add("one");
c@x6<S%* [font="Times] list.add("second");
Wg=q lux- [font="Times] list.add("3rd");
NArr2o2 [font="Times] list.add(new Integer(4));
t[p/65L>8 [font="Times] list.add(new Float( 5.0F ));
`4-m$ab [font="Times] list.add("second");
:U,-v [font="Times] list.add(new Integer(4));
M~g{}_0Z [font="Times] System.out.println(list);
/kV5~i<1S [font="Times] }}
JOo+RA5d ke2}@|?t w3FEX$`_ [font="Times]Map举例
42=/$V [font="Times]import java.util.Map;
qHu\3@px [font="Times]import java.util.HashMap;
nq:'jdY5| [font="Times]import java.util.Iterator;
"""pe+Y [font="Times]import java.io.FileReader;
&F.L*M [font="Times]
vRLWs`1j [font="Times]public class MapExample {
AWDy_11Nm [font="Times] public static void main(String[] args) throws java.io.FileNotFoundException {
\xKhbpO~ [font="Times] Map word_count_map = new HashMap();
v+W4wD [font="Times] FileReader reader = new FileReader(args[0]);
{'6-;2&f [font="Times] Iterator words = new WordStreamIterator(reader);
5P[urOvV [font="Times]
#c(BBTuX [font="Times] while ( words.hasNext() ) {
TTJj=KPA [font="Times] String word = (String) words.next();
>/'/^h [font="Times] String word_lowercase = word.toLowerCase();
`V{'GF&[ [font="Times] Integer frequency = (Integer)word_count_map.get(word_lowercase);
h]WW?. [font="Times]if ( frequency == null ) {
hR,5U=+M7 [font="Times] frequency = new Integer(1);
u@T,8 [font="Times] } else {
x{3q'2 [font="Times] int value = frequency.intValue();
r85j/YK [font="Times] frequency = new Integer(value + 1);}
d5'4RYfkQ [font="Times] word_count_map.put(word_lowercase, frequency);
}CvhLjo [font="Times] }
OBf$0 [font="Times] System.out.println(word_count_map);
BOme`0A [font="Times] }}
r#w.yg4EX [_HOD^ `.F3&pA [font="Times]Queue举例:
AtP!.p"j [font="Times]import java.io.IOException;
;O {"\H6 [font="Times]import java.io.PrintStream;
CK8!7=>}^ [font="Times]import java.util.LinkedList;
^4=%~Yx [font="Times]import java.util.Queue;
`%$+rbo~ [font="Times]
Q+dLWFI [font="Times]public class QueueTester {
+0M0g_sk [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
"]B%V!@ [font="Times] public QueueTester() {q = new LinkedList();}
MI>_wG5P@ [font="Times]public void testFIFO(PrintStream out) throws IOException {
uE'Kk8 [font="Times] q.add("First");
*mkL>v & [font="Times] q.add("Second");
x<=R?4@rq [font="Times] q.add("Third");
b5W(}ka+ [font="Times] Object o;
vA*Q}]Ov [font="Times] while ((o = q.poll()) != null) {
V(0V$&qipc [font="Times] out.println(o);}}
%(>,eee_ [font="Times] public static void main(String[] args) {
[#;CBs5o [font="Times] QueueTester tester = new QueueTester();
m:c .dei5 [font="Times] try { tester.testFIFO(System.out);
WU~L#Ih.V [font="Times] } catch (IOException e) {
_z53r+A [font="Times] e.printStackTrace(); } }}
s/Q8(sF5 NZZc[P dg4q+ 上述例子和一些相关例子(共6个例子)打包下载[font="Times]
.DHQJ|J-1 总结:
[BFPIVD)h] 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。
unt{RVR% 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。