全新java初学者实践教程10(java SE5.0版)
gE-tjoJ jdk5的集合类
>[f?vrz :x3QRF t}_r]E,{u cx,+k]9D 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。
39c2pV[ 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。
*YI98 [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。
yHYsZ,GE `K"L /I9 [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]
v4<nI;Ux /*~EO{o [font="Times](1) Set
$B+8Of 集([font="Times]set)是最简单的一种集合,它的对象不按特定方式排序,只是简单的把对象加入集合中,就像往口袋里放东西。对集中成员的访问和操作是通过集中对象的引用进行的,所以集中不能有重复对象。我们知道数学上的集合也是[font="Times]Set这个,集合里面一定是没有重复的元素的。
D'>_I. |*Yr<zt ([font="Times]2)[font="Times]List
H.P_]3f 列表([font="Times]List)的主要特征是其对象以线性方式存储,没有特定顺序,只有一个开头和一个结尾,当然,它与根本没有顺序的[font="Times]Set是不同的。它是链表嘛,一条链肯定有顺序这个顺序就不一定了。
a"1t-x #&+{mCjs T}Tp$.gB [font="Times] ([font="Times]3)[font="Times]Map
3=#<X-); [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。
E#RDqL*J xH4m| xa'*P=<)C' [font="Times] ([font="Times]4)[font="Times]Queue
F-Qzrqu S [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。
.D~;u-%|F [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:
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`o ]2A^1Del 实践:[font="Times] Set举例
;7*[Bcj. [font="Times] import java.util.*;
=}^9 wP [font="Times]public class SetExample {
AD>e?u [font="Times] public static void main(String[] args) {
:]K4KFM [font="Times] Set set = new HashSet(); //HashSet是[font="Times]Set的子接口
Z9E\,Ly [font="Times] set.add("one");
`%bypHeSp [font="Times] set.add("second");
Xfc-UP|} [font="Times] set.add("3rd");
q_lKKzA [font="Times] set.add(new Integer(4));
Q>qUk@ [font="Times] set.add(new Float( 5.0F ));
d7^}tM [font="Times] set.add("second");
yZ7&b&2nLn [font="Times] set.add(new Integer(4));
(y'hyJo [font="Times] System.out.println(set);
Y;eZ9|Ht9 [font="Times] }}
[|wZ77\ Z{.8^u1I [font="Times]List举例:
NSMyliM1Y [font="Times] import java.util.*;
BU)U/A8iS [font="Times]public class ListExample {
wVXS%4|v [font="Times] public static void main(String[] args) {
&<g|gsG` [font="Times] List list = new ArrayList();
f^ZRT@`O [font="Times] list.add("one");
>~rTqtKd [font="Times] list.add("second");
O^PKn_OJ [font="Times] list.add("3rd");
?5__oT [font="Times] list.add(new Integer(4));
3d8L6GJ [font="Times] list.add(new Float( 5.0F ));
eu|YCYj)g [font="Times] list.add("second");
y8Ir@qp5 [font="Times] list.add(new Integer(4));
?PxP% $hS [font="Times] System.out.println(list);
~hH REI& [font="Times] }}
;1W6G=m <V'@ks% wo}H'Q}Hj [font="Times]Map举例
}v;V=%N+v [font="Times]import java.util.Map;
~Gp[_ %K [font="Times]import java.util.HashMap;
.<?GS{6
N [font="Times]import java.util.Iterator;
yF:1( 4 [font="Times]import java.io.FileReader;
0JS?; fk [font="Times]
bRDYGuC [font="Times]public class MapExample {
e
,'_xV [font="Times] public static void main(String[] args) throws java.io.FileNotFoundException {
]]![EHi(\ [font="Times] Map word_count_map = new HashMap();
TprTWod2]t [font="Times] FileReader reader = new FileReader(args[0]);
M.D1XX1/ [font="Times] Iterator words = new WordStreamIterator(reader);
# +>oZWVc [font="Times]
ldcqe$7, [font="Times] while ( words.hasNext() ) {
68|E9^`l [font="Times] String word = (String) words.next();
iU918!!N [font="Times] String word_lowercase = word.toLowerCase();
LP^$AAy [font="Times] Integer frequency = (Integer)word_count_map.get(word_lowercase);
H'5)UX@LP [font="Times]if ( frequency == null ) {
uC vj! [font="Times] frequency = new Integer(1);
"!P3R1;% [font="Times] } else {
je-!4r, [font="Times] int value = frequency.intValue();
y1 DL,%j [font="Times] frequency = new Integer(value + 1);}
B
IEO,W| [font="Times] word_count_map.put(word_lowercase, frequency);
HWAdhDZ [font="Times] }
m@j?za9s [font="Times] System.out.println(word_count_map);
M^Yh|%M [font="Times] }}
ssA`I<p # ,,.QfUj/& FXCMR\BsQ [font="Times]Queue举例:
7"D",1h [font="Times]import java.io.IOException;
2|y"!JqE1 [font="Times]import java.io.PrintStream;
+/7?HGf [font="Times]import java.util.LinkedList;
SR
hiQ [font="Times]import java.util.Queue;
yzn%<H~ [font="Times]
,KZ~?3$yj [font="Times]public class QueueTester {
!n!*/[}X [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
8nqG<!,q [font="Times] public QueueTester() {q = new LinkedList();}
c):/!Q [font="Times]public void testFIFO(PrintStream out) throws IOException {
539>WyG5 [font="Times] q.add("First");
VCfl`Aq'l [font="Times] q.add("Second");
s)t@ol [font="Times] q.add("Third");
M?49TOQA [font="Times] Object o;
*R,5h2; [font="Times] while ((o = q.poll()) != null) {
`hm-.@f,9 [font="Times] out.println(o);}}
//MUeTxR [font="Times] public static void main(String[] args) {
**0~K" ;\ [font="Times] QueueTester tester = new QueueTester();
sdrfsrNvB- [font="Times] try { tester.testFIFO(System.out);
%0?KMRr [font="Times] } catch (IOException e) {
0auYG><= [font="Times] e.printStackTrace(); } }}
FUzzB94a By,eETU] b_krk\e@S 上述例子和一些相关例子(共6个例子)打包下载[font="Times]
iyE7V_O T 总结:
Q*cf( 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。
XJB)rP 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。