全新java初学者实践教程10(java SE5.0版)
Mo @C9Y0 jdk5的集合类
*"n vX2iz okv 1K rZ^v?4Z\ YY!6/5*/] 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。
+0q>fp_K(+ 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。
p2udm! )J [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。
q**G(}K OYmR<x5y/ [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]
<l9-;2L4 !\L/[:n [font="Times](1) Set
+g]yA3 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。
ugx%_x6 fUQ6Z,9 ([font="Times]2)[font="Times]List
?Poq2 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。
ehG/zVgn Zrr5csE !M]\I & [font="Times] ([font="Times]3)[font="Times]Map
sZm$|T0 [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。
i21Gw41p: e `,ds~ F^LZeF[#t [font="Times] ([font="Times]4)[font="Times]Queue
FMkzrs [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。
x61 U[/r [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:
H;fxxu`cS z0*_^MH MxcFvo*LCp 实践:[font="Times] Set举例
wz.6du6- [font="Times] import java.util.*;
eT8} [font="Times]public class SetExample {
H4!+q:< [font="Times] public static void main(String[] args) {
/E5 5Pec [font="Times] Set set = new HashSet(); //HashSet是[font="Times]Set的子接口
^:* 1d
\ [font="Times] set.add("one");
?Wt$6{) [font="Times] set.add("second");
pd8Nke [font="Times] set.add("3rd");
'ao"9-c [font="Times] set.add(new Integer(4));
s)2fG\1 [font="Times] set.add(new Float( 5.0F ));
{aC!~qR [font="Times] set.add("second");
&F5@6nJ` [font="Times] set.add(new Integer(4));
y>|{YWbp? [font="Times] System.out.println(set);
\qR %%S [font="Times] }}
ADk8{L{UU H0R&2#YD [font="Times]List举例:
aKJQm'9Ks [font="Times] import java.util.*;
DHT&,= [font="Times]public class ListExample {
TdGnf [font="Times] public static void main(String[] args) {
BQ2wnGc [font="Times] List list = new ArrayList();
BC;: [font="Times] list.add("one");
,b;{emX h [font="Times] list.add("second");
_#}n~}d [font="Times] list.add("3rd");
PF7&p~O(Z [font="Times] list.add(new Integer(4));
TkO[rAC [font="Times] list.add(new Float( 5.0F ));
-,{-bi [font="Times] list.add("second");
xOHgp=#D [font="Times] list.add(new Integer(4));
x5WW--YR+ [font="Times] System.out.println(list);
4[-*~C|W5 [font="Times] }}
p6XtTx xvSuPP4 m &gE 75B [font="Times]Map举例
mA@Me7m} [font="Times]import java.util.Map;
Iu*^xn [font="Times]import java.util.HashMap;
~ Yngkt [font="Times]import java.util.Iterator;
f5==";eP [font="Times]import java.io.FileReader;
wL^%w9q- [font="Times]
NwR}yb6 [font="Times]public class MapExample {
k8uvNLA)a [font="Times] public static void main(String[] args) throws java.io.FileNotFoundException {
LJTQaItdqJ [font="Times] Map word_count_map = new HashMap();
j]m|7] [font="Times] FileReader reader = new FileReader(args[0]);
ed_FiQd [font="Times] Iterator words = new WordStreamIterator(reader);
zb
Z4|_ [font="Times]
'vaLUy9] [font="Times] while ( words.hasNext() ) {
_:B1_rz7, [font="Times] String word = (String) words.next();
vuFBET, [font="Times] String word_lowercase = word.toLowerCase();
\Ro^*4B [font="Times] Integer frequency = (Integer)word_count_map.get(word_lowercase);
K[7EOXLy [font="Times]if ( frequency == null ) {
z|(+|pV( [font="Times] frequency = new Integer(1);
ii0Ce}8d~ [font="Times] } else {
y{a$y}7#X [font="Times] int value = frequency.intValue();
.+([ [font="Times] frequency = new Integer(value + 1);}
^+9sG$T_EV [font="Times] word_count_map.put(word_lowercase, frequency);
`H3.,] [font="Times] }
`3'0I /d"z [font="Times] System.out.println(word_count_map);
~b|`'kU [font="Times] }}
ZB[Qs ?`Y\)'} J!*/a'Cv [font="Times]Queue举例:
02k4N% [font="Times]import java.io.IOException;
xlR2|4|8 [font="Times]import java.io.PrintStream;
&X]\)`j0 [font="Times]import java.util.LinkedList;
2. X" f [font="Times]import java.util.Queue;
UP{j5gR:_ [font="Times]
Y}D onF [font="Times]public class QueueTester {
=0'q!}._! [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
]k8/#@19 [font="Times] public QueueTester() {q = new LinkedList();}
irZFV
[font="Times]public void testFIFO(PrintStream out) throws IOException {
Kw`VrcwjT [font="Times] q.add("First");
eb8w~ [font="Times] q.add("Second");
s$*'^: [font="Times] q.add("Third");
x)_@9ldYv [font="Times] Object o;
m%8qZzqk [font="Times] while ((o = q.poll()) != null) {
DBs*Fx[ [font="Times] out.println(o);}}
1]T`n /d V [font="Times] public static void main(String[] args) {
2qO3XI [font="Times] QueueTester tester = new QueueTester();
{3Vk p5%l [font="Times] try { tester.testFIFO(System.out);
U\?g* [font="Times] } catch (IOException e) {
g3%t8O/M [font="Times] e.printStackTrace(); } }}
ro[Y-o5Q0 Fequm+ h
!(>7/Gi 上述例子和一些相关例子(共6个例子)打包下载[font="Times]
.M4IGOvOS 总结:
5b6s4ZyV 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。
,s^<X85gp\ 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。