全新java初学者实践教程10(java SE5.0版)
Mg#j3W}] jdk5的集合类
yqSs,vz 65ly2gl fC}R4f7C L6>pGx 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。
,G#.BLH
cX 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。
g'];Estb~ [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。
mAk{"65V .qk]$LJF7 [font="Times] 集合类型主要有[font="Times]3种:[font="Times]set(集)、[font="Times]list(列表)、[font="Times]map(映射[font="Times])和[font="Times]Queue(队列)。[font="Times]//队列为[font="Times]jdk5中的加上的[font="Times]
rbT)=-( p;?*}xa [font="Times](1) Set
S4witIK5 集([font="Times]set)是最简单的一种集合,它的对象不按特定方式排序,只是简单的把对象加入集合中,就像往口袋里放东西。对集中成员的访问和操作是通过集中对象的引用进行的,所以集中不能有重复对象。我们知道数学上的集合也是[font="Times]Set这个,集合里面一定是没有重复的元素的。
jlFk@:y4 VF&Z%O3n ([font="Times]2)[font="Times]List
]pEV}@7 列表([font="Times]List)的主要特征是其对象以线性方式存储,没有特定顺序,只有一个开头和一个结尾,当然,它与根本没有顺序的[font="Times]Set是不同的。它是链表嘛,一条链肯定有顺序这个顺序就不一定了。
^\B:R, Kb =@ =Xta Z ,^9Z [font="Times] ([font="Times]3)[font="Times]Map
^IKO2Ft [font="Times] 映射([font="Times]Map),这个在[font="Times]java里不是地图的意思,其实地图也是映射哈。它里面的东西是键-值对([font="Times]key-value)出现的,键值对是什么呢?举个例子,比如我们查字典,用部首查字法。目录那个字就是键,这个字的解释就是值。键和值成对出现。这样说可以理解吧。这也是很常用的数据结构哦。
`IYuz: p0.|< M4ozTp<$O [font="Times] ([font="Times]4)[font="Times]Queue
CIjZG ?A [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。
'WHHc 9rG, [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:
)} DUMq7 pf4 ^Bk}e oJKa"H-jL 实践:[font="Times] Set举例
"m{,~'x [font="Times] import java.util.*;
7VK}Dy/Vvn [font="Times]public class SetExample {
.oEmU+ [font="Times] public static void main(String[] args) {
X0{/ydGF8 [font="Times] Set set = new HashSet(); //HashSet是[font="Times]Set的子接口
k`". [font="Times] set.add("one");
:V)lbn\ [font="Times] set.add("second");
xf%4, JQ [font="Times] set.add("3rd");
}FF W|f [font="Times] set.add(new Integer(4));
H"2uxhdLK3 [font="Times] set.add(new Float( 5.0F ));
F_xbwa*= [font="Times] set.add("second");
#S%Q*k<hw [font="Times] set.add(new Integer(4));
y]%w )4PS [font="Times] System.out.println(set);
;X ,1I [font="Times] }}
m8623DB" QZ
`tNq :/ [font="Times]List举例:
3Rm#-T s [font="Times] import java.util.*;
d2X[(3 [font="Times]public class ListExample {
[<`SfE [font="Times] public static void main(String[] args) {
|%~+2m [font="Times] List list = new ArrayList();
QrApxiw [font="Times] list.add("one");
Hn,:`mj4-6 [font="Times] list.add("second");
K.gEj*@ [font="Times] list.add("3rd");
@?C#r.vgp [font="Times] list.add(new Integer(4));
4[ryKPa, [font="Times] list.add(new Float( 5.0F ));
{%w!@- [font="Times] list.add("second");
co_oMc [font="Times] list.add(new Integer(4));
Oo?,fw [font="Times] System.out.println(list);
4E44Hzs [font="Times] }}
p8wyEHB 2tayP@$ \b[9ebME [font="Times]Map举例
W"L&fV+3 [font="Times]import java.util.Map;
JcJmds [font="Times]import java.util.HashMap;
~_9"3,~o5 [font="Times]import java.util.Iterator;
jClj_E [font="Times]import java.io.FileReader;
7\o!HMfK [font="Times]
H1!iP$1#V [font="Times]public class MapExample {
SM[Bv9|0 [font="Times] public static void main(String[] args) throws java.io.FileNotFoundException {
HxK$ 4I` [font="Times] Map word_count_map = new HashMap();
2?@j~I=s2h [font="Times] FileReader reader = new FileReader(args[0]);
&Bx
J [font="Times] Iterator words = new WordStreamIterator(reader);
-Xz?s [font="Times]
OT
%nr zP [font="Times] while ( words.hasNext() ) {
|L_wX:d`9 [font="Times] String word = (String) words.next();
uGdp@]z&8Q [font="Times] String word_lowercase = word.toLowerCase();
BiE08,nj [font="Times] Integer frequency = (Integer)word_count_map.get(word_lowercase);
AvR2_ [font="Times]if ( frequency == null ) {
=Cqv= [font="Times] frequency = new Integer(1);
DN4#H` [font="Times] } else {
%}2@rLP [font="Times] int value = frequency.intValue();
4^6.~6a [font="Times] frequency = new Integer(value + 1);}
r!}al5~& [font="Times] word_count_map.put(word_lowercase, frequency);
Dc~,D1xWj [font="Times] }
66snC{gU [font="Times] System.out.println(word_count_map);
\EoX8b}$b0 [font="Times] }}
4
;Qlu A5#y?Aq v"+k~:t* [font="Times]Queue举例:
XwM611 [font="Times]import java.io.IOException;
ujW1+Oj=~ [font="Times]import java.io.PrintStream;
fpM#XFj [font="Times]import java.util.LinkedList;
o/[ [font="Times]import java.util.Queue;
o6"*4P| [font="Times]
*cWmS\h| [font="Times]public class QueueTester {
`Lyq[zg8 [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
xChI,~i [font="Times] public QueueTester() {q = new LinkedList();}
lA>\Ko [font="Times]public void testFIFO(PrintStream out) throws IOException {
j:5%ppIY [font="Times] q.add("First");
,1Qd\8N9 [font="Times] q.add("Second");
31Cq22" [font="Times] q.add("Third");
{5c]Mn"r [font="Times] Object o;
N#N0Q0W= [font="Times] while ((o = q.poll()) != null) {
HOt>}x [font="Times] out.println(o);}}
'#\D]5 [font="Times] public static void main(String[] args) {
K|W^l\Lt [font="Times] QueueTester tester = new QueueTester();
SM[{BH< [font="Times] try { tester.testFIFO(System.out);
tXF]t
[font="Times] } catch (IOException e) {
(yQ
5` [font="Times] e.printStackTrace(); } }}
{u7##Vrgt8 $ &5w\P g1DmV,W-Q 上述例子和一些相关例子(共6个例子)打包下载[font="Times]
T+"f]v 总结:
8F;>5i 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。
zIQzmvf 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。