全新java初学者实践教程10(java SE5.0版)
rL3<r jdk5的集合类
hEi]-N\X Umjt~K^Z SeN4gr* $}_N379& 上次课我们学过了数组,知道它只是一组数(或是对象),但是有些自己的特性。在[font="Times]java里还有一类东西与数组类似,也是有着特性的一组数[font="Times](或是对象[font="Times]),叫做集合类。
U`N|pPe:w 我们上节课讲到了,数组的长度在创建时已经确定了,但是有时候我们事先根本不知道长度是多少啊,比如我们做电子商务网站时,有个购物车程序。你总不能用数组规定,人家只能买[font="Times]5样东西吧。你就是把长度定为[font="Times]10000也不行,万一遇上个特别有钱的呢!呵呵,这只是开玩笑的。我们会使用集合类解决这个问题。
Ef{rY|E [font="Times] 集合类是放在[font="Times]java.util.*;这个包里。集合类存放的都是对象的引用,而非对象本身,为了说起来方便些,我们称集合中的对象就是指集合中对象的引用([font="Times]reference)。引用的概念大家不会忘了吧,在前边我们讲
数据类型时讲的。
L^bt-QbhO 'bJ!~ML& [font="Times] 集合类型主要有[font="Times]3种:[font="Times]set(集)、[font="Times]list(列表)、[font="Times]map(映射[font="Times])和[font="Times]Queue(队列)。[font="Times]//队列为[font="Times]jdk5中的加上的[font="Times]
#"-?+F=rk D9,609w [font="Times](1) Set
to,DN2rN 集([font="Times]set)是最简单的一种集合,它的对象不按特定方式排序,只是简单的把对象加入集合中,就像往口袋里放东西。对集中成员的访问和操作是通过集中对象的引用进行的,所以集中不能有重复对象。我们知道数学上的集合也是[font="Times]Set这个,集合里面一定是没有重复的元素的。
JBfDz0P [zv>Wlf,% ([font="Times]2)[font="Times]List
RO[Ko-m|/N 列表([font="Times]List)的主要特征是其对象以线性方式存储,没有特定顺序,只有一个开头和一个结尾,当然,它与根本没有顺序的[font="Times]Set是不同的。它是链表嘛,一条链肯定有顺序这个顺序就不一定了。
TP o%zZo fZ1v| psyxNM=dN# [font="Times] ([font="Times]3)[font="Times]Map
)]WWx-Uf' [font="Times] 映射([font="Times]Map),这个在[font="Times]java里不是地图的意思,其实地图也是映射哈。它里面的东西是键-值对([font="Times]key-value)出现的,键值对是什么呢?举个例子,比如我们查字典,用部首查字法。目录那个字就是键,这个字的解释就是值。键和值成对出现。这样说可以理解吧。这也是很常用的数据结构哦。
KiYO,nD;\ `q|&;wP. cpvN
}G [font="Times] ([font="Times]4)[font="Times]Queue
D,q=?~ [font="Times] 在[font="Times]jdk5.0以前,通常的实现方式是使用[font="Times]java.util.List集合来模仿[font="Times]Queue。[font="Times]Queue的概念通过把对象添加(称为[font="Times]enqueuing的操作)到[font="Times]List的尾部(即[font="Times]Queue的后部)并通过从[font="Times]List的头部(即[font="Times]Queue的前部)提取对象而从[font="Times] List中移除(称为[font="Times]dequeuing的操作)来模拟。你需要执行先进先出的动作时可以直接使用[font="Times]Queue接口就可以了。
|$"2R3 [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:
gw~%jD-2 QR4rQu qcMVY\gi 实践:[font="Times] Set举例
2gz}]_ [font="Times] import java.util.*;
L08>9tf` [font="Times]public class SetExample {
Ay)q %:qx [font="Times] public static void main(String[] args) {
F x,08 [font="Times] Set set = new HashSet(); //HashSet是[font="Times]Set的子接口
AQn>K{M [font="Times] set.add("one");
S^q)DuF5! [font="Times] set.add("second");
Ww5c9orXn [font="Times] set.add("3rd");
,:g.B\'Q [font="Times] set.add(new Integer(4));
xw_VK1 [font="Times] set.add(new Float( 5.0F ));
j
!^Tw.Ty [font="Times] set.add("second");
(t&]u7Atr [font="Times] set.add(new Integer(4));
xg=}MoX [font="Times] System.out.println(set);
x,$N!X [font="Times] }}
HUqG)t*c1 ! |SPOk [font="Times]List举例:
jv$Y]nf [font="Times] import java.util.*;
Ci%u =%( [font="Times]public class ListExample {
<;O=h;
~| [font="Times] public static void main(String[] args) {
H`u8}{7 [font="Times] List list = new ArrayList();
kwxb~~S}h( [font="Times] list.add("one");
GT\,
@$r [font="Times] list.add("second");
sX&.8 [font="Times] list.add("3rd");
C!Cg.^; [font="Times] list.add(new Integer(4));
QZy+` [font="Times] list.add(new Float( 5.0F ));
`nBCCz'Y! [font="Times] list.add("second");
+pYrA qmO- [font="Times] list.add(new Integer(4));
V]b1cDx{ [font="Times] System.out.println(list);
D|8vS8p [font="Times] }}
TymE(,1 ]zp5 6U|xa 9pPb]v,6 [font="Times]Map举例
}e3M5LI1L [font="Times]import java.util.Map;
8N<0|u [font="Times]import java.util.HashMap;
\s<7!NAE4 [font="Times]import java.util.Iterator;
#_yQv? J [font="Times]import java.io.FileReader;
cBf9-k [font="Times]
^NJ]~h{n$ [font="Times]public class MapExample {
'Cy^G; [font="Times] public static void main(String[] args) throws java.io.FileNotFoundException {
?DcR D)X [font="Times] Map word_count_map = new HashMap();
lB!`,>"c [font="Times] FileReader reader = new FileReader(args[0]);
"zW3dKVc [font="Times] Iterator words = new WordStreamIterator(reader);
34VyR
a [font="Times]
4)D#kP [font="Times] while ( words.hasNext() ) {
*S_e:^ [font="Times] String word = (String) words.next();
hW*2Le!I [font="Times] String word_lowercase = word.toLowerCase();
M=[ /v/M= [font="Times] Integer frequency = (Integer)word_count_map.get(word_lowercase);
j]7|5mC78 [font="Times]if ( frequency == null ) {
@rs(`4QEh [font="Times] frequency = new Integer(1);
EubR]ckB [font="Times] } else {
\8\)5#? [font="Times] int value = frequency.intValue();
"%~\kJ(G [font="Times] frequency = new Integer(value + 1);}
A^7!:^%K [font="Times] word_count_map.put(word_lowercase, frequency);
SsA;T5:6 [font="Times] }
Ore$yI}!m [font="Times] System.out.println(word_count_map);
?]gZg[ [font="Times] }}
0P`wh=") g-'y_'%0G '^ '4C'J [font="Times]Queue举例:
3oX%tx [font="Times]import java.io.IOException;
{9TWPB/> [font="Times]import java.io.PrintStream;
&SS"A*xg [font="Times]import java.util.LinkedList;
XfA3Ez,} [font="Times]import java.util.Queue;
Hv%(9)-8 [font="Times]
{ZFa
+ [font="Times]public class QueueTester {
Vzy]N6QT{ [font="Times] public Queue q; //发现了一个奇怪的语法,这个尖括号是泛型声明
L/(e/Jalg [font="Times] public QueueTester() {q = new LinkedList();}
GZT}aMMSJ [font="Times]public void testFIFO(PrintStream out) throws IOException {
ga#,42)H [font="Times] q.add("First");
9dA(f~ [font="Times] q.add("Second");
Z+ubc"MVb [font="Times] q.add("Third");
U`q[5U" [font="Times] Object o;
ZCPK{Ru QE [font="Times] while ((o = q.poll()) != null) {
T#Pz_
hAu [font="Times] out.println(o);}}
j`Lf/S!} [font="Times] public static void main(String[] args) {
2'g< H-[ [font="Times] QueueTester tester = new QueueTester();
|Hn[XRsf [font="Times] try { tester.testFIFO(System.out);
zO{$kT\r& [font="Times] } catch (IOException e) {
20m6-rkI<} [font="Times] e.printStackTrace(); } }}
Fk D .jMm-vox} Pb?H cg 上述例子和一些相关例子(共6个例子)打包下载[font="Times]
wo2^,Y2z+ 总结:
EZ #UdK_ 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。
~J2-B2S! 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。