全新java初学者实践教程10(java SE5.0版)
!W=2ZlzS jdk5的集合类
rmI@ #' }yCgd 5+_ i'#%t/ u *I9G"R8 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。
Z \- 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。
_&xkj8O [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。
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"M]`>eixL [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]
,z0E2 :/NP8$~@j [font="Times](1) Set
mOYXd,xd 集([font="Times]set)是最简单的一种集合,它的对象不按特定方式排序,只是简单的把对象加入集合中,就像往口袋里放东西。对集中成员的访问和操作是通过集中对象的引用进行的,所以集中不能有重复对象。我们知道数学上的集合也是[font="Times]Set这个,集合里面一定是没有重复的元素的。
'9|R7 Zo,]Dx ([font="Times]2)[font="Times]List
q?&JS 列表([font="Times]List)的主要特征是其对象以线性方式存储,没有特定顺序,只有一个开头和一个结尾,当然,它与根本没有顺序的[font="Times]Set是不同的。它是链表嘛,一条链肯定有顺序这个顺序就不一定了。
8ZO~=e xtp55"g )/tdiRpn [font="Times] ([font="Times]3)[font="Times]Map
E95VR?nUg [font="Times] 映射([font="Times]Map),这个在[font="Times]java里不是地图的意思,其实地图也是映射哈。它里面的东西是键-值对([font="Times]key-value)出现的,键值对是什么呢?举个例子,比如我们查字典,用部首查字法。目录那个字就是键,这个字的解释就是值。键和值成对出现。这样说可以理解吧。这也是很常用的数据结构哦。
wtGb3D"am Q9t.*+ h^b= [font="Times] ([font="Times]4)[font="Times]Queue
KhbbGdmfS$ [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。
]}AyDy6C [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:
z.kBQ{P :#W>lq@H 8L(KdDY 实践:[font="Times] Set举例
Cd7d-'EQn [font="Times] import java.util.*;
.ZH5^Sv$vp [font="Times]public class SetExample {
fP1fm [font="Times] public static void main(String[] args) {
wijY]$ [font="Times] Set set = new HashSet(); //HashSet是[font="Times]Set的子接口
*(<3 oIRS [font="Times] set.add("one");
#.\X%! [font="Times] set.add("second");
kc:2ID& [font="Times] set.add("3rd");
~bJ*LM?wOP [font="Times] set.add(new Integer(4));
{pB9T3ry] [font="Times] set.add(new Float( 5.0F ));
Bk~M^AK@~ [font="Times] set.add("second");
QgX[?2 [font="Times] set.add(new Integer(4));
/?QBMI [font="Times] System.out.println(set);
.w]S!=h [font="Times] }}
8f|+045E@ $d7{q3K&1 [font="Times]List举例:
H=9\B} [font="Times] import java.util.*;
OAz-w [font="Times]public class ListExample {
Tk4"qGC. [font="Times] public static void main(String[] args) {
=").W\, [font="Times] List list = new ArrayList();
bjq2XP?LL [font="Times] list.add("one");
u@zBE?
g [font="Times] list.add("second");
L"!BN/i_ [font="Times] list.add("3rd");
9Uh nr]J. [font="Times] list.add(new Integer(4));
bpeWK& [font="Times] list.add(new Float( 5.0F ));
\>-%OcYlM [font="Times] list.add("second");
Ss7XjWP.} [font="Times] list.add(new Integer(4));
|4a#O8d [font="Times] System.out.println(list);
D<cHa | [font="Times] }}
YEqWTB|w (wIpq<% v4Q8RE? [font="Times]Map举例
!xK`:[B [font="Times]import java.util.Map;
/1lUFL2D [font="Times]import java.util.HashMap;
Gwkp(9d [font="Times]import java.util.Iterator;
!P0Oq)q [font="Times]import java.io.FileReader;
0qV!-i [font="Times]
AHR[i%3W [font="Times]public class MapExample {
SSF4P& [font="Times] public static void main(String[] args) throws java.io.FileNotFoundException {
z7J2O [font="Times] Map word_count_map = new HashMap();
j7 D\O [font="Times] FileReader reader = new FileReader(args[0]);
I(Yyg,1Z [font="Times] Iterator words = new WordStreamIterator(reader);
,9p
4(jjX [font="Times]
QY<2i-A [font="Times] while ( words.hasNext() ) {
K(HP PM\ [font="Times] String word = (String) words.next();
R@r"a&{/ [font="Times] String word_lowercase = word.toLowerCase();
HIi5kv]}| [font="Times] Integer frequency = (Integer)word_count_map.get(word_lowercase);
PGHl:4`Es! [font="Times]if ( frequency == null ) {
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p{|>3 [font="Times] frequency = new Integer(1);
x*[\$E`v [font="Times] } else {
~$i36" [font="Times] int value = frequency.intValue();
BS;_l"? [font="Times] frequency = new Integer(value + 1);}
~8Z0{^ [font="Times] word_count_map.put(word_lowercase, frequency);
``}EbOMG [font="Times] }
Dw^d!%Ala [font="Times] System.out.println(word_count_map);
rr#&0`] [font="Times] }}
}4kd=]Nk \5Jpr'mY5 ><[($Gq`g [font="Times]Queue举例:
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[font="Times]import java.io.IOException;
P/?` [font="Times]import java.io.PrintStream;
\0m[Ch}~ey [font="Times]import java.util.LinkedList;
^l6q [font="Times]import java.util.Queue;
(&FSoe/![' [font="Times]
g~Zel}h# [font="Times]public class QueueTester {
@twClk.s [font="Times] public Queue q; //发现了一个奇怪的语法,这个尖括号是泛型声明
'kK}9VKl [font="Times] public QueueTester() {q = new LinkedList();}
,.uPlnB_ [font="Times]public void testFIFO(PrintStream out) throws IOException {
ucyz>TL0 [font="Times] q.add("First");
C-$S]6 [font="Times] q.add("Second");
m-UI^M,@< [font="Times] q.add("Third");
fNc3&=]] [font="Times] Object o;
q7C>A`w [font="Times] while ((o = q.poll()) != null) {
ZUb6d*B [font="Times] out.println(o);}}
uHUicZf. [font="Times] public static void main(String[] args) {
QdL
;|3K9 [font="Times] QueueTester tester = new QueueTester();
DGTSk9iK( [font="Times] try { tester.testFIFO(System.out);
l-"$a8jn2 [font="Times] } catch (IOException e) {
aO?(ZL [font="Times] e.printStackTrace(); } }}
}r6SV%]: =9e()j _U)DL=a' 上述例子和一些相关例子(共6个例子)打包下载[font="Times]
#z)@T 总结:
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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。
XS]=sfN 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。