全新java初学者实践教程10(java SE5.0版)
tD,I7%|@ jdk5的集合类
uvi&! )x g"\JiBb5 )!;20Po N|/gwcKe 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。
Hxn#vAc 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。
!t?5U_on [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。
|O;vWn'U2 #q5tG\gnM [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]
ndw&F'.r >u]9(o7I [font="Times](1) Set
((M>To_l 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。
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G`|) ([font="Times]2)[font="Times]List
V`G^Jyj 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。
G[n;%c~`+ )_}xK={ f/"IC;<~t> [font="Times] ([font="Times]3)[font="Times]Map
7Dw.9EQ [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。
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nJ [font="Times] ([font="Times]4)[font="Times]Queue
COj^pdE3 [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。
;WgzR_'!' [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:
w*X(bua@ *nEG<Y) Y Azj>c& 实践:[font="Times] Set举例
'Z)#SzY [font="Times] import java.util.*;
AYDAt5K_ [font="Times]public class SetExample {
}|)T<|Y; [font="Times] public static void main(String[] args) {
*\*]:BIe&v [font="Times] Set set = new HashSet(); //HashSet是[font="Times]Set的子接口
gUYTVp Vf [font="Times] set.add("one");
(0Jr<16si$ [font="Times] set.add("second");
0v``4z2Z [font="Times] set.add("3rd");
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zwS [font="Times] set.add(new Integer(4));
I:1Pz|$` [font="Times] set.add(new Float( 5.0F ));
xpI8QV$# [font="Times] set.add("second");
qHPinxewx [font="Times] set.add(new Integer(4));
(3=bKcD' [font="Times] System.out.println(set);
I1JL`\;4 [font="Times] }}
=L`PP>"rW 5UX-Qqr [font="Times]List举例:
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[font="Times] import java.util.*;
z>b^Ui0 [font="Times]public class ListExample {
# wyjb:Ql [font="Times] public static void main(String[] args) {
[}4\CWM [font="Times] List list = new ArrayList();
l-5O5|C [font="Times] list.add("one");
|W*2L]& [font="Times] list.add("second");
j$4lyDfD [font="Times] list.add("3rd");
*%%n9T [font="Times] list.add(new Integer(4));
yM7FR); [font="Times] list.add(new Float( 5.0F ));
rS+ >oP} [font="Times] list.add("second");
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[font="Times] list.add(new Integer(4));
ZgmK~iJ [font="Times] System.out.println(list);
{fY(zHC [font="Times] }}
}%<cFi & -s^cy+jd D;OPsNQ [font="Times]Map举例
{mLv?"M] [font="Times]import java.util.Map;
+~7[T/v+n [font="Times]import java.util.HashMap;
[8vqw(2Tm( [font="Times]import java.util.Iterator;
=FMrVE [font="Times]import java.io.FileReader;
Z7 ++c<|p [font="Times]
OaY]}4tI$ [font="Times]public class MapExample {
3h6,x0AG [font="Times] public static void main(String[] args) throws java.io.FileNotFoundException {
Equ%6x [font="Times] Map word_count_map = new HashMap();
6=x]20 [font="Times] FileReader reader = new FileReader(args[0]);
hMgk+4* [font="Times] Iterator words = new WordStreamIterator(reader);
Fxn=+Xgg [font="Times]
gx2v(1?S [font="Times] while ( words.hasNext() ) {
<~e*YrJ?- [font="Times] String word = (String) words.next();
5f75r [font="Times] String word_lowercase = word.toLowerCase();
Hi U/fi` [font="Times] Integer frequency = (Integer)word_count_map.get(word_lowercase);
#v4^,$k> [font="Times]if ( frequency == null ) {
fT<3~Z>m [font="Times] frequency = new Integer(1);
T0cm+|S [font="Times] } else {
dvglh?7d [font="Times] int value = frequency.intValue();
!:~C/B{ [font="Times] frequency = new Integer(value + 1);}
waG &3m [font="Times] word_count_map.put(word_lowercase, frequency);
DLO#_t^v. [font="Times] }
c~d*SDca [font="Times] System.out.println(word_count_map);
yr)e."#S [font="Times] }}
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= P<9T.l )=5*iWe [font="Times]Queue举例:
}ee3'LUPX [font="Times]import java.io.IOException;
slLTZ] [font="Times]import java.io.PrintStream;
xscR Bx [font="Times]import java.util.LinkedList;
I]~s{I(EK [font="Times]import java.util.Queue;
ncpA\E;ff^ [font="Times]
T,B%iZgCh [font="Times]public class QueueTester {
QRF:6bAxsL [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
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"JH=< [font="Times] public QueueTester() {q = new LinkedList();}
\?^ EFA+; [font="Times]public void testFIFO(PrintStream out) throws IOException {
S)"vyGv [font="Times] q.add("First");
GT3}'`f B [font="Times] q.add("Second");
m-qOyt [font="Times] q.add("Third");
CljEC1S# [font="Times] Object o;
[TT:^F(Y [font="Times] while ((o = q.poll()) != null) {
r%LG>c`^ [font="Times] out.println(o);}}
[p)2!]y [font="Times] public static void main(String[] args) {
y }h2 [font="Times] QueueTester tester = new QueueTester();
YL[y3&K [font="Times] try { tester.testFIFO(System.out);
N8!e(YK_ [font="Times] } catch (IOException e) {
r)<n)eXeD [font="Times] e.printStackTrace(); } }}
5^Lbc.h ]agdVr^ k;.<DN 上述例子和一些相关例子(共6个例子)打包下载[font="Times]
UYpln[S 总结:
>_X(rar0 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。
wHQYBYKcd 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。