全新java初学者实践教程10(java SE5.0版)
u-W6 hZ$ jdk5的集合类
FzOr#(^ `aFy2x`3 A>"v1Wk 4(aDi;x "w 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。
7m;2M]BRi 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。
4X2XSK4 [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。
SnK j:|bV {(}Mu R [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]
>wK ^W{ r7tN(2;5 [font="Times](1) Set
={9G.%W 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。
[\o+I:,}wi 1vTncU! ([font="Times]2)[font="Times]List
uN`{; Av 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。
`{g8A P3 ^}XKhn.S' AL.zF\? [font="Times] ([font="Times]3)[font="Times]Map
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( [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。
K\ww,S 2Wlk] 0dKI+zgr [font="Times] ([font="Times]4)[font="Times]Queue
kl.)A-6V [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。
_=c>>X [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:
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S@ 实践:[font="Times] Set举例
Sd;/yC 8 [font="Times] import java.util.*;
0G/VbS [font="Times]public class SetExample {
_(J 7^rN [font="Times] public static void main(String[] args) {
{mPaloA [font="Times] Set set = new HashSet(); //HashSet是[font="Times]Set的子接口
}?,Gn]] [font="Times] set.add("one");
IAt;?4 [font="Times] set.add("second");
Cc:4n1|]> [font="Times] set.add("3rd");
q #f
U* [font="Times] set.add(new Integer(4));
:$&%Pxm [font="Times] set.add(new Float( 5.0F ));
$tyF(RybG [font="Times] set.add("second");
+w Oa [font="Times] set.add(new Integer(4));
,jWMJ0X/N= [font="Times] System.out.println(set);
i/rdPbq [font="Times] }}
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M.K-)r, [font="Times]List举例:
.xT8@] [font="Times] import java.util.*;
:sRV]!Iw [font="Times]public class ListExample {
iw6qNV:\Z [font="Times] public static void main(String[] args) {
LOyL:~$ [font="Times] List list = new ArrayList();
xq:.|{HUk [font="Times] list.add("one");
<dx
xXzLT [font="Times] list.add("second");
?PNG@OK [font="Times] list.add("3rd");
!Gu,X'#Ab [font="Times] list.add(new Integer(4));
u49zc9 [font="Times] list.add(new Float( 5.0F ));
tE0DST/ [font="Times] list.add("second");
3 Oy-\09 [font="Times] list.add(new Integer(4));
8tWOVLquJ [font="Times] System.out.println(list);
yp=Hxf [font="Times] }}
LTu
c s} 03*` T aG7QLCL [font="Times]Map举例
qu[ ~# [font="Times]import java.util.Map;
Gx?p,Fj [font="Times]import java.util.HashMap;
q/xMM`{ [font="Times]import java.util.Iterator;
RQI? \?o [font="Times]import java.io.FileReader;
!|`G<WD [font="Times]
]trVlmZXH} [font="Times]public class MapExample {
ReOp,A/y [font="Times] public static void main(String[] args) throws java.io.FileNotFoundException {
2=X 2M [font="Times] Map word_count_map = new HashMap();
-ea>}S [font="Times] FileReader reader = new FileReader(args[0]);
8P r H"pI [font="Times] Iterator words = new WordStreamIterator(reader);
@NGK2J [font="Times]
>W"gr]R< [font="Times] while ( words.hasNext() ) {
(#* 7LdZ [font="Times] String word = (String) words.next();
d%?+q0j [font="Times] String word_lowercase = word.toLowerCase();
'1A S66k [font="Times] Integer frequency = (Integer)word_count_map.get(word_lowercase);
g(t"+
P [font="Times]if ( frequency == null ) {
&| %<=\ [font="Times] frequency = new Integer(1);
.lfKS!m2 [font="Times] } else {
ud K)F$7 [font="Times] int value = frequency.intValue();
'v^CA} [font="Times] frequency = new Integer(value + 1);}
c[]_gUp8 [font="Times] word_count_map.put(word_lowercase, frequency);
; >3q@9\D [font="Times] }
i(9=` A} [font="Times] System.out.println(word_count_map);
e&f9/rfx [font="Times] }}
gB@Xi* 2"lD Kjj FjIS:9^)t5 [font="Times]Queue举例:
<=8REA? [font="Times]import java.io.IOException;
6k;__@B, [font="Times]import java.io.PrintStream;
*vFVXJo [font="Times]import java.util.LinkedList;
FblwQ-D [font="Times]import java.util.Queue;
/_E8'qlx [font="Times]
LZm6\x [font="Times]public class QueueTester {
@sJ[<V [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
Pw/Z;N;:V [font="Times] public QueueTester() {q = new LinkedList();}
+MPM^ m [font="Times]public void testFIFO(PrintStream out) throws IOException {
zVe@`gc [font="Times] q.add("First");
W
HO;;j [font="Times] q.add("Second");
}l&Uh&B` [font="Times] q.add("Third");
Vh^fbv`? [font="Times] Object o;
J&}/Xw) [font="Times] while ((o = q.poll()) != null) {
)n 1b [font="Times] out.println(o);}}
Ddde,WJA [font="Times] public static void main(String[] args) {
~H/|J^ J [font="Times] QueueTester tester = new QueueTester();
yiGq?WA7 [font="Times] try { tester.testFIFO(System.out);
naCPSsei [font="Times] } catch (IOException e) {
2bxkZS] [font="Times] e.printStackTrace(); } }}
'EJ8)2 /*g3TbUs Ed ,`1+ 上述例子和一些相关例子(共6个例子)打包下载[font="Times]
Eq^k @ 总结:
k|Vq-w 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。
Zh`lC1l' 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。