全新java初学者实践教程10(java SE5.0版)
e'FBV[e jdk5的集合类
=q1=.VTn Nq`@ >Ml g ni=S~u `K ,{Y_ 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 r}5)ZR. 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。
o=t@83Fh5 [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。
UE-< g[Z$\A?ZbZ [font="Times] 集合类型主要有[font="Times]3种:[font="Times]set(集)、[font="Times]list(列表)、[font="Times]map(映射[font="Times])和[font="Times]Queue(队列)。[font="Times]//队列为[font="Times]jdk5中的加上的[font="Times]
cmLGMlFT ,f4VV\ [font="Times](1) Set
=6XJr7Ay8u 集([font="Times]set)是最简单的一种集合,它的对象不按特定方式排序,只是简单的把对象加入集合中,就像往口袋里放东西。对集中成员的访问和操作是通过集中对象的引用进行的,所以集中不能有重复对象。我们知道数学上的集合也是[font="Times]Set这个,集合里面一定是没有重复的元素的。
4GA9oLl SXYH#p ([font="Times]2)[font="Times]List
_2eRH@T 列表([font="Times]List)的主要特征是其对象以线性方式存储,没有特定顺序,只有一个开头和一个结尾,当然,它与根本没有顺序的[font="Times]Set是不同的。它是链表嘛,一条链肯定有顺序这个顺序就不一定了。
>UUcKq1M: 5`TbM !Ez5@ [font="Times] ([font="Times]3)[font="Times]Map
#Tz$ona [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。
F/ZB%;O9 C 2f=9n/ m#}41< [font="Times] ([font="Times]4)[font="Times]Queue
tx,_0[hZi [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。
y&ZyThqg [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:
OcA_m. #ujry.m :%6OFO$z 实践:[font="Times] Set举例
BBV"nm_(/ [font="Times] import java.util.*;
B]KR * [font="Times]public class SetExample {
b^*9m PP [font="Times] public static void main(String[] args) {
L701j.7" [font="Times] Set set = new HashSet(); //HashSet是[font="Times]Set的子接口
JPH! .@ [font="Times] set.add("one");
7U9*-9 [font="Times] set.add("second");
zBQV2.@ [font="Times] set.add("3rd");
FAkrM?0/ [font="Times] set.add(new Integer(4));
)8cb @N [font="Times] set.add(new Float( 5.0F ));
C+0MzfLgf [font="Times] set.add("second");
SmpYH@ [font="Times] set.add(new Integer(4));
,4Q1[K35B [font="Times] System.out.println(set);
_>8rTk`/h [font="Times] }}
IP LKOT~ %Z8'h\| [font="Times]List举例:
&RJ*DAmL [font="Times] import java.util.*;
-wh?9?W [font="Times]public class ListExample {
%E R"Udh [font="Times] public static void main(String[] args) {
1fK]A*{p [font="Times] List list = new ArrayList();
:17Pc\:DS [font="Times] list.add("one");
vGCvJ*4! [font="Times] list.add("second");
]c9\[Kdq}H [font="Times] list.add("3rd");
YE*%Y[" [font="Times] list.add(new Integer(4));
%`_Rl>@K= [font="Times] list.add(new Float( 5.0F ));
;u8a%h! [font="Times] list.add("second");
[u $X.=( [font="Times] list.add(new Integer(4));
h-f`as"d [font="Times] System.out.println(list);
hCOCX_ [font="Times] }}
|JL?"cc O"GuVC}B f:K>o. [font="Times]Map举例
BXb=NE [font="Times]import java.util.Map;
\{a!Z&df [font="Times]import java.util.HashMap;
V0<g$,W= [font="Times]import java.util.Iterator;
)n&6= Li [font="Times]import java.io.FileReader;
h6Q~Di [font="Times]
eJ%b"H! [font="Times]public class MapExample {
Y#5v5
[font="Times] public static void main(String[] args) throws java.io.FileNotFoundException {
~m'8<B5+ [font="Times] Map word_count_map = new HashMap();
+8|9&v` [font="Times] FileReader reader = new FileReader(args[0]);
#z1/VZ [font="Times] Iterator words = new WordStreamIterator(reader);
:O'QL, [font="Times]
i'QR-B&Z [font="Times] while ( words.hasNext() ) {
iG ,z3/~v [font="Times] String word = (String) words.next();
s&4&\Aq}x# [font="Times] String word_lowercase = word.toLowerCase();
~>#?.f [font="Times] Integer frequency = (Integer)word_count_map.get(word_lowercase);
Xw#"?B(M] [font="Times]if ( frequency == null ) {
cy(4g-b]@e [font="Times] frequency = new Integer(1);
9SBTeJ$RZ [font="Times] } else {
.0rTk$B
[font="Times] int value = frequency.intValue();
SsEpuEn [font="Times] frequency = new Integer(value + 1);}
? *v*fs0 [font="Times] word_count_map.put(word_lowercase, frequency);
Yr*!T= z [font="Times] }
vqAEF^HYry [font="Times] System.out.println(word_count_map);
c{IL"B6> [font="Times] }}
zm{`+boH< AQ?;UDqU nMJ(tQ [font="Times]Queue举例:
f5Hv![x [font="Times]import java.io.IOException;
>"+ho [font="Times]import java.io.PrintStream;
bzr QQQ [font="Times]import java.util.LinkedList;
Hr7?#ZX;e [font="Times]import java.util.Queue;
-<ome~| [font="Times]
RrT`]1". [font="Times]public class QueueTester {
O#k eoC4 [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
x_x_TEyy h [font="Times] public QueueTester() {q = new LinkedList();}
=A*a9c2
[font="Times]public void testFIFO(PrintStream out) throws IOException {
N^M6*,F,J [font="Times] q.add("First");
1%C EUE [font="Times] q.add("Second");
qg/FI#r [font="Times] q.add("Third");
Dkx}}E:< [font="Times] Object o;
BCuoFw) [font="Times] while ((o = q.poll()) != null) {
FUXJy{n6"2 [font="Times] out.println(o);}}
01&@8z'E [font="Times] public static void main(String[] args) {
2acTw# [font="Times] QueueTester tester = new QueueTester();
n2~WUK [font="Times] try { tester.testFIFO(System.out);
rvU^W+d [font="Times] } catch (IOException e) {
2rW9ja [font="Times] e.printStackTrace(); } }}
psse^rFg J(K/z,4h \*&?o51!e 上述例子和一些相关例子(共6个例子)打包下载[font="Times]
Uq @].3nf 总结:
@u`W(Ow 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。
OFBEJacy 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。