全新java初学者实践教程10(java SE5.0版)
x/!5K|c jdk5的集合类
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e"jw#B Fv74bC% h[o6-f<D zZ=pP5y8 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。
#P<N^[m 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。
0@I S [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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WW [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]
3"N)xO- \xv;sl$f [font="Times](1) Set
Fqy\CMC 集([font="Times]set)是最简单的一种集合,它的对象不按特定方式排序,只是简单的把对象加入集合中,就像往口袋里放东西。对集中成员的访问和操作是通过集中对象的引用进行的,所以集中不能有重复对象。我们知道数学上的集合也是[font="Times]Set这个,集合里面一定是没有重复的元素的。
t.p~\6Yi 5Xn.CBd] ([font="Times]2)[font="Times]List
lVOu)q@l7g 列表([font="Times]List)的主要特征是其对象以线性方式存储,没有特定顺序,只有一个开头和一个结尾,当然,它与根本没有顺序的[font="Times]Set是不同的。它是链表嘛,一条链肯定有顺序这个顺序就不一定了。
sEMQ p]T<HGJ P +N`ua [font="Times] ([font="Times]3)[font="Times]Map
9h&R]yz; [font="Times] 映射([font="Times]Map),这个在[font="Times]java里不是地图的意思,其实地图也是映射哈。它里面的东西是键-值对([font="Times]key-value)出现的,键值对是什么呢?举个例子,比如我们查字典,用部首查字法。目录那个字就是键,这个字的解释就是值。键和值成对出现。这样说可以理解吧。这也是很常用的数据结构哦。
i4JqT \q Fz#X=gmG bKg8rK u [font="Times] ([font="Times]4)[font="Times]Queue
2i;7{7 [font="Times] 在[font="Times]jdk5.0以前,通常的实现方式是使用[font="Times]java.util.List集合来模仿[font="Times]Queue。[font="Times]Queue的概念通过把对象添加(称为[font="Times]enqueuing的操作)到[font="Times]List的尾部(即[font="Times]Queue的后部)并通过从[font="Times]List的头部(即[font="Times]Queue的前部)提取对象而从[font="Times] List中移除(称为[font="Times]dequeuing的操作)来模拟。你需要执行先进先出的动作时可以直接使用[font="Times]Queue接口就可以了。
Ae]sGU|?' [font="Times] 这[font="Times]4个东西,有时候功能还不太完善,需要有些子类继承它的特性。[font="Times]Set的子接口有[font="Times]TreeSet,SortedSet,[font="Times]List的有[font="Times]ArrayList等,[font="Times]Map里有[font="Times]HashMap,HashTable等,[font="Times]Queue里面有[font="Times]BlockingQueue等。我们来看看例子吧:
NIdZ El\%E"Tk% yAL[[ 实践:[font="Times] Set举例
GZI`jS"lU [font="Times] import java.util.*;
'k;rH!R [font="Times]public class SetExample {
^=>Tk$ _2 [font="Times] public static void main(String[] args) {
?POUtRN [font="Times] Set set = new HashSet(); //HashSet是[font="Times]Set的子接口
$odso;Hn [font="Times] set.add("one");
LUB${0BrA [font="Times] set.add("second");
y!tC20Q [font="Times] set.add("3rd");
(T`E!A0I\? [font="Times] set.add(new Integer(4));
yY?b.ty [font="Times] set.add(new Float( 5.0F ));
f+8wl!M+6 [font="Times] set.add("second");
o1M$.* [font="Times] set.add(new Integer(4));
n3AaZp[ [font="Times] System.out.println(set);
#(An6itl [font="Times] }}
^8.]d~j ^nbnbU4' [font="Times]List举例:
>
pI;%' [font="Times] import java.util.*;
hxQqa 0B [font="Times]public class ListExample {
y@0E[/O [font="Times] public static void main(String[] args) {
BauU{:Sh [font="Times] List list = new ArrayList();
i&{DOI%w [font="Times] list.add("one");
k0Ol*L!p [font="Times] list.add("second");
{~Rk2:gx [font="Times] list.add("3rd");
X}+>!%W!} [font="Times] list.add(new Integer(4));
4R18A=X [font="Times] list.add(new Float( 5.0F ));
[w%
qV 6 [font="Times] list.add("second");
hjZKUMG(k [font="Times] list.add(new Integer(4));
!1e6Ss [font="Times] System.out.println(list);
vZ.x{"n'~ [font="Times] }}
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,SiA-3* [font="Times]Map举例
H\TI[JPAl [font="Times]import java.util.Map;
JvY}-}?c [font="Times]import java.util.HashMap;
H$y-8-&) [font="Times]import java.util.Iterator;
0`^&9nR [font="Times]import java.io.FileReader;
xlQBe-Wg [font="Times]
4$P0 : [font="Times]public class MapExample {
}GeSu|m( [font="Times] public static void main(String[] args) throws java.io.FileNotFoundException {
Y1]n^ [font="Times] Map word_count_map = new HashMap();
:j+ ZI3@ [font="Times] FileReader reader = new FileReader(args[0]);
<J^MCqp!v [font="Times] Iterator words = new WordStreamIterator(reader);
%i5M77#Z [font="Times]
vM5k_D [font="Times] while ( words.hasNext() ) {
6I%5Q4Ll [font="Times] String word = (String) words.next();
e)(wss+d7P [font="Times] String word_lowercase = word.toLowerCase();
/5u<78GW1 [font="Times] Integer frequency = (Integer)word_count_map.get(word_lowercase);
4O35"1 [font="Times]if ( frequency == null ) {
ZMel{w`n [font="Times] frequency = new Integer(1);
v%q0OX>9X" [font="Times] } else {
<yd{tD$A* [font="Times] int value = frequency.intValue();
3\XU_Xs(] [font="Times] frequency = new Integer(value + 1);}
Za1QC;7 [font="Times] word_count_map.put(word_lowercase, frequency);
K*~0"F>"0 [font="Times] }
cXKjrL[b [font="Times] System.out.println(word_count_map);
+F^^c2E [font="Times] }}
\--8lH -K 3.*8)NW ))"6ern [font="Times]Queue举例:
[n:<8ho [font="Times]import java.io.IOException;
zx)z/1 [font="Times]import java.io.PrintStream;
+mn,F}; [font="Times]import java.util.LinkedList;
Le\?+h42> [font="Times]import java.util.Queue;
PpAu!2lt9 [font="Times]
"hsb8- [font="Times]public class QueueTester {
<i&_ooX [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
~vyf4TF<# [font="Times] public QueueTester() {q = new LinkedList();}
[5SD_dN [font="Times]public void testFIFO(PrintStream out) throws IOException {
>Z'NXha [font="Times] q.add("First");
2+e}*&iQpp [font="Times] q.add("Second");
nCdR EXw [font="Times] q.add("Third");
V=o
t-1,j7 [font="Times] Object o;
EwvoQ$#jv [font="Times] while ((o = q.poll()) != null) {
g\&g N [font="Times] out.println(o);}}
eS~LF.^Jw [font="Times] public static void main(String[] args) {
-w"VK|SGm [font="Times] QueueTester tester = new QueueTester();
5fd]v< [font="Times] try { tester.testFIFO(System.out);
t:'Mh9h7u [font="Times] } catch (IOException e) {
wY[+ZT [font="Times] e.printStackTrace(); } }}
'F9 jq tM'P m =Jyu4j *} 上述例子和一些相关例子(共6个例子)打包下载[font="Times]
iMDM1}b 总结:
~kEI4}O 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。
\RC'XKQ*n 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。