全新java初学者实践教程10(java SE5.0版)
sKK*{+,kh; jdk5的集合类
_R 6+bB$ ySEhi_)9^ ToVi; ;&N=t64" 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。
vL,:Yn@b 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。
&+v!mw > [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。
Xbp~cn v3`k?jAaI [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]
ZFNn(n &rmXz6F [font="Times](1) Set
SL O~ 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。
I}S~,4 9AgTrP ([font="Times]2)[font="Times]List
X>W2aDuEZ 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。
h/a|-V}m& /P>t3E2c ZgP~VB0)$ [font="Times] ([font="Times]3)[font="Times]Map
?mCino [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。
n8dJ6"L<" qij<XNZU"& I\DH [font="Times] ([font="Times]4)[font="Times]Queue
XFiP8aX< [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。
&=-ZNWNo [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:
%gb4(~E+N (WISf}[l; z9B""ws 实践:[font="Times] Set举例
[$<\*d/ [font="Times] import java.util.*;
..5rW0lr [font="Times]public class SetExample {
(&)PlIi7 [font="Times] public static void main(String[] args) {
8wXnc% [font="Times] Set set = new HashSet(); //HashSet是[font="Times]Set的子接口
CC8)yO [font="Times] set.add("one");
g]V_)} [font="Times] set.add("second");
m@Vz42g~+ [font="Times] set.add("3rd");
@*VfG CQ( [font="Times] set.add(new Integer(4));
;;#_[Zl [font="Times] set.add(new Float( 5.0F ));
nH=8I~jp [font="Times] set.add("second");
R;]z/|8 [font="Times] set.add(new Integer(4));
mz'r<v2Tc [font="Times] System.out.println(set);
BM,]Wjfdj [font="Times] }}
%]m/fo4b \pVmSac, [font="Times]List举例:
z{N~AaY [font="Times] import java.util.*;
-szSA [font="Times]public class ListExample {
m/T3Um [font="Times] public static void main(String[] args) {
P~H?[
; [font="Times] List list = new ArrayList();
lI<Q=gd [font="Times] list.add("one");
nbMxQODk [font="Times] list.add("second");
;
m]KKB [font="Times] list.add("3rd");
iW
#|N^ [font="Times] list.add(new Integer(4));
+'lj\_n [font="Times] list.add(new Float( 5.0F ));
rEF0A&5 [font="Times] list.add("second");
a^ __Z3g, [font="Times] list.add(new Integer(4));
:Q=tGj\G [font="Times] System.out.println(list);
lzE{e6 [font="Times] }}
D\ ;(BB 5(+PIKCjC U_8 Z& [font="Times]Map举例
fVXZfq6 [font="Times]import java.util.Map;
Wz%b,! [font="Times]import java.util.HashMap;
R.(fo:ve> [font="Times]import java.util.Iterator;
0,z3A>C [font="Times]import java.io.FileReader;
dx&!RK+ [font="Times]
P"%QFt, [font="Times]public class MapExample {
E0s|eA& [font="Times] public static void main(String[] args) throws java.io.FileNotFoundException {
Oe/&Ryj=mm [font="Times] Map word_count_map = new HashMap();
g"dq;H [font="Times] FileReader reader = new FileReader(args[0]);
hp$/O4fD [font="Times] Iterator words = new WordStreamIterator(reader);
.yF@Ow [font="Times]
cOq'MDr [font="Times] while ( words.hasNext() ) {
0'3f^Ajf [font="Times] String word = (String) words.next();
&&daQg4Ha [font="Times] String word_lowercase = word.toLowerCase();
nhu;e}[> [font="Times] Integer frequency = (Integer)word_count_map.get(word_lowercase);
c&mLK1A6 [font="Times]if ( frequency == null ) {
L/Ytk ag [font="Times] frequency = new Integer(1);
WCdl 25L# [font="Times] } else {
o
_G,Ph!7 [font="Times] int value = frequency.intValue();
aWCZ1F [font="Times] frequency = new Integer(value + 1);}
M&v;#CV [font="Times] word_count_map.put(word_lowercase, frequency);
j TyR+#Wn [font="Times] }
?^Q8#Y^M [font="Times] System.out.println(word_count_map);
2d# 3LnO [font="Times] }}
Q:5^K 4!</JZX~$ bih%hqny [font="Times]Queue举例:
+QZ}c@'r [font="Times]import java.io.IOException;
H:k?#7D( [font="Times]import java.io.PrintStream;
nr{#Krkb [font="Times]import java.util.LinkedList;
@CTSvTt$ [font="Times]import java.util.Queue;
0ap_tCY [font="Times]
^xt @ [font="Times]public class QueueTester {
X7g@.Oy` [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
AL;z's(F? [font="Times] public QueueTester() {q = new LinkedList();}
#B!HPlrv [font="Times]public void testFIFO(PrintStream out) throws IOException {
'nMj<:0wlD [font="Times] q.add("First");
6L!/#d0 [font="Times] q.add("Second");
sy]hMGH:3W [font="Times] q.add("Third");
x_+-TC4IXn [font="Times] Object o;
k',#T932x1 [font="Times] while ((o = q.poll()) != null) {
%4QpDt [font="Times] out.println(o);}}
;}dvc7 [font="Times] public static void main(String[] args) {
s?5vJ:M
Xr [font="Times] QueueTester tester = new QueueTester();
mp:xR ^5c [font="Times] try { tester.testFIFO(System.out);
Ct<]('Hm( [font="Times] } catch (IOException e) {
KL<,avC/ [font="Times] e.printStackTrace(); } }}
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F%tV^$% 上述例子和一些相关例子(共6个例子)打包下载[font="Times]
)yt_i'D} 总结:
~z kzuh 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。
#
E{2 !Z 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。