全新java初学者实践教程10(java SE5.0版)
r"7n2 jdk5的集合类
#.Rn6|V/4 XjX /)P}[Q4 /(N/DMl[ 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。
isQ(O 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。
'YL[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。
FwCb$yE#M @YJI'Hf67 [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]
:D.0\.p z|l*5@p [font="Times](1) Set
+ ?1GscJ 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。
U ZM #O j|eA*UE ([font="Times]2)[font="Times]List
EYAaK^ & 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。
\(o"/* f-b],YE /R)wM#& [font="Times] ([font="Times]3)[font="Times]Map
>[}oH2oi [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。
hx;f/EPx y>^a~}Zq G95,J/w [font="Times] ([font="Times]4)[font="Times]Queue
+#W94s~0V [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。
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~6a [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:
wKV4-uyr ud1M-lY\U .Eao|; 实践:[font="Times] Set举例
\CbJU [font="Times] import java.util.*;
w:~*wv [font="Times]public class SetExample {
C-'hXh;hQ [font="Times] public static void main(String[] args) {
{1W:@6tl [font="Times] Set set = new HashSet(); //HashSet是[font="Times]Set的子接口
ccD+AGM.
[font="Times] set.add("one");
g)D_!iz [font="Times] set.add("second");
Fnw:alWr [font="Times] set.add("3rd");
Ha'[uEDb [font="Times] set.add(new Integer(4));
yIMqQSt79z [font="Times] set.add(new Float( 5.0F ));
P]_d;\
!"v [font="Times] set.add("second");
2eT?qCxqc [font="Times] set.add(new Integer(4));
dUI5,3* [font="Times] System.out.println(set);
MmuT~d/ [font="Times] }}
kB\{1; E~'mxx~i [font="Times]List举例:
{T){!UVp! [font="Times] import java.util.*;
*b~6 B M$ [font="Times]public class ListExample {
p?@ %/!S [font="Times] public static void main(String[] args) {
ZL MH~cc [font="Times] List list = new ArrayList();
xmW~R*^ [font="Times] list.add("one");
(\V
i_ [font="Times] list.add("second");
"q@m6fs [font="Times] list.add("3rd");
c OYDN[k [font="Times] list.add(new Integer(4));
okNo-\Dh! [font="Times] list.add(new Float( 5.0F ));
G0cG%sIl [font="Times] list.add("second");
TkbaoD [font="Times] list.add(new Integer(4));
I[\~pi, [font="Times] System.out.println(list);
UM}u(;oo%) [font="Times] }}
}pc9uvmIJ O] _4pP =OVDJ0ozZ [font="Times]Map举例
G#M)5'Q]U [font="Times]import java.util.Map;
C0rf [font="Times]import java.util.HashMap;
!40>LpL[ [font="Times]import java.util.Iterator;
!3ggQG!e [font="Times]import java.io.FileReader;
d[ N1zQW [font="Times]
~%TWF+ [font="Times]public class MapExample {
nla6QlFYn* [font="Times] public static void main(String[] args) throws java.io.FileNotFoundException {
[}RoZB&I [font="Times] Map word_count_map = new HashMap();
GK(CuwJe [font="Times] FileReader reader = new FileReader(args[0]);
2ZH+fV?. [font="Times] Iterator words = new WordStreamIterator(reader);
Wboh2:TH: [font="Times]
ZykMri3bi [font="Times] while ( words.hasNext() ) {
I`>U#x* [font="Times] String word = (String) words.next();
v9$!v^U"D [font="Times] String word_lowercase = word.toLowerCase();
rr<E#w [font="Times] Integer frequency = (Integer)word_count_map.get(word_lowercase);
>ZA=9v [font="Times]if ( frequency == null ) {
bp1AN9~ [font="Times] frequency = new Integer(1);
.8hI
ad [font="Times] } else {
2hE(h [font="Times] int value = frequency.intValue();
Ia&R/I [font="Times] frequency = new Integer(value + 1);}
Uv^\[ [font="Times] word_count_map.put(word_lowercase, frequency);
6Rd4waj_,U [font="Times] }
vDy&sgS$< [font="Times] System.out.println(word_count_map);
p7h#.m~Qu [font="Times] }}
WWT1= #" 5{Cz!ut;tE uOxHa>h [font="Times]Queue举例:
{4"V)9o-1> [font="Times]import java.io.IOException;
|"H 2'L$ [font="Times]import java.io.PrintStream;
~z,o):q1} [font="Times]import java.util.LinkedList;
(!j#u)O [font="Times]import java.util.Queue;
6CJMQi,kn [font="Times]
8;PkuJR_] [font="Times]public class QueueTester {
5J5si<v25 [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
IThd\#= [font="Times] public QueueTester() {q = new LinkedList();}
&W `xZyb3 [font="Times]public void testFIFO(PrintStream out) throws IOException {
R>Ra~b [font="Times] q.add("First");
_IH" SVub [font="Times] q.add("Second");
V+d_1]
l [font="Times] q.add("Third");
Z$oy;j99y [font="Times] Object o;
h}bfZL [font="Times] while ((o = q.poll()) != null) {
E?m~DYnU [font="Times] out.println(o);}}
q76POytV| [font="Times] public static void main(String[] args) {
'CLZ7pV [font="Times] QueueTester tester = new QueueTester();
i`,FXF) [font="Times] try { tester.testFIFO(System.out);
;C]Ufk [font="Times] } catch (IOException e) {
h}b:-a [font="Times] e.printStackTrace(); } }}
en<mm#Ab Lu.zc='\ *kr/,_K 上述例子和一些相关例子(共6个例子)打包下载[font="Times]
K^-1M? 总结:
Io6/Fv>! 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。
f|RmAP;X, 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。