全新java初学者实践教程10(java SE5.0版)
A'HFpsa jdk5的集合类
h5e(Avk $014/IB lM~ 3yBy OaY.T 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。
\C $LjSS- 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。
oOlqlv [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。
_]@ NKd}g [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]
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s [font="Times](1) Set
zgXg-cr 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。
(`\ DDJ[ 'hN_H}U ([font="Times]2)[font="Times]List
mN?y\GB 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。
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! 6e0tA ()F Zvz Zs [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。
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m r"b/oM{ ]Jz2[F"J [font="Times] ([font="Times]4)[font="Times]Queue
Is1P,`*! [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。
Cp4 U`] [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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~*#wE `Y>'*4a\ *:S_v.Y3" 实践:[font="Times] Set举例
vqO d`_) [font="Times] import java.util.*;
DSjEoWj [font="Times]public class SetExample {
R8LJC]6Bh [font="Times] public static void main(String[] args) {
3m%oXT [font="Times] Set set = new HashSet(); //HashSet是[font="Times]Set的子接口
dt&m YSZ} [font="Times] set.add("one");
(7Su{tq [font="Times] set.add("second");
P/i{_r [font="Times] set.add("3rd");
hOZ:r =% [font="Times] set.add(new Integer(4));
O*0%AjT6 [font="Times] set.add(new Float( 5.0F ));
3-x ;_ [font="Times] set.add("second");
*\Z9=8yK [font="Times] set.add(new Integer(4));
s^f7w [font="Times] System.out.println(set);
K#Ia19au5 [font="Times] }}
yp}J+/PX} Buc{dcL/ [font="Times]List举例:
NULew]:5 [font="Times] import java.util.*;
U'~M(9uv: [font="Times]public class ListExample {
J5dwd,FQ [font="Times] public static void main(String[] args) {
skr dL.5 [font="Times] List list = new ArrayList();
by07l5 [font="Times] list.add("one");
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[font="Times] list.add("second");
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QF [font="Times] list.add("3rd");
AXnRAW [font="Times] list.add(new Integer(4));
CjR!dh1w_ [font="Times] list.add(new Float( 5.0F ));
eX)'C>4W [font="Times] list.add("second");
u}I-#j)wap [font="Times] list.add(new Integer(4));
{A^ 3<=| [font="Times] System.out.println(list);
wwh1aV * [font="Times] }}
NM
FgCL uuHg=8( /bdL.Y# V [font="Times]Map举例
2<$pai"yl [font="Times]import java.util.Map;
'q>2WP|UY9 [font="Times]import java.util.HashMap;
hTfq>jIB_ [font="Times]import java.util.Iterator;
lw+54lZX| [font="Times]import java.io.FileReader;
3CL1Z\8To [font="Times]
X LHi [font="Times]public class MapExample {
pLYLHS`* [font="Times] public static void main(String[] args) throws java.io.FileNotFoundException {
|D*a"*1+A [font="Times] Map word_count_map = new HashMap();
+O$`8a)m [font="Times] FileReader reader = new FileReader(args[0]);
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C<a [font="Times] Iterator words = new WordStreamIterator(reader);
74_':,u;]~ [font="Times]
}%75Wety [font="Times] while ( words.hasNext() ) {
z)%Ke~)<\@ [font="Times] String word = (String) words.next();
mD5Vsy{Pb [font="Times] String word_lowercase = word.toLowerCase();
]{Y7mpdB [font="Times] Integer frequency = (Integer)word_count_map.get(word_lowercase);
<JUumrEo [font="Times]if ( frequency == null ) {
c,>y1%V*S{ [font="Times] frequency = new Integer(1);
'=AqC,\# [font="Times] } else {
{CH5`& [font="Times] int value = frequency.intValue();
/1@py~ZX [font="Times] frequency = new Integer(value + 1);}
)FQxVT,. [font="Times] word_count_map.put(word_lowercase, frequency);
cr,fyAvX [font="Times] }
Qg6tJB [font="Times] System.out.println(word_count_map);
xAwP [font="Times] }}
t,NE`LC tJe5`L -HwqR Ys [font="Times]Queue举例:
-%fc)y&$ [font="Times]import java.io.IOException;
Kz~E"? [font="Times]import java.io.PrintStream;
6;C2^J @ [font="Times]import java.util.LinkedList;
d9iVuw0u< [font="Times]import java.util.Queue;
[n]C [font="Times]
Six2{b)p [font="Times]public class QueueTester {
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1V?0 [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
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C\< [font="Times] public QueueTester() {q = new LinkedList();}
D`'h8:\ [font="Times]public void testFIFO(PrintStream out) throws IOException {
G^ShN45 [font="Times] q.add("First");
:3N6Ej [font="Times] q.add("Second");
VwN=AFk
Oj [font="Times] q.add("Third");
\h>6k [font="Times] Object o;
1y3)ogL [font="Times] while ((o = q.poll()) != null) {
n\GN}?4 [font="Times] out.println(o);}}
x)R1aq [font="Times] public static void main(String[] args) {
y(<+= [font="Times] QueueTester tester = new QueueTester();
'}l7=r [font="Times] try { tester.testFIFO(System.out);
o,rK8x [font="Times] } catch (IOException e) {
<=~*`eWV [font="Times] e.printStackTrace(); } }}
GX+Gqj. I*z|_}$ ,UYe OM2Ao 上述例子和一些相关例子(共6个例子)打包下载[font="Times]
aH7@:=B 总结:
3mQ3mV: 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。
QsX`IYk 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。