全新java初学者实践教程10(java SE5.0版)
ja$>>5<q jdk5的集合类
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a E#"QaI8` \C.%S +u 上次课我们学过了数组,知道它只是一组数(或是对象),但是有些自己的特性。在[font="Times]java里还有一类东西与数组类似,也是有着特性的一组数[font="Times](或是对象[font="Times]),叫做集合类。
1A^iUC5) 我们上节课讲到了,数组的长度在创建时已经确定了,但是有时候我们事先根本不知道长度是多少啊,比如我们做电子商务网站时,有个购物车程序。你总不能用数组规定,人家只能买[font="Times]5样东西吧。你就是把长度定为[font="Times]10000也不行,万一遇上个特别有钱的呢!呵呵,这只是开玩笑的。我们会使用集合类解决这个问题。
N_<sCRd]9 [font="Times] 集合类是放在[font="Times]java.util.*;这个包里。集合类存放的都是对象的引用,而非对象本身,为了说起来方便些,我们称集合中的对象就是指集合中对象的引用([font="Times]reference)。引用的概念大家不会忘了吧,在前边我们讲
数据类型时讲的。
`;fh<kv PK1j$&F [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]
J*6I@_{/U E%eao$ [font="Times](1) Set
3ojK2F(1D 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。
1wUZ0r1' Cw?AP6f% ([font="Times]2)[font="Times]List
xrx{8pf 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。
m}.ru)^p 7=XQgbY/ l|`FW [font="Times] ([font="Times]3)[font="Times]Map
XuJwZN!( [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。
q3Umqvl)oe G],+?E_, O<4i)Lx2 [font="Times] ([font="Times]4)[font="Times]Queue
2>Kq)Ii [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。
S-Va_t$ [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:
N>`+{ zF/}s_><* [rqe;00] 实践:[font="Times] Set举例
Y~1}B_ [font="Times] import java.util.*;
etf ft8 [font="Times]public class SetExample {
BEvSX|M>x [font="Times] public static void main(String[] args) {
n? "ti [font="Times] Set set = new HashSet(); //HashSet是[font="Times]Set的子接口
, >Y.! [font="Times] set.add("one");
~C5iyXR [font="Times] set.add("second");
$gDp-7 [font="Times] set.add("3rd");
X[k-J\ [font="Times] set.add(new Integer(4));
A(_AOoA' [font="Times] set.add(new Float( 5.0F ));
B%6bk. [font="Times] set.add("second");
5X&<+{bX [font="Times] set.add(new Integer(4));
[e)81yZG> [font="Times] System.out.println(set);
:w_F<2d0
0 [font="Times] }}
vVyO}Q` "b%hAdR [font="Times]List举例:
mDE{s",q/ [font="Times] import java.util.*;
9BI5qHEp [font="Times]public class ListExample {
L:YsAv [font="Times] public static void main(String[] args) {
1hZM)) [font="Times] List list = new ArrayList();
OfTcF_% [font="Times] list.add("one");
xmKa8']x [font="Times] list.add("second");
V.WfP*~NJ [font="Times] list.add("3rd");
/6{`6(p [font="Times] list.add(new Integer(4));
|}Mkn4 [font="Times] list.add(new Float( 5.0F ));
sxL;o>{ [font="Times] list.add("second");
*"
<tFQ [font="Times] list.add(new Integer(4));
{N5g52MN [font="Times] System.out.println(list);
ulfpop*2 [font="Times] }}
.u7d >|0yH9af B0p>' O2 [font="Times]Map举例
UQ|0Aqwq [font="Times]import java.util.Map;
_zh}%#6L [font="Times]import java.util.HashMap;
Oi
BK [font="Times]import java.util.Iterator;
{\|? {8f [font="Times]import java.io.FileReader;
yE9JMi0 [font="Times]
6(9Ta'ywZ [font="Times]public class MapExample {
|+6Z+-.Hg [font="Times] public static void main(String[] args) throws java.io.FileNotFoundException {
};o R x) [font="Times] Map word_count_map = new HashMap();
zQ{ Q>"- [font="Times] FileReader reader = new FileReader(args[0]);
t{~"vD9Am [font="Times] Iterator words = new WordStreamIterator(reader);
5YS`v#+ [font="Times]
nWFp$tJ/R [font="Times] while ( words.hasNext() ) {
mMN oR] [font="Times] String word = (String) words.next();
u;J= g [font="Times] String word_lowercase = word.toLowerCase();
@<vDR"> [font="Times] Integer frequency = (Integer)word_count_map.get(word_lowercase);
0IDHoNaT< [font="Times]if ( frequency == null ) {
1+eC'&@Xjt [font="Times] frequency = new Integer(1);
-D:J$d
6R< [font="Times] } else {
"]c:V4S#`A [font="Times] int value = frequency.intValue();
K'?ab 0 [font="Times] frequency = new Integer(value + 1);}
h>mQ; L [font="Times] word_count_map.put(word_lowercase, frequency);
4n3QW%# [font="Times] }
{(a@3m~a% [font="Times] System.out.println(word_count_map);
3kR- WgVF, [font="Times] }}
nOQ+oqM< mf}?z21vD 3 tXtt@Yy [font="Times]Queue举例:
9}}D -&Mc [font="Times]import java.io.IOException;
)Xd=EWGUS [font="Times]import java.io.PrintStream;
);))kYr [font="Times]import java.util.LinkedList;
zN5i}U=|r [font="Times]import java.util.Queue;
e}[$ = [font="Times]
4]
? [font="Times]public class QueueTester {
dUO~dV1 [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
EzNmsbtZ( [font="Times] public QueueTester() {q = new LinkedList();}
hNx`=D9[7 [font="Times]public void testFIFO(PrintStream out) throws IOException {
#S<>+,Lk [font="Times] q.add("First");
}GkEv}~t [font="Times] q.add("Second");
$Z{ fKr [font="Times] q.add("Third");
DVSYH{U4 [font="Times] Object o;
1pzU=!R?-O [font="Times] while ((o = q.poll()) != null) {
D%^EG8i n. [font="Times] out.println(o);}}
\XRViG,|5 [font="Times] public static void main(String[] args) {
$6y1';A [font="Times] QueueTester tester = new QueueTester();
G Q8I |E [font="Times] try { tester.testFIFO(System.out);
Z?nMt [font="Times] } catch (IOException e) {
z[t$[Qg [font="Times] e.printStackTrace(); } }}
ybS7uo AO']Kmm 5 yA^ n6 上述例子和一些相关例子(共6个例子)打包下载[font="Times]
#{h4lte 总结:
|{9"n<JW 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。
2A`EFk7_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。