全新java初学者实践教程10(java SE5.0版)
s~Wj h7' jdk5的集合类
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;no>=* 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。
21GjRPs\ 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。
,c"_X8Fkx$ [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。
QytqO{B^ FH}n]T [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]
]g-(|X~> #M*h)/d[A [font="Times](1) Set
f XxdOn. 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。
sKIWr{D j>~^jz: ([font="Times]2)[font="Times]List
uy\<t 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。
E :*!an [dFxW6n #(3w6l2 [font="Times] ([font="Times]3)[font="Times]Map
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Sy0Of [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。
+]H9:ARI 9)l-5o:D X>OO4SV [font="Times] ([font="Times]4)[font="Times]Queue
Acr\2!)) [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。
dA>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:
F.c`0u;= .$)'7 #C,M8~Q7 实践:[font="Times] Set举例
STu!v5XY}- [font="Times] import java.util.*;
6gwjrGje\ [font="Times]public class SetExample {
{55{YDqx [font="Times] public static void main(String[] args) {
'1;Q'-/J [font="Times] Set set = new HashSet(); //HashSet是[font="Times]Set的子接口
aWek<Y~+ [font="Times] set.add("one");
@uz&]~+` [font="Times] set.add("second");
yCkfAx8] [font="Times] set.add("3rd");
'-3AWBWI1 [font="Times] set.add(new Integer(4));
!> b>"\b [font="Times] set.add(new Float( 5.0F ));
]O',Ei^ [font="Times] set.add("second");
QU16X [font="Times] set.add(new Integer(4));
XyJ*>;q [font="Times] System.out.println(set);
le yhiL< [font="Times] }}
CJg & }MY7<sMDOy [font="Times]List举例:
#T
Cz$_=t [font="Times] import java.util.*;
z=<T[Uy [font="Times]public class ListExample {
a#FkoA~M [font="Times] public static void main(String[] args) {
CyO2Z
[font="Times] List list = new ArrayList();
p%,:U8fOR [font="Times] list.add("one");
o%X_V!B{V [font="Times] list.add("second");
`x$d8(1J`# [font="Times] list.add("3rd");
X!&DKE [font="Times] list.add(new Integer(4));
M_+&XLnzsJ [font="Times] list.add(new Float( 5.0F ));
!y$Hr[v [font="Times] list.add("second");
;YNN)P%" [font="Times] list.add(new Integer(4));
53P\OG^G` [font="Times] System.out.println(list);
2<>n8 K [font="Times] }}
7^'TU=ss_ YQ X+lE 1;3oGuHj8 [font="Times]Map举例
[&t3xC, [font="Times]import java.util.Map;
"C.'_H!Ex [font="Times]import java.util.HashMap;
CCfuz & [font="Times]import java.util.Iterator;
z*ZEw [font="Times]import java.io.FileReader;
2\l7=9 ]\3 [font="Times]
G& cm5 [font="Times]public class MapExample {
G U~?S'{ [font="Times] public static void main(String[] args) throws java.io.FileNotFoundException {
@!fy24R]D [font="Times] Map word_count_map = new HashMap();
0#F3@/1h [font="Times] FileReader reader = new FileReader(args[0]);
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ng[& [font="Times] Iterator words = new WordStreamIterator(reader);
N72z5[.. [font="Times]
85$MHod}[, [font="Times] while ( words.hasNext() ) {
pBiC [font="Times] String word = (String) words.next();
[J\5DctX;c [font="Times] String word_lowercase = word.toLowerCase();
9_JK. [font="Times] Integer frequency = (Integer)word_count_map.get(word_lowercase);
'VFxg, [font="Times]if ( frequency == null ) {
]Rohf WHX [font="Times] frequency = new Integer(1);
o,9E~Q '`{ [font="Times] } else {
u /JEQz1 [font="Times] int value = frequency.intValue();
ESiNW&u2 [font="Times] frequency = new Integer(value + 1);}
|;'V":yDs [font="Times] word_count_map.put(word_lowercase, frequency);
YNc%[S[u^1 [font="Times] }
?|TVz!3 [font="Times] System.out.println(word_count_map);
ur={+0
y [font="Times] }}
1c&/&6#5 Jx1oK 6[wej$u [font="Times]Queue举例:
~[Mk QJxe [font="Times]import java.io.IOException;
#9EpQc[4 [font="Times]import java.io.PrintStream;
GV6!`@< [font="Times]import java.util.LinkedList;
W*;~(hDz [font="Times]import java.util.Queue;
'IP'g,o++ [font="Times]
NZ9=hI;iM [font="Times]public class QueueTester {
;j=/2vU~@ [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
n9gj{]% [font="Times] public QueueTester() {q = new LinkedList();}
xB]~%nC[O [font="Times]public void testFIFO(PrintStream out) throws IOException {
0z&3jWWY@ [font="Times] q.add("First");
pD##lkJr [font="Times] q.add("Second");
;[0<QmeI! [font="Times] q.add("Third");
u91;GBY [font="Times] Object o;
\hQ[5> [font="Times] while ((o = q.poll()) != null) {
ncb?iJ/b^ [font="Times] out.println(o);}}
\ [font="Times] public static void main(String[] args) {
+N"A5U [font="Times] QueueTester tester = new QueueTester();
5FtbZ1L [font="Times] try { tester.testFIFO(System.out);
zCL/^^# [font="Times] } catch (IOException e) {
[%YA42_`LD [font="Times] e.printStackTrace(); } }}
ye KzI~ Un^QNd> !jMa%;/ 上述例子和一些相关例子(共6个例子)打包下载[font="Times]
H:#b(&qw2 总结:
?(Dkh${@ 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。
9H2^4D8 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。