全新java初学者实践教程10(java SE5.0版)
>h?!6L- d jdk5的集合类
jA&ZO>4 s<f<:BC ;<j[0~qp: ?Vy%<f$ 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。
v1s.j2T 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]?KDID; [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。
jea{BhdUr D"x;/I [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]
`1AVw]k oa4{s&db- [font="Times](1) Set
PBXRey7>D 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。
bi^[Eh rHzwSR@}1 ([font="Times]2)[font="Times]List
&!|' EW 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。
P4&3jQ[o i&%~:K* -@6R`m=> [font="Times] ([font="Times]3)[font="Times]Map
^lB=O [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。
kj$Ks2!W ,4O|{Iu#n fC$Rz#5? [font="Times] ([font="Times]4)[font="Times]Queue
O;bnyB$ [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。
_"b[UT}m [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:
kx6AMx!nX ZCP
r`H :Pa^/i 实践:[font="Times] Set举例
}XJA#@ [font="Times] import java.util.*;
/$w,8pV= [font="Times]public class SetExample {
,".1![b [font="Times] public static void main(String[] args) {
|ia#Elavo [font="Times] Set set = new HashSet(); //HashSet是[font="Times]Set的子接口
P2U^%_~ [font="Times] set.add("one");
`7v"( [font="Times] set.add("second");
>(>,*zP<9 [font="Times] set.add("3rd");
xL-]gwq [font="Times] set.add(new Integer(4));
JDp"!x{O [font="Times] set.add(new Float( 5.0F ));
zEHX:-f8 [font="Times] set.add("second");
<'{*6f@n [font="Times] set.add(new Integer(4));
V 6DWYs> [font="Times] System.out.println(set);
'T!^H [font="Times] }}
Pdq}~um3{ /2%646 [font="Times]List举例:
})v`` + [font="Times] import java.util.*;
)=~OP>7B [font="Times]public class ListExample {
c#-o@`Po [font="Times] public static void main(String[] args) {
v-
793pr [font="Times] List list = new ArrayList();
z(00"ei [font="Times] list.add("one");
>-%tvrS% [font="Times] list.add("second");
}Kt`du= [font="Times] list.add("3rd");
-rn%ASye [font="Times] list.add(new Integer(4));
QD2;JI2 [font="Times] list.add(new Float( 5.0F ));
]0Y5 Z)3:z [font="Times] list.add("second");
O,a1?_m8 [font="Times] list.add(new Integer(4));
-2o_ L? [font="Times] System.out.println(list);
DG%vEM,y [font="Times] }}
LqZsH0C yYdow.b! n<GTc{>Z [font="Times]Map举例
Gx&o3^ t [font="Times]import java.util.Map;
QfdATK P [font="Times]import java.util.HashMap;
dH5 Go9`~R [font="Times]import java.util.Iterator;
4l2/eh]Hc( [font="Times]import java.io.FileReader;
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[font="Times]
bS1?I@ [font="Times]public class MapExample {
5_H`6-q [font="Times] public static void main(String[] args) throws java.io.FileNotFoundException {
_l{`lQ} [font="Times] Map word_count_map = new HashMap();
*VuiEBG [font="Times] FileReader reader = new FileReader(args[0]);
>/BMA;` [font="Times] Iterator words = new WordStreamIterator(reader);
[>IV#6 $ [font="Times]
'<Fr}Cn [font="Times] while ( words.hasNext() ) {
!_yWe [font="Times] String word = (String) words.next();
e&R?9z-* [font="Times] String word_lowercase = word.toLowerCase();
S)?V;@p6 [font="Times] Integer frequency = (Integer)word_count_map.get(word_lowercase);
%0y_WIjz [font="Times]if ( frequency == null ) {
D1ep7ykY [font="Times] frequency = new Integer(1);
43'!<[?x [font="Times] } else {
efT@A}sV [font="Times] int value = frequency.intValue();
xWiR7~E [font="Times] frequency = new Integer(value + 1);}
Wb1?>q [font="Times] word_count_map.put(word_lowercase, frequency);
4#^E$N: [font="Times] }
DN$[rCi7 [font="Times] System.out.println(word_count_map);
|E}-j;( [font="Times] }}
P]~apMi: `X8wnD /WxCsQn [font="Times]Queue举例:
QC,LHt?6 [font="Times]import java.io.IOException;
}zO>y%eI [font="Times]import java.io.PrintStream;
#CV;Np [font="Times]import java.util.LinkedList;
*^+]`S [font="Times]import java.util.Queue;
j5Cf\*B4J [font="Times]
hFQ*50n} [font="Times]public class QueueTester {
(:9=M5d [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
PxvD0GTW [font="Times] public QueueTester() {q = new LinkedList();}
>WcOY7 [font="Times]public void testFIFO(PrintStream out) throws IOException {
O>KrTK-AV [font="Times] q.add("First");
x+Ws lN2a [font="Times] q.add("Second");
CVAX?c{ [font="Times] q.add("Third");
N 4!18{/2 [font="Times] Object o;
Ib&]1ger#= [font="Times] while ((o = q.poll()) != null) {
_ ;!$1lM[ [font="Times] out.println(o);}}
)wM881_! [font="Times] public static void main(String[] args) {
l^cz&k=+ [font="Times] QueueTester tester = new QueueTester();
9OS~;9YR [font="Times] try { tester.testFIFO(System.out);
j=up7395 [font="Times] } catch (IOException e) {
?!Wh ^su- [font="Times] e.printStackTrace(); } }}
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tsu"G y0R9[;b07 * YR>u@ 上述例子和一些相关例子(共6个例子)打包下载[font="Times]
%7S{g 总结:
yADX^r( 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。
N hY`_?) 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。