全新java初学者实践教程10(java SE5.0版)
Y|",.~ jdk5的集合类
PeOgXg)L`z TSeAC[%pL G8@%)$A \I>,j,c 上次课我们学过了数组,知道它只是一组数(或是对象),但是有些自己的特性。在[font="Times]java里还有一类东西与数组类似,也是有着特性的一组数[font="Times](或是对象[font="Times]),叫做集合类。
'(?@R5a 我们上节课讲到了,数组的长度在创建时已经确定了,但是有时候我们事先根本不知道长度是多少啊,比如我们做电子商务网站时,有个购物车程序。你总不能用数组规定,人家只能买[font="Times]5样东西吧。你就是把长度定为[font="Times]10000也不行,万一遇上个特别有钱的呢!呵呵,这只是开玩笑的。我们会使用集合类解决这个问题。
zT@vji%Y [font="Times] 集合类是放在[font="Times]java.util.*;这个包里。集合类存放的都是对象的引用,而非对象本身,为了说起来方便些,我们称集合中的对象就是指集合中对象的引用([font="Times]reference)。引用的概念大家不会忘了吧,在前边我们讲
数据类型时讲的。
Rh39x-`Z SRMy#j- [font="Times] 集合类型主要有[font="Times]3种:[font="Times]set(集)、[font="Times]list(列表)、[font="Times]map(映射[font="Times])和[font="Times]Queue(队列)。[font="Times]//队列为[font="Times]jdk5中的加上的[font="Times]
zK}.Bhj# 9E+^FZ e [font="Times](1) Set
7J)-WXk 集([font="Times]set)是最简单的一种集合,它的对象不按特定方式排序,只是简单的把对象加入集合中,就像往口袋里放东西。对集中成员的访问和操作是通过集中对象的引用进行的,所以集中不能有重复对象。我们知道数学上的集合也是[font="Times]Set这个,集合里面一定是没有重复的元素的。
1'h?qv^( wx<DzC ([font="Times]2)[font="Times]List
gxF3gM 列表([font="Times]List)的主要特征是其对象以线性方式存储,没有特定顺序,只有一个开头和一个结尾,当然,它与根本没有顺序的[font="Times]Set是不同的。它是链表嘛,一条链肯定有顺序这个顺序就不一定了。
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2:P~ + IMP< [font="Times] ([font="Times]3)[font="Times]Map
U`D"L4},. [font="Times] 映射([font="Times]Map),这个在[font="Times]java里不是地图的意思,其实地图也是映射哈。它里面的东西是键-值对([font="Times]key-value)出现的,键值对是什么呢?举个例子,比如我们查字典,用部首查字法。目录那个字就是键,这个字的解释就是值。键和值成对出现。这样说可以理解吧。这也是很常用的数据结构哦。
F$.M2*9 'Z';$N ] ~*&_zPTN [font="Times] ([font="Times]4)[font="Times]Queue
{Tx+m;5F [font="Times] 在[font="Times]jdk5.0以前,通常的实现方式是使用[font="Times]java.util.List集合来模仿[font="Times]Queue。[font="Times]Queue的概念通过把对象添加(称为[font="Times]enqueuing的操作)到[font="Times]List的尾部(即[font="Times]Queue的后部)并通过从[font="Times]List的头部(即[font="Times]Queue的前部)提取对象而从[font="Times] List中移除(称为[font="Times]dequeuing的操作)来模拟。你需要执行先进先出的动作时可以直接使用[font="Times]Queue接口就可以了。
c#_%|gg [font="Times] 这[font="Times]4个东西,有时候功能还不太完善,需要有些子类继承它的特性。[font="Times]Set的子接口有[font="Times]TreeSet,SortedSet,[font="Times]List的有[font="Times]ArrayList等,[font="Times]Map里有[font="Times]HashMap,HashTable等,[font="Times]Queue里面有[font="Times]BlockingQueue等。我们来看看例子吧:
Yk }zN_v 7p{lDQ 7:]I@Gc' 实践:[font="Times] Set举例
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A(W [font="Times] import java.util.*;
FJ/kumq [font="Times]public class SetExample {
VXC_Y [font="Times] public static void main(String[] args) {
zdtzR<X [font="Times] Set set = new HashSet(); //HashSet是[font="Times]Set的子接口
}pA0mW9 [font="Times] set.add("one");
RP6QS )| [font="Times] set.add("second");
NVP~`sxiZ [font="Times] set.add("3rd");
z`{x1*w_ [font="Times] set.add(new Integer(4));
Fq~de%y [font="Times] set.add(new Float( 5.0F ));
U<Ag=vsZE [font="Times] set.add("second");
*T 6<'a [font="Times] set.add(new Integer(4));
m%akx@{WL [font="Times] System.out.println(set);
J-=&B5"O> [font="Times] }}
bjbm"~ .DHPKz`W0 [font="Times]List举例:
*PD7H9m [font="Times] import java.util.*;
i9$
-lk [font="Times]public class ListExample {
1--5ok
h [font="Times] public static void main(String[] args) {
5E4np`J [font="Times] List list = new ArrayList();
.F |yxj;I7 [font="Times] list.add("one");
OF!(BJL [font="Times] list.add("second");
r-=#C1eY& [font="Times] list.add("3rd");
&!_>J0 [font="Times] list.add(new Integer(4));
MK,#"Ty}zK [font="Times] list.add(new Float( 5.0F ));
%vgn>A?]1 [font="Times] list.add("second");
z;D[7tT [font="Times] list.add(new Integer(4));
^k/@y@% [font="Times] System.out.println(list);
j%0D:jOY] [font="Times] }}
o#CNr5/ bvi
Y.G3 *$Bx#0J8 [font="Times]Map举例
<t\!g [font="Times]import java.util.Map;
Sw%^&*J [font="Times]import java.util.HashMap;
1)w^.8f [font="Times]import java.util.Iterator;
l 'm!e '7_ [font="Times]import java.io.FileReader;
7;5SK:X%dm [font="Times]
AfB,`l`k [font="Times]public class MapExample {
:.J Ad$>P [font="Times] public static void main(String[] args) throws java.io.FileNotFoundException {
?31#:Mg6g+ [font="Times] Map word_count_map = new HashMap();
=wtu [font="Times] FileReader reader = new FileReader(args[0]);
e+Vn@-L; [font="Times] Iterator words = new WordStreamIterator(reader);
BRyrdt*_e [font="Times]
SN'j?- [font="Times] while ( words.hasNext() ) {
v*U OD'tk [font="Times] String word = (String) words.next();
]?M3X_Mq [font="Times] String word_lowercase = word.toLowerCase();
@vs+)aRa [font="Times] Integer frequency = (Integer)word_count_map.get(word_lowercase);
~x4]^XS [font="Times]if ( frequency == null ) {
k>SPtiAs [font="Times] frequency = new Integer(1);
wF[%+n (* [font="Times] } else {
QbrR=[8b [font="Times] int value = frequency.intValue();
sYE| [font="Times] frequency = new Integer(value + 1);}
;auT!a~a# [font="Times] word_count_map.put(word_lowercase, frequency);
_X;xW#go [font="Times] }
> &tmdE [font="Times] System.out.println(word_count_map);
}6b" JoC [font="Times] }}
UXgeL2`; POB6#x yI$KBx/]n [font="Times]Queue举例:
OXDEU. [font="Times]import java.io.IOException;
xU9T8Lw [font="Times]import java.io.PrintStream;
C( wZjO?N [font="Times]import java.util.LinkedList;
ON [F [font="Times]import java.util.Queue;
r"YOA@ [font="Times]
M$,4B [font="Times]public class QueueTester {
[SJ3FZ< [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
l_$le [font="Times] public QueueTester() {q = new LinkedList();}
46[k9T [font="Times]public void testFIFO(PrintStream out) throws IOException {
r|av|7R [font="Times] q.add("First");
[$]qJ~kz [font="Times] q.add("Second");
wjy<{I [font="Times] q.add("Third");
nOTe 3?i> [font="Times] Object o;
BMi5F?Q'G [font="Times] while ((o = q.poll()) != null) {
?
R!Pf: t [font="Times] out.println(o);}}
++\s0A(e [font="Times] public static void main(String[] args) {
_D"V^4^yqu [font="Times] QueueTester tester = new QueueTester();
MMU>55+- [font="Times] try { tester.testFIFO(System.out);
T0|hp7WM [font="Times] } catch (IOException e) {
'/2)I8 [font="Times] e.printStackTrace(); } }}
FQ_a=v adY ,Nz U2G[uDa; 上述例子和一些相关例子(共6个例子)打包下载[font="Times]
'+&!;Jj, 总结:
.C|dGE?, 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。
Q3<ctd\]Y 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。