全新java初学者实践教程10(java SE5.0版)
'xLXj> jdk5的集合类
8([ MR c:aW"U C8x9 Jrc -Fq`#" 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。
U"=Lzo.0 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。
1,P\dGmu [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。
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-KQ 0O[le*3b [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]
d$"?8r4:K ,^RZ1tLz [font="Times](1) Set
n?U^vK_ 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。
fN/;BT (&Rql7](8 ([font="Times]2)[font="Times]List
7>= 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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[font="Times] ([font="Times]3)[font="Times]Map
<HtGp6q [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。
yf7|/M Mh{244|o[ _PcF/Gyk [font="Times] ([font="Times]4)[font="Times]Queue
HX)]@qL [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。
QQHQ3\ [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:
TF9A4 et"Pb_-U bB>.dC 实践:[font="Times] Set举例
xS>vmnW [font="Times] import java.util.*;
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a'[2L [font="Times]public class SetExample {
!nq`Py MR [font="Times] public static void main(String[] args) {
#m17cDL [font="Times] Set set = new HashSet(); //HashSet是[font="Times]Set的子接口
{Kf5a
m [font="Times] set.add("one");
A{e>7Z72 [font="Times] set.add("second");
w3z'ZCcr;" [font="Times] set.add("3rd");
':3[?d1Es [font="Times] set.add(new Integer(4));
G<*
Iw>ep [font="Times] set.add(new Float( 5.0F ));
C1+f\A|9FP [font="Times] set.add("second");
.9N7` [font="Times] set.add(new Integer(4));
#uF`|M$u [font="Times] System.out.println(set);
~KRS0^ [font="Times] }}
KK6fRtKv>q P*H0Hwn; [font="Times]List举例:
S}a]Bt [font="Times] import java.util.*;
8Ihl}aguW [font="Times]public class ListExample {
e"_kH_7sv [font="Times] public static void main(String[] args) {
JEaTDV_ [font="Times] List list = new ArrayList();
+xvn n [font="Times] list.add("one");
;6~5FTmV [font="Times] list.add("second");
Eh)VT{vp [font="Times] list.add("3rd");
l4dG=x}M] [font="Times] list.add(new Integer(4));
#4JLWg [font="Times] list.add(new Float( 5.0F ));
T:@7EL [font="Times] list.add("second");
k~gOL#$ [font="Times] list.add(new Integer(4));
XK\3"`kd [font="Times] System.out.println(list);
X&kp1Ih<^ [font="Times] }}
K7([Gc9 DVVyWn[ T9XUNR{& [font="Times]Map举例
.xuzu#- [font="Times]import java.util.Map;
jRd$Vt [font="Times]import java.util.HashMap;
#lg R"% [font="Times]import java.util.Iterator;
]Vmo> [font="Times]import java.io.FileReader;
gO)":!_n W [font="Times]
)$1>6C\ [font="Times]public class MapExample {
d^J)Mhju [font="Times] public static void main(String[] args) throws java.io.FileNotFoundException {
PZ`11#bbm [font="Times] Map word_count_map = new HashMap();
EZN!3y| m [font="Times] FileReader reader = new FileReader(args[0]);
g8l6bh$} [font="Times] Iterator words = new WordStreamIterator(reader);
L~^5Ez6U [font="Times]
q2s0g*z [font="Times] while ( words.hasNext() ) {
cdh0b7tjn [font="Times] String word = (String) words.next();
%1:caa@_p [font="Times] String word_lowercase = word.toLowerCase();
-- FzRO{D [font="Times] Integer frequency = (Integer)word_count_map.get(word_lowercase);
JSi0-S[Y{ [font="Times]if ( frequency == null ) {
ZX64kk+ [font="Times] frequency = new Integer(1);
)UM^#<- [font="Times] } else {
Mn/@?K?y [font="Times] int value = frequency.intValue();
s'oNW [font="Times] frequency = new Integer(value + 1);}
tv.<pP9-C [font="Times] word_count_map.put(word_lowercase, frequency);
S1I.l">P [font="Times] }
k=[s%O6H [font="Times] System.out.println(word_count_map);
~D>pu%F [font="Times] }}
R^$|D)( ;Xy=;Z.]i 2,F9P+ [font="Times]Queue举例:
! xG*W6IT [font="Times]import java.io.IOException;
\Dy|}LE [font="Times]import java.io.PrintStream;
b0YEIV<$ [font="Times]import java.util.LinkedList;
DJ@n$G`^^ [font="Times]import java.util.Queue;
|oke)w=gn [font="Times]
#XA`n@2Uoo [font="Times]public class QueueTester {
(y 7X1Qc) [font="Times] public Queue q; //发现了一个奇怪的语法,这个尖括号是泛型声明
F -,chp [font="Times] public QueueTester() {q = new LinkedList();}
tV`=o$` [font="Times]public void testFIFO(PrintStream out) throws IOException {
)0iN2L]U; [font="Times] q.add("First");
.1jiANY [font="Times] q.add("Second");
8\!E )M|4 [font="Times] q.add("Third");
BjsT 9?6W/ [font="Times] Object o;
qSB&Q0T [font="Times] while ((o = q.poll()) != null) {
Pv/v=s>X [font="Times] out.println(o);}}
XWnP(C9? [font="Times] public static void main(String[] args) {
w$6Z}M1d [font="Times] QueueTester tester = new QueueTester();
4$[o; t> [font="Times] try { tester.testFIFO(System.out);
CDRbYO [font="Times] } catch (IOException e) {
{\(MMTQ [font="Times] e.printStackTrace(); } }}
j&oRj6;Ha+ #}FUa u$ V(F9=r<X 上述例子和一些相关例子(共6个例子)打包下载[font="Times]
8ZVQM7O 总结:
a
\1QnCy 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。
%Qlc?Wl: 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。