全新java初学者实践教程10(java SE5.0版)
"?F[]8F.b jdk5的集合类
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l`N4P T7"QwA 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。
MH#"dGGu 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。
;4ybkOD [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。
Im\ ~x~{ b@&uwS v [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]
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8d| =.;ib6M [font="Times](1) Set
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& 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。
D ka8[z7 3o[(pfcU ([font="Times]2)[font="Times]List
hHTt-x# 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。
$x`HmL3Sb T)sIV5bk g*My1+J! [font="Times] ([font="Times]3)[font="Times]Map
wkGr} [font="Times] 映射([font="Times]Map),这个在[font="Times]java里不是地图的意思,其实地图也是映射哈。它里面的东西是键-值对([font="Times]key-value)出现的,键值对是什么呢?举个例子,比如我们查字典,用部首查字法。目录那个字就是键,这个字的解释就是值。键和值成对出现。这样说可以理解吧。这也是很常用的数据结构哦。
V\x'w*FP vT0Op e6m >oWPwXA [font="Times] ([font="Times]4)[font="Times]Queue
'DVn /3?X [font="Times] 在[font="Times]jdk5.0以前,通常的实现方式是使用[font="Times]java.util.List集合来模仿[font="Times]Queue。[font="Times]Queue的概念通过把对象添加(称为[font="Times]enqueuing的操作)到[font="Times]List的尾部(即[font="Times]Queue的后部)并通过从[font="Times]List的头部(即[font="Times]Queue的前部)提取对象而从[font="Times] List中移除(称为[font="Times]dequeuing的操作)来模拟。你需要执行先进先出的动作时可以直接使用[font="Times]Queue接口就可以了。
(kB [font="Times] 这[font="Times]4个东西,有时候功能还不太完善,需要有些子类继承它的特性。[font="Times]Set的子接口有[font="Times]TreeSet,SortedSet,[font="Times]List的有[font="Times]ArrayList等,[font="Times]Map里有[font="Times]HashMap,HashTable等,[font="Times]Queue里面有[font="Times]BlockingQueue等。我们来看看例子吧:
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U`=r.> "hI"4xSg 实践:[font="Times] Set举例
{Kr}RR*{X [font="Times] import java.util.*;
[KYq01cj [font="Times]public class SetExample {
S$hxR [font="Times] public static void main(String[] args) {
6]%=q)oL[ [font="Times] Set set = new HashSet(); //HashSet是[font="Times]Set的子接口
SQ> Yf\ [font="Times] set.add("one");
]:D&kTc [font="Times] set.add("second");
C(v'7H{4cW [font="Times] set.add("3rd");
_g/d/{-{Q [font="Times] set.add(new Integer(4));
Yb5@W/' [font="Times] set.add(new Float( 5.0F ));
&odQ&%X [font="Times] set.add("second");
Ncbe{}<md [font="Times] set.add(new Integer(4));
5I6?gv/ [font="Times] System.out.println(set);
,"`3N2!Y} [font="Times] }}
5]]QW3 guYP| [font="Times]List举例:
u`Nrg< [font="Times] import java.util.*;
A\rY~$Vr [font="Times]public class ListExample {
flqr["czwK [font="Times] public static void main(String[] args) {
m`fdf>gWp [font="Times] List list = new ArrayList();
vb0Ca+}} [font="Times] list.add("one");
y~#5!:Be [font="Times] list.add("second");
/];N 1 [font="Times] list.add("3rd");
<ZZfN@6 [font="Times] list.add(new Integer(4));
~I>|f [font="Times] list.add(new Float( 5.0F ));
J+ :3==, [font="Times] list.add("second");
DL~!
^fx [font="Times] list.add(new Integer(4));
~l'[P=R+8 [font="Times] System.out.println(list);
E#m^.B-} [font="Times] }}
W3\+51P M_E$w$l2< YGChVROG~ [font="Times]Map举例
._x"b5C [font="Times]import java.util.Map;
AM!P?${a [font="Times]import java.util.HashMap;
#oEtLb@O [font="Times]import java.util.Iterator;
j^ 8Hjg [font="Times]import java.io.FileReader;
<LBCu; [font="Times]
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7h [font="Times]public class MapExample {
}W " i{s/ [font="Times] public static void main(String[] args) throws java.io.FileNotFoundException {
+gndW [font="Times] Map word_count_map = new HashMap();
9} C(M?d [font="Times] FileReader reader = new FileReader(args[0]);
:X9;KoJl-V [font="Times] Iterator words = new WordStreamIterator(reader);
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hZ<FCY,/? [font="Times] while ( words.hasNext() ) {
NODE`VFu [font="Times] String word = (String) words.next();
T^|6{ S\ [font="Times] String word_lowercase = word.toLowerCase();
^|rzqXW [font="Times] Integer frequency = (Integer)word_count_map.get(word_lowercase);
u^ wGVg [font="Times]if ( frequency == null ) {
Us&~d"n [font="Times] frequency = new Integer(1);
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[font="Times] } else {
#CeWk$)m [font="Times] int value = frequency.intValue();
REJBm [font="Times] frequency = new Integer(value + 1);}
pDr/8HEh [font="Times] word_count_map.put(word_lowercase, frequency);
6 ;\>, [font="Times] }
O'k<4'TC [font="Times] System.out.println(word_count_map);
+@Kq [font="Times] }}
O'~^wu. `kOp9(Q{ =snJ+yn! [font="Times]Queue举例:
"\afIYS I [font="Times]import java.io.IOException;
,8p-EH [font="Times]import java.io.PrintStream;
![%:X)? [font="Times]import java.util.LinkedList;
viS7+E|O [font="Times]import java.util.Queue;
A sf]sU.. [font="Times]
[DSzhi] [font="Times]public class QueueTester {
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F [font="Times] public Queue q; //发现了一个奇怪的语法,这个尖括号是泛型声明
Dm;aTe [font="Times] public QueueTester() {q = new LinkedList();}
_ =O;Lz$x [font="Times]public void testFIFO(PrintStream out) throws IOException {
>Cr'dKZ} [font="Times] q.add("First");
Z=s]@r [font="Times] q.add("Second");
8g_GXtn(z [font="Times] q.add("Third");
g7LS [font="Times] Object o;
5@Xy) z [font="Times] while ((o = q.poll()) != null) {
Kv\uBMJNW [font="Times] out.println(o);}}
Wf$P+i* [font="Times] public static void main(String[] args) {
_3Q8R} [font="Times] QueueTester tester = new QueueTester();
%i9*2{e#~ [font="Times] try { tester.testFIFO(System.out);
8bEii1EM [font="Times] } catch (IOException e) {
yD&UH_ 1g [font="Times] e.printStackTrace(); } }}
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(>x_fDv 上述例子和一些相关例子(共6个例子)打包下载[font="Times]
0(!=N1l 总结:
Zo5.Yse 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。
lLeN`{? 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。