全新java初学者实践教程10(java SE5.0版)
EU?qLj': jdk5的集合类
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>OP 5CkG^9 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。
~`OX}h/Z 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。
H`JFXMa< [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。
$xbC^ k cfLF@LW!]) [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]
\]tBwa u;`]U$Qq9 [font="Times](1) Set
n$/|r 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。
#aP#r4$ 3Zb%-_%j ([font="Times]2)[font="Times]List
@Po5AK3cy 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。
kt6x"'"1 +K4XMf AWR :~{ [font="Times] ([font="Times]3)[font="Times]Map
uk):z$x [font="Times] 映射([font="Times]Map),这个在[font="Times]java里不是地图的意思,其实地图也是映射哈。它里面的东西是键-值对([font="Times]key-value)出现的,键值对是什么呢?举个例子,比如我们查字典,用部首查字法。目录那个字就是键,这个字的解释就是值。键和值成对出现。这样说可以理解吧。这也是很常用的数据结构哦。
RaJTya^ v ccH(T N"S`9B1eD( [font="Times] ([font="Times]4)[font="Times]Queue
,MY7h8V/ [font="Times] 在[font="Times]jdk5.0以前,通常的实现方式是使用[font="Times]java.util.List集合来模仿[font="Times]Queue。[font="Times]Queue的概念通过把对象添加(称为[font="Times]enqueuing的操作)到[font="Times]List的尾部(即[font="Times]Queue的后部)并通过从[font="Times]List的头部(即[font="Times]Queue的前部)提取对象而从[font="Times] List中移除(称为[font="Times]dequeuing的操作)来模拟。你需要执行先进先出的动作时可以直接使用[font="Times]Queue接口就可以了。
U zy@\ [font="Times] 这[font="Times]4个东西,有时候功能还不太完善,需要有些子类继承它的特性。[font="Times]Set的子接口有[font="Times]TreeSet,SortedSet,[font="Times]List的有[font="Times]ArrayList等,[font="Times]Map里有[font="Times]HashMap,HashTable等,[font="Times]Queue里面有[font="Times]BlockingQueue等。我们来看看例子吧:
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fzSkl`K} 实践:[font="Times] Set举例
Dp'urf\*$ [font="Times] import java.util.*;
sl' 4AK~\ [font="Times]public class SetExample {
\`n(JV [font="Times] public static void main(String[] args) {
iGW|j>N [font="Times] Set set = new HashSet(); //HashSet是[font="Times]Set的子接口
:R;w<Tbz" [font="Times] set.add("one");
#dauXUKH [font="Times] set.add("second");
.hT>a< [font="Times] set.add("3rd");
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. [font="Times] set.add(new Integer(4));
yZleots1 [font="Times] set.add(new Float( 5.0F ));
;2f=d_/x [font="Times] set.add("second");
VeA@HC`?" [font="Times] set.add(new Integer(4));
(Z),gxt [font="Times] System.out.println(set);
dN{At- [font="Times] }}
7H7
Xbi@ p _gN}v [font="Times]List举例:
1WUSp;JMl [font="Times] import java.util.*;
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z3j [font="Times]public class ListExample {
R3>q ] [font="Times] public static void main(String[] args) {
72<9xNcB!} [font="Times] List list = new ArrayList();
Kr}RFJ"d [font="Times] list.add("one");
xWLvx'8W [font="Times] list.add("second");
uzd7v, [font="Times] list.add("3rd");
iQG!-.aX [font="Times] list.add(new Integer(4));
_H+]G"k/r [font="Times] list.add(new Float( 5.0F ));
|+cz\+ [font="Times] list.add("second");
X|of87 [font="Times] list.add(new Integer(4));
$S6(V}yh [font="Times] System.out.println(list);
bxXpw& [font="Times] }}
yzS]FwW7 *6s_7{; V+gZjuN$ [font="Times]Map举例
[OC(~b [font="Times]import java.util.Map;
?8>a;0 [font="Times]import java.util.HashMap;
JX59n%$@ [font="Times]import java.util.Iterator;
NFT:$>83` [font="Times]import java.io.FileReader;
N63?4'_W [font="Times]
Ia2WBs= [font="Times]public class MapExample {
N*IroT3 [font="Times] public static void main(String[] args) throws java.io.FileNotFoundException {
>.?yz [font="Times] Map word_count_map = new HashMap();
j=0kxvp [font="Times] FileReader reader = new FileReader(args[0]);
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J< [font="Times] Iterator words = new WordStreamIterator(reader);
Jf^3nBZ [font="Times]
,ri&zbB [font="Times] while ( words.hasNext() ) {
t^#1=nK [font="Times] String word = (String) words.next();
+t7HlAXB# [font="Times] String word_lowercase = word.toLowerCase();
-laH^<jm5 [font="Times] Integer frequency = (Integer)word_count_map.get(word_lowercase);
+ai3 [font="Times]if ( frequency == null ) {
YD4I2'E [font="Times] frequency = new Integer(1);
bmT%?it [font="Times] } else {
Tuvs} [font="Times] int value = frequency.intValue();
*DJsY/9d}' [font="Times] frequency = new Integer(value + 1);}
uKx:7"KD [font="Times] word_count_map.put(word_lowercase, frequency);
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S( [font="Times] }
d6i}xnmC [font="Times] System.out.println(word_count_map);
Qf:e;1F! [font="Times] }}
2[lP ,;! }?m0bM z~H1f$} [font="Times]Queue举例:
&8VH m?h [font="Times]import java.io.IOException;
vn.5X [font="Times]import java.io.PrintStream;
\'O/3Y7?X [font="Times]import java.util.LinkedList;
frcAXh9 [font="Times]import java.util.Queue;
gwaSgV$z [font="Times]
xF_u:}7` [font="Times]public class QueueTester {
HNZ$CaJh [font="Times] public Queue q; //发现了一个奇怪的语法,这个尖括号是泛型声明
iM .yen_vp [font="Times] public QueueTester() {q = new LinkedList();}
o$@/@r [font="Times]public void testFIFO(PrintStream out) throws IOException {
PMQTcQ^ [font="Times] q.add("First");
<"Y>|X [font="Times] q.add("Second");
+w0Wg.4V [font="Times] q.add("Third");
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