全新java初学者实践教程10(java SE5.0版)
(HSgEs1d jdk5的集合类
D<Zp!J1o oiX+l5`pz tl><"6AIP 7{I h_.# 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。
1[jb)j1 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。
(y M^ [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。
T'LIrf sgO'wXcoP [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]
dw TMq*e I('Un@hS [font="Times](1) Set
v>Mnl 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。
$6CwkM: 7^Ns&Q ([font="Times]2)[font="Times]List
v{9t]s>B 列表([font="Times]List)的主要特征是其对象以线性方式存储,没有特定顺序,只有一个开头和一个结尾,当然,它与根本没有顺序的[font="Times]Set是不同的。它是链表嘛,一条链肯定有顺序这个顺序就不一定了。
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C&6IU8l\ 7f~Sf [font="Times] ([font="Times]3)[font="Times]Map
_L@2_#h! [font="Times] 映射([font="Times]Map),这个在[font="Times]java里不是地图的意思,其实地图也是映射哈。它里面的东西是键-值对([font="Times]key-value)出现的,键值对是什么呢?举个例子,比如我们查字典,用部首查字法。目录那个字就是键,这个字的解释就是值。键和值成对出现。这样说可以理解吧。这也是很常用的数据结构哦。
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5vw{b? ^|TG$`M(w [font="Times] ([font="Times]4)[font="Times]Queue
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B}o9r [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。
Gkp<o [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:
kiRa+w: CYKr\DA =IUUeFv +r 实践:[font="Times] Set举例
_>v<(7 [font="Times] import java.util.*;
fgBM_c&9T [font="Times]public class SetExample {
1&P< [font="Times] public static void main(String[] args) {
cKn`/\.H [font="Times] Set set = new HashSet(); //HashSet是[font="Times]Set的子接口
`\m*+Bk[5 [font="Times] set.add("one");
:OW;?{ ~j [font="Times] set.add("second");
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[font="Times] set.add("3rd");
#?XQ7Im [font="Times] set.add(new Integer(4));
l2&`J_" [font="Times] set.add(new Float( 5.0F ));
(XXheC [font="Times] set.add("second");
P9S2?Q [font="Times] set.add(new Integer(4));
|QMhMGjV [font="Times] System.out.println(set);
V=lfl1Ev0J [font="Times] }}
*bxzCI7b > ]8a3x [font="Times]List举例:
%/>Y/!; [font="Times] import java.util.*;
9JWa$iBH@ [font="Times]public class ListExample {
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Y [font="Times] public static void main(String[] args) {
JXw^/Y$ [font="Times] List list = new ArrayList();
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J3 [font="Times] list.add("one");
!H2QjW [font="Times] list.add("second");
+Y
V|ij [font="Times] list.add("3rd");
yB3; [font="Times] list.add(new Integer(4));
l/Vo-# [font="Times] list.add(new Float( 5.0F ));
@]![o % [font="Times] list.add("second");
!wWJ^Oz= [font="Times] list.add(new Integer(4));
]r-C1bKD` [font="Times] System.out.println(list);
11,!XD*" [font="Times] }}
efD)S92 ;A'":vXmc cW{1
Pz^_ [font="Times]Map举例
iR\Hv'| [font="Times]import java.util.Map;
D)@YI.T [font="Times]import java.util.HashMap;
HFq m6| [font="Times]import java.util.Iterator;
4<x'ocKlD [font="Times]import java.io.FileReader;
/'hC i]b@v [font="Times]
Gdg)9 [font="Times]public class MapExample {
HXoX [font="Times] public static void main(String[] args) throws java.io.FileNotFoundException {
b]7GmRekl [font="Times] Map word_count_map = new HashMap();
PC qZNBN [font="Times] FileReader reader = new FileReader(args[0]);
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9Su^:1 [font="Times] Iterator words = new WordStreamIterator(reader);
@rHK(25+d [font="Times]
YhRWz=l [font="Times] while ( words.hasNext() ) {
/5#rADOS [font="Times] String word = (String) words.next();
>+mD$:L [font="Times] String word_lowercase = word.toLowerCase();
)NO<s0?& [font="Times] Integer frequency = (Integer)word_count_map.get(word_lowercase);
F|&{Rt [font="Times]if ( frequency == null ) {
k2xHH$+{#= [font="Times] frequency = new Integer(1);
7y`}PMn [font="Times] } else {
(708H_ [font="Times] int value = frequency.intValue();
c)Ic#<e( [font="Times] frequency = new Integer(value + 1);}
fi+R2p~vs [font="Times] word_count_map.put(word_lowercase, frequency);
n 3lE,b [font="Times] }
^0#;YOk [font="Times] System.out.println(word_count_map);
z`Hy'{1 [font="Times] }}
)~V4+*< X{^}\,cVtG TyKWy0x-3 [font="Times]Queue举例:
.^bft P\ [font="Times]import java.io.IOException;
5qf
BEPJ [font="Times]import java.io.PrintStream;
zvvP81$W [font="Times]import java.util.LinkedList;
m{7^EF [font="Times]import java.util.Queue;
xP=/N!,# [font="Times]
r|*&GHo L [font="Times]public class QueueTester {
ql GW.jY. [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
`y+-H|%? [font="Times] public QueueTester() {q = new LinkedList();}
$HG}[XD? [font="Times]public void testFIFO(PrintStream out) throws IOException {
fA=#Fzk 2 [font="Times] q.add("First");
n$aA)"A # [font="Times] q.add("Second");
J>^\oAgpE [font="Times] q.add("Third");
f""`cdqAOh [font="Times] Object o;
ms_ VM>l [font="Times] while ((o = q.poll()) != null) {
`+#G+Vu5 [font="Times] out.println(o);}}
xBFJ} v [font="Times] public static void main(String[] args) {
}P3tn [font="Times] QueueTester tester = new QueueTester();
'u4ezwF; [font="Times] try { tester.testFIFO(System.out);
4 V1bLm [font="Times] } catch (IOException e) {
,+;:3gRk9 [font="Times] e.printStackTrace(); } }}
@R m-CWa D{v8q)5r -AYA~O(& 上述例子和一些相关例子(共6个例子)打包下载[font="Times]
7n o5b]
\ 总结:
XM<KF&pVB 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。
x"4} isp< 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。