全新java初学者实践教程10(java SE5.0版)
SJ@_eir\o jdk5的集合类
A*;h}\n V@f#/"u' P .( X]+ Us.jyg7_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。
1Xc%%j 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。
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fb [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。
8;rS"!qM 1!#ZEI C [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]
Pw.+DA Mpx.n]O. [font="Times](1) Set
xoaQ5u 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。
!1R ,/9|j*9H ([font="Times]2)[font="Times]List
Jq)k?WS 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。
5o#8DIal _;W|iUreb }qPo%T [font="Times] ([font="Times]3)[font="Times]Map
,_O[;L [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。
5~QB.m,> p(%x&*)f U5=J;[w}N [font="Times] ([font="Times]4)[font="Times]Queue
Ccmbdw,Z5 [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。
na)ceN2h [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:
mQJ GKh&Pk dGjvSK<1@ K2Zy6lGOZ 实践:[font="Times] Set举例
I*"]!z1 [font="Times] import java.util.*;
qsx1:Ny1 [font="Times]public class SetExample {
ktRdf6:~ [font="Times] public static void main(String[] args) {
R)z|("%ec [font="Times] Set set = new HashSet(); //HashSet是[font="Times]Set的子接口
3iTjM>+> [font="Times] set.add("one");
4F?1,-X [font="Times] set.add("second");
qZG >FC37 [font="Times] set.add("3rd");
]W,g>91m [font="Times] set.add(new Integer(4));
kEh# 0 [font="Times] set.add(new Float( 5.0F ));
!5-[kG& [font="Times] set.add("second");
V>Cf
8>m [font="Times] set.add(new Integer(4));
6D<A@DR9J [font="Times] System.out.println(set);
!$HWUxM;p [font="Times] }}
jL<.?HE X(9Ff=0.~ [font="Times]List举例:
KNhH4K2iP8 [font="Times] import java.util.*;
OMU#Sx!6 [font="Times]public class ListExample {
Hn)=:lI [font="Times] public static void main(String[] args) {
RZjR d [font="Times] List list = new ArrayList();
sMK/l @7 [font="Times] list.add("one");
B^{DCHu/ [font="Times] list.add("second");
sYzG_*) [font="Times] list.add("3rd");
P.0-( [font="Times] list.add(new Integer(4));
`Ii>wb [font="Times] list.add(new Float( 5.0F ));
.wywO| [font="Times] list.add("second");
>xN^#$ng} [font="Times] list.add(new Integer(4));
gUcE,L [font="Times] System.out.println(list);
sh1fz 6g [font="Times] }}
j06DP _9M ?}.(k/ {U9jA_XX [font="Times]Map举例
Df9}YI;? [font="Times]import java.util.Map;
Bv3v;^ [font="Times]import java.util.HashMap;
"7DPsPs [font="Times]import java.util.Iterator;
[B[ J%?NS [font="Times]import java.io.FileReader;
PZ s [font="Times]
yLP0w^Q [font="Times]public class MapExample {
M<729M [font="Times] public static void main(String[] args) throws java.io.FileNotFoundException {
>*MB_m2| [font="Times] Map word_count_map = new HashMap();
6-~ [font="Times] FileReader reader = new FileReader(args[0]);
"?!IPX2\S [font="Times] Iterator words = new WordStreamIterator(reader);
b8Qm4 b?:4 [font="Times]
~oI49Q&{ [font="Times] while ( words.hasNext() ) {
G+}LLm.wX [font="Times] String word = (String) words.next();
}|d:(* [font="Times] String word_lowercase = word.toLowerCase();
v|xlI4 [font="Times] Integer frequency = (Integer)word_count_map.get(word_lowercase);
iX28+weH [font="Times]if ( frequency == null ) {
T7v8}_"- [font="Times] frequency = new Integer(1);
t65!2G"< [font="Times] } else {
\ gN) GR [font="Times] int value = frequency.intValue();
|w5#a_adM [font="Times] frequency = new Integer(value + 1);}
<}=D ?bXw [font="Times] word_count_map.put(word_lowercase, frequency);
E$T#o{pai [font="Times] }
_rM%N+$&d_ [font="Times] System.out.println(word_count_map);
fITml6mbE [font="Times] }}
Vswi /( _:z~P<%s 7]Egu D4 [font="Times]Queue举例:
! 9e>J [font="Times]import java.io.IOException;
d dPJx< [font="Times]import java.io.PrintStream;
z} %to0W [font="Times]import java.util.LinkedList;
8Xr3q eh+ [font="Times]import java.util.Queue;
qhtc?A/0} [font="Times]
)q,}jeM8 [font="Times]public class QueueTester {
sM-*[Q=_ [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
MG6Tk(3S [font="Times] public QueueTester() {q = new LinkedList();}
Qmrcng}P [font="Times]public void testFIFO(PrintStream out) throws IOException {
gOk O8P6P8 [font="Times] q.add("First");
La!PGZ{ [font="Times] q.add("Second");
p4[W@JV [font="Times] q.add("Third");
~?p
> L [font="Times] Object o;
ms$o,[ [font="Times] while ((o = q.poll()) != null) {
[Xrq+O, [font="Times] out.println(o);}}
cE3co(j [font="Times] public static void main(String[] args) {
Fri5_rxLl [font="Times] QueueTester tester = new QueueTester();
+0OQ"2^& [font="Times] try { tester.testFIFO(System.out);
GHkSU;}) [font="Times] } catch (IOException e) {
17 GyE=Uu [font="Times] e.printStackTrace(); } }}
0\mzGfd G`n-WP L=$P 上述例子和一些相关例子(共6个例子)打包下载[font="Times]
WM#!X!Vo 总结:
YP7<j*s8 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。
eZv0"FK
X 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。