全新java初学者实践教程10(java SE5.0版)
w}QU;rl8q jdk5的集合类
;Quk%6;[N y@Ga9bI7 YumHECej hj-#pL-t 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。
3SWO_ 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。
}Dc?Emb [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。
;AK@Kb R4g% $} [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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*U`_ [font="Times](1) Set
#1` lJ 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。
TYh_uox6 6(.]TEu0 ([font="Times]2)[font="Times]List
G8voqP 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。
3a]Omuu|= ZU-vZD> N|L Ey [font="Times] ([font="Times]3)[font="Times]Map
vL:tuEE3 [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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d365{ )'gO?cN [font="Times] ([font="Times]4)[font="Times]Queue
C'jE'B5b [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。
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E56J-*}x [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:
Oe*+pReSD 2OJ=Xb1 Epf[8La 实践:[font="Times] Set举例
P~"`Og+ [font="Times] import java.util.*;
Yhkn(k2 [font="Times]public class SetExample {
L[FNr& [font="Times] public static void main(String[] args) {
c|^#v8x^/ [font="Times] Set set = new HashSet(); //HashSet是[font="Times]Set的子接口
H+ Y+8 [font="Times] set.add("one");
VY=c_Gl [font="Times] set.add("second");
g<r'f"^ [font="Times] set.add("3rd");
F(Iq8DV [font="Times] set.add(new Integer(4));
r% ]^( [font="Times] set.add(new Float( 5.0F ));
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" [font="Times] set.add("second");
;/phZ$l [font="Times] set.add(new Integer(4));
H6PS7g" [font="Times] System.out.println(set);
BVpRkUC" [font="Times] }}
L=wg"$ hhVyz{u [font="Times]List举例:
m;"i4! [font="Times] import java.util.*;
=9ISsI\Y6 [font="Times]public class ListExample {
_U`_;=( [font="Times] public static void main(String[] args) {
3Vj,O?(Z [font="Times] List list = new ArrayList();
h (`Erb [font="Times] list.add("one");
{|~22UkF[V [font="Times] list.add("second");
z"C+r'39d= [font="Times] list.add("3rd");
$|yO
mh [font="Times] list.add(new Integer(4));
tzeS D C [font="Times] list.add(new Float( 5.0F ));
aN5w [font="Times] list.add("second");
b8@gv OB [font="Times] list.add(new Integer(4));
S%7^7MSqA [font="Times] System.out.println(list);
BiUOjQC# [font="Times] }}
.v3~2r*& YQI&8~z T]%:+_,
[font="Times]Map举例
MQv2C@K9F [font="Times]import java.util.Map;
Ux Yb[Nbc [font="Times]import java.util.HashMap;
M)oy3y^& [font="Times]import java.util.Iterator;
62X;gb [font="Times]import java.io.FileReader;
ag$mc8-p[ [font="Times]
6(`Bl$M9 [font="Times]public class MapExample {
dm&vLQVS [font="Times] public static void main(String[] args) throws java.io.FileNotFoundException {
7]~65@%R-& [font="Times] Map word_count_map = new HashMap();
)"IBw0] [font="Times] FileReader reader = new FileReader(args[0]);
qdFYf/y [font="Times] Iterator words = new WordStreamIterator(reader);
)NwIEk>Tf [font="Times]
|hprk-R*OH [font="Times] while ( words.hasNext() ) {
k2xOu9ncEj [font="Times] String word = (String) words.next();
3UBg"1IC [font="Times] String word_lowercase = word.toLowerCase();
{T]^C [font="Times] Integer frequency = (Integer)word_count_map.get(word_lowercase);
6-@
X [font="Times]if ( frequency == null ) {
Y!6,ty' [font="Times] frequency = new Integer(1);
]~SOGAFW [font="Times] } else {
}!-K)j. [font="Times] int value = frequency.intValue();
C>vp
oCA [font="Times] frequency = new Integer(value + 1);}
9*+%Qt,{B [font="Times] word_count_map.put(word_lowercase, frequency);
V+mTo^ [font="Times] }
JZ5NQ)sX [font="Times] System.out.println(word_count_map);
"@JSF [font="Times] }}
X~O2!F xsq+RBJi F~cvob{ [font="Times]Queue举例:
:Z@!*F [font="Times]import java.io.IOException;
S;vE% [font="Times]import java.io.PrintStream;
Z[DiLXHL [font="Times]import java.util.LinkedList;
{ L(Q|bB [font="Times]import java.util.Queue;
G6N$^HkW? [font="Times]
,h'q}5 [font="Times]public class QueueTester {
XujVOf [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
YJlpP0;++ [font="Times] public QueueTester() {q = new LinkedList();}
(YbRYu [font="Times]public void testFIFO(PrintStream out) throws IOException {
S[bFS7[ [font="Times] q.add("First");
j#TtY|Po [font="Times] q.add("Second");
'Lw4jq [font="Times] q.add("Third");
z@nJ-*'U8 [font="Times] Object o;
pm-SDp>s [font="Times] while ((o = q.poll()) != null) {
tkFGGc}w\ [font="Times] out.println(o);}}
H
h4G3h0 [font="Times] public static void main(String[] args) {
F]hKi`@ [font="Times] QueueTester tester = new QueueTester();
s:j"8ZH [font="Times] try { tester.testFIFO(System.out);
\#.@*?fk [font="Times] } catch (IOException e) {
9}{i8
<$= [font="Times] e.printStackTrace(); } }}
|Go?A/' qFo'"z`84 5V5E,2+
0 上述例子和一些相关例子(共6个例子)打包下载[font="Times]
,haCZH{ 总结:
E\V-<]o 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。
(~~*PT- 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。