全新java初学者实践教程10(java SE5.0版)
soOfk!b jdk5的集合类
.U#oN_D P>EG;u@. cwE?+vB SveP:uJA[ 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。
%O9P|04]3 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。
gI/SA [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。
I6i qC"BK jZk dTiI [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]
!{F\\D/ W'PW;., [font="Times](1) Set
=j%ORD[ 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。
*JOp)e0b )}J}d) ([font="Times]2)[font="Times]List
TB_OFbI2 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。
=, 64Qbau pmiC|F83!8 yujv^2/ [font="Times] ([font="Times]3)[font="Times]Map
A
|P
wm` [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。
z(#CO<C.t _ xM}*_<VP Lh-+i [font="Times] ([font="Times]4)[font="Times]Queue
Tdxc%'l [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。
)`#SMLMy~ [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:
PKZMuEEy, * $|9e jA3xDbM 实践:[font="Times] Set举例
3F9 dr@I.7 [font="Times] import java.util.*;
,Vy_%f [font="Times]public class SetExample {
$\aJ.N6rb [font="Times] public static void main(String[] args) {
4|hfzCjMI [font="Times] Set set = new HashSet(); //HashSet是[font="Times]Set的子接口
7g4IAsoD [font="Times] set.add("one");
?NxaJ^ [font="Times] set.add("second");
|[@v+koq [font="Times] set.add("3rd");
0?''v>% [font="Times] set.add(new Integer(4));
:cA8[! [font="Times] set.add(new Float( 5.0F ));
Hv*+HUc(: [font="Times] set.add("second");
;73{n*a$ [font="Times] set.add(new Integer(4));
`^)oVs [font="Times] System.out.println(set);
v<ati c [font="Times] }}
nFjaV`6`@ ,_bG'Hmt [font="Times]List举例:
WS+uK b^< [font="Times] import java.util.*;
;vUw_M{P=) [font="Times]public class ListExample {
+;a\
gF^ [font="Times] public static void main(String[] args) {
c^~R%Bx [font="Times] List list = new ArrayList();
km,@yU [font="Times] list.add("one");
nu X`>Oy [font="Times] list.add("second");
UAds$9 [font="Times] list.add("3rd");
hM[I}$M&O [font="Times] list.add(new Integer(4));
1`9'.w+r [font="Times] list.add(new Float( 5.0F ));
fS4 Ru [font="Times] list.add("second");
EdCcnl?R6 [font="Times] list.add(new Integer(4));
SpMHq_MLM [font="Times] System.out.println(list);
xgIb4Y% [font="Times] }}
eMjW^-RgE5 )gG_K$08? eK/rsr [font="Times]Map举例
&ZJ$V [font="Times]import java.util.Map;
wx^1lC2 [font="Times]import java.util.HashMap;
U3p Mv|b [font="Times]import java.util.Iterator;
rs@qC>_C0 [font="Times]import java.io.FileReader;
`jT1R!$3F [font="Times]
s-S|#5 [font="Times]public class MapExample {
{'o\#4Wk [font="Times] public static void main(String[] args) throws java.io.FileNotFoundException {
H6Kt^s<6xu [font="Times] Map word_count_map = new HashMap();
Cp]q>lM" [font="Times] FileReader reader = new FileReader(args[0]);
N#^o,/ [font="Times] Iterator words = new WordStreamIterator(reader);
1ifPc5j} [font="Times]
?dvcmXR [font="Times] while ( words.hasNext() ) {
960rbxKy3 [font="Times] String word = (String) words.next();
fn.}LeeS> [font="Times] String word_lowercase = word.toLowerCase();
t7/a5x [font="Times] Integer frequency = (Integer)word_count_map.get(word_lowercase);
(y?`|=G-xT [font="Times]if ( frequency == null ) {
wTn" [font="Times] frequency = new Integer(1);
\P9HAz'6 [font="Times] } else {
*KPNWY9!W [font="Times] int value = frequency.intValue();
<< aAYkx< [font="Times] frequency = new Integer(value + 1);}
{ pu .l4nk [font="Times] word_count_map.put(word_lowercase, frequency);
{&h= [font="Times] }
@qB1:==@7 [font="Times] System.out.println(word_count_map);
gal.<SVW [font="Times] }}
ETaLE[T%1 ~ym-Szo &Fl*, [font="Times]Queue举例:
.*L_*}tno [font="Times]import java.io.IOException;
'Inqa;TQz [font="Times]import java.io.PrintStream;
88+J(^y> [font="Times]import java.util.LinkedList;
p"q4R2_/jh [font="Times]import java.util.Queue;
tH9BC5+r} [font="Times]
`BY&&Bv#? [font="Times]public class QueueTester {
&uxwz@RC0 [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
Mh5 =]O+ [font="Times] public QueueTester() {q = new LinkedList();}
y_Nn%(j [font="Times]public void testFIFO(PrintStream out) throws IOException {
+WSM<