全新java初学者实践教程10(java SE5.0版)
flLC\ jdk5的集合类
n\ma5"n0=\ F,e_ ` O;:8mm%( %f@VOSs 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。
C/[2?[ 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。
OZ_'&CZ [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。
~R) Km`t S&V5zB""n [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]
}d)>pH Z\{WBUR;4t [font="Times](1) Set
^n<p#0)+a 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。
];1z%. <9/oqp{C4 ([font="Times]2)[font="Times]List
h2KXW}y"4 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。
6kjBd3 |J`YFv 3;j?i<kM [font="Times] ([font="Times]3)[font="Times]Map
}_M.-Xm [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。
/PIU@$DV A"C%.InZ JPiC/ [font="Times] ([font="Times]4)[font="Times]Queue
'&3Sl?E [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。
yaA9*k [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:
5in6Y5c kj wLU w'Ai $;+B)# 实践:[font="Times] Set举例
q[b-vTzI [font="Times] import java.util.*;
slHlfWHq [font="Times]public class SetExample {
i<1w*yu [font="Times] public static void main(String[] args) {
T{|'<KT [font="Times] Set set = new HashSet(); //HashSet是[font="Times]Set的子接口
P,~a'_w:|D [font="Times] set.add("one");
qEf)TW( [font="Times] set.add("second");
PF!Q2t5c3 [font="Times] set.add("3rd");
f b_tda",} [font="Times] set.add(new Integer(4));
s iv
KXd [font="Times] set.add(new Float( 5.0F ));
.$4DK* [font="Times] set.add("second");
5<a)SP 0 [font="Times] set.add(new Integer(4));
J
C1T033 r [font="Times] System.out.println(set);
tB[(o%k [font="Times] }}
d+ih]? !?ayZ5G([ [font="Times]List举例:
!HqIi@>8 [font="Times] import java.util.*;
,US~p_M! [font="Times]public class ListExample {
f<WnPoV [font="Times] public static void main(String[] args) {
rP;Fh|w# [font="Times] List list = new ArrayList();
3T Q#3h [font="Times] list.add("one");
,vW.vq<{q3 [font="Times] list.add("second");
; ZL<7tLDb [font="Times] list.add("3rd");
=}r&>|rrJ [font="Times] list.add(new Integer(4));
QKZm<lUL [font="Times] list.add(new Float( 5.0F ));
[gzw<b:` [font="Times] list.add("second");
Q_6./.GQ
[font="Times] list.add(new Integer(4));
Gr?"okaA [font="Times] System.out.println(list);
C3bZ3vcW$ [font="Times] }}
?GD{}f33 ozkN&0 rgIJ]vmy<H [font="Times]Map举例
J}`K&DtM9 [font="Times]import java.util.Map;
9T|7edl [font="Times]import java.util.HashMap;
D/{Tl [font="Times]import java.util.Iterator;
o|l)oc6{ [font="Times]import java.io.FileReader;
n1uJQt [font="Times]
v2EM| Q xp [font="Times]public class MapExample {
w>H!H6Q [font="Times] public static void main(String[] args) throws java.io.FileNotFoundException {
\fU{$ [font="Times] Map word_count_map = new HashMap();
x7Ly, [font="Times] FileReader reader = new FileReader(args[0]);
zmf5!77 [font="Times] Iterator words = new WordStreamIterator(reader);
A>OL5TCl [font="Times]
xJ>hN@5}i [font="Times] while ( words.hasNext() ) {
c2?(.UV [font="Times] String word = (String) words.next();
;csAhkf:S [font="Times] String word_lowercase = word.toLowerCase();
xYM/{[ [font="Times] Integer frequency = (Integer)word_count_map.get(word_lowercase);
^lRXc.c z [font="Times]if ( frequency == null ) {
x}N+vK [font="Times] frequency = new Integer(1);
fPK|Nw]b [font="Times] } else {
&!/L^Y*+ [font="Times] int value = frequency.intValue();
Ax0u \(p<^ [font="Times] frequency = new Integer(value + 1);}
qg:1 [font="Times] word_count_map.put(word_lowercase, frequency);
N_q7ip%z [font="Times] }
pR 1 v^m| [font="Times] System.out.println(word_count_map);
Wz:MPdz3( [font="Times] }}
k%NY,(:( }%$9nq3 IOTHk+w [font="Times]Queue举例:
M29[\@zL [font="Times]import java.io.IOException;
1.yw\ZC\ [font="Times]import java.io.PrintStream;
$hn_4$ [font="Times]import java.util.LinkedList;
!&SUoa [font="Times]import java.util.Queue;
<B$Lu4b@c [font="Times]
9S&6u1 [font="Times]public class QueueTester {
Mk|h ><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
'$1-A%e$1 [font="Times] public QueueTester() {q = new LinkedList();}
F2oY_mA [font="Times]public void testFIFO(PrintStream out) throws IOException {
&E {/s [font="Times] q.add("First");
6$)Yqg`X [font="Times] q.add("Second");
L V33vy [font="Times] q.add("Third");
W|D'S}J [font="Times] Object o;
g6QkF41nG [font="Times] while ((o = q.poll()) != null) {
Gu*;z% b2 [font="Times] out.println(o);}}
faD(,H [font="Times] public static void main(String[] args) {
nsw.\(# [font="Times] QueueTester tester = new QueueTester();
79:x>i= [font="Times] try { tester.testFIFO(System.out);
T"9`[Lzva [font="Times] } catch (IOException e) {
&ks>.l\ [font="Times] e.printStackTrace(); } }}
b4ORDU \^N9Q9{7]
6=A++H@ 上述例子和一些相关例子(共6个例子)打包下载[font="Times]
0\o0(eHCQz 总结:
@WBy:gV" 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。
UTin0k 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。