全新java初学者实践教程10(java SE5.0版)
@8aV*zjB jdk5的集合类
dWi.V?K4z g3Hi5[-H W >}T$a}\ g`.H)36 上次课我们学过了数组,知道它只是一组数(或是对象),但是有些自己的特性。在[font="Times]java里还有一类东西与数组类似,也是有着特性的一组数[font="Times](或是对象[font="Times]),叫做集合类。
~ oq.y n/1 我们上节课讲到了,数组的长度在创建时已经确定了,但是有时候我们事先根本不知道长度是多少啊,比如我们做电子商务网站时,有个购物车程序。你总不能用数组规定,人家只能买[font="Times]5样东西吧。你就是把长度定为[font="Times]10000也不行,万一遇上个特别有钱的呢!呵呵,这只是开玩笑的。我们会使用集合类解决这个问题。
q@VIFmqY! [font="Times] 集合类是放在[font="Times]java.util.*;这个包里。集合类存放的都是对象的引用,而非对象本身,为了说起来方便些,我们称集合中的对象就是指集合中对象的引用([font="Times]reference)。引用的概念大家不会忘了吧,在前边我们讲
数据类型时讲的。
E[zq<&P@ saQo]6# [font="Times] 集合类型主要有[font="Times]3种:[font="Times]set(集)、[font="Times]list(列表)、[font="Times]map(映射[font="Times])和[font="Times]Queue(队列)。[font="Times]//队列为[font="Times]jdk5中的加上的[font="Times]
oOubqx Z0'LD< [font="Times](1) Set
mF4OLG3L0 集([font="Times]set)是最简单的一种集合,它的对象不按特定方式排序,只是简单的把对象加入集合中,就像往口袋里放东西。对集中成员的访问和操作是通过集中对象的引用进行的,所以集中不能有重复对象。我们知道数学上的集合也是[font="Times]Set这个,集合里面一定是没有重复的元素的。
)$a6l8
E KN<KnU% ([font="Times]2)[font="Times]List
55] MRv 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。
u WdKG({][ cG@Wo8+ kJNg>SN*@# [font="Times] ([font="Times]3)[font="Times]Map
usoyH0t!? [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。
qx*b\6Rt [0kZyjCq@ QG
L~?? [font="Times] ([font="Times]4)[font="Times]Queue
<m{#u4FC' [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。
gXn`! [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:
gQu!(7WLI X>o*eN Ky8,HdAq 实践:[font="Times] Set举例
$/(``8li_ [font="Times] import java.util.*;
[(TmAEON [font="Times]public class SetExample {
I4UsDs*BD [font="Times] public static void main(String[] args) {
d>#X+;-k [font="Times] Set set = new HashSet(); //HashSet是[font="Times]Set的子接口
s,/C^E [font="Times] set.add("one");
;<+Z}d/g9 [font="Times] set.add("second");
4 R8Qn^ [font="Times] set.add("3rd");
Ic&YiATj [font="Times] set.add(new Integer(4));
H+gB| [font="Times] set.add(new Float( 5.0F ));
T-7(3#& [font="Times] set.add("second");
k{lX K\zN [font="Times] set.add(new Integer(4));
3KkJQ5a [font="Times] System.out.println(set);
R `ob;>[Q [font="Times] }}
/S^>06{-+ ^HTvw~]5 [font="Times]List举例:
|m*l/@1 [font="Times] import java.util.*;
$DnJ/hg;qD [font="Times]public class ListExample {
%X%f0J [font="Times] public static void main(String[] args) {
yw7bIcs|#b [font="Times] List list = new ArrayList();
meThjCC [font="Times] list.add("one");
Z
R~2Y?Wt9 [font="Times] list.add("second");
%=s2>vv9 [font="Times] list.add("3rd");
[x`),3qD [font="Times] list.add(new Integer(4));
E0!0 uSg& [font="Times] list.add(new Float( 5.0F ));
V}Q`dEk2r [font="Times] list.add("second");
9j-;-`$S [font="Times] list.add(new Integer(4));
W4( [font="Times] System.out.println(list);
HB.:/5\ [font="Times] }}
-sDl[ YT)1_>*\ Su
+<mW [font="Times]Map举例
NQiu>Sg [font="Times]import java.util.Map;
U!BZsVx [font="Times]import java.util.HashMap;
,LLx&jS [font="Times]import java.util.Iterator;
&Akw V- [font="Times]import java.io.FileReader;
jSdC1,wR [font="Times]
*Csxf[O [font="Times]public class MapExample {
WigTNg4 [font="Times] public static void main(String[] args) throws java.io.FileNotFoundException {
2sEG#/Y= [font="Times] Map word_count_map = new HashMap();
t)O]0)
s [font="Times] FileReader reader = new FileReader(args[0]);
'b >3:& [font="Times] Iterator words = new WordStreamIterator(reader);
h{jm [font="Times]
o{hZjn- [font="Times] while ( words.hasNext() ) {
3(*vZ [font="Times] String word = (String) words.next();
i_`Po% [font="Times] String word_lowercase = word.toLowerCase();
7<2^8` [font="Times] Integer frequency = (Integer)word_count_map.get(word_lowercase);
F`Z?$ 1 [font="Times]if ( frequency == null ) {
*lheF>^ [font="Times] frequency = new Integer(1);
NNJQDkO-I [font="Times] } else {
{D,-
Whi [font="Times] int value = frequency.intValue();
q"f7$ [font="Times] frequency = new Integer(value + 1);}
$t5>1G1j7 [font="Times] word_count_map.put(word_lowercase, frequency);
m>!o
Yy_ [font="Times] }
:r:x|[3. [font="Times] System.out.println(word_count_map);
~0Zy$L/D [font="Times] }}
N!\1O, EVLDP\w{ *rV{(%\m [font="Times]Queue举例:
lX50JJwk [font="Times]import java.io.IOException;
7(o:J [font="Times]import java.io.PrintStream;
Gu2=+?i?h [font="Times]import java.util.LinkedList;
2J3y
1 [font="Times]import java.util.Queue;
=4!m]*y [font="Times]
^0I" [font="Times]public class QueueTester {
Gsm.a [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
u:wf:^ [font="Times] public QueueTester() {q = new LinkedList();}
<<@F{B7h [font="Times]public void testFIFO(PrintStream out) throws IOException {
Ca/N'|}^ [font="Times] q.add("First");
]4lC/&nm [font="Times] q.add("Second");
{9Q**U`w [font="Times] q.add("Third");
DoICf1 [font="Times] Object o;
[8acan+
2l [font="Times] while ((o = q.poll()) != null) {
#??[;xjs! [font="Times] out.println(o);}}
T7Ju7_q} [font="Times] public static void main(String[] args) {
~eiD(04^r* [font="Times] QueueTester tester = new QueueTester();
e_S,N0 [font="Times] try { tester.testFIFO(System.out);
(8N E'd8 [font="Times] } catch (IOException e) {
<Y;w
I#C [font="Times] e.printStackTrace(); } }}
I-Hg6WtB ;1r|Bx <5 }%y5<n*v\ 上述例子和一些相关例子(共6个例子)打包下载[font="Times]
nLrCy5R: 总结:
&$~irI 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。
yi -0CHo 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。