全新java初学者实践教程10(java SE5.0版)
QE"$Lc) jdk5的集合类
vh"';L_*37 l+6@,TY1U 4J,6cOuW4 M6MxY\uM 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。
mQ}\ptdfV 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。
Eyf17 [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。
b?0WA.[{ J6EzD\.Y) [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]
hU( NM9ViYm>P [font="Times](1) Set
Rq| 5%;1 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。
RgFpc*.T M6cybEk` ([font="Times]2)[font="Times]List
n5xG4.#G 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。
anz7ae&P'K `::j\3B&Y- pvt/{ [font="Times] ([font="Times]3)[font="Times]Map
#q34>}O< O [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。
6T~+vT Kg2@]J9m ( AA@sN [font="Times] ([font="Times]4)[font="Times]Queue
xF) .S@ [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。
*]q`:~u2 [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:
lbIW1z%:sy N0lFx?4 `,pBOh|' 实践:[font="Times] Set举例
^{yb4yQ
0 [font="Times] import java.util.*;
P/~dY[6m [font="Times]public class SetExample {
5r8
[" [font="Times] public static void main(String[] args) {
G2[2y-Rv [font="Times] Set set = new HashSet(); //HashSet是[font="Times]Set的子接口
0j;|IU\ [font="Times] set.add("one");
HSG9|}$ [font="Times] set.add("second");
#F
.8x@ [font="Times] set.add("3rd");
_kOuD}_| [font="Times] set.add(new Integer(4));
i-0AcN./p [font="Times] set.add(new Float( 5.0F ));
'%Oo1:wJ [font="Times] set.add("second");
G@ybx[_[@ [font="Times] set.add(new Integer(4));
+A,cdi9z [font="Times] System.out.println(set);
z&GGa`T" [font="Times] }}
mNe908Yw 79Q,XRWh| [font="Times]List举例:
3s:)CXO [font="Times] import java.util.*;
<C"}OW8 [font="Times]public class ListExample {
gcX [font="Times] public static void main(String[] args) {
N]c:8dOj [font="Times] List list = new ArrayList();
h;K9}w [font="Times] list.add("one");
:1iXBG\ [font="Times] list.add("second");
uTbMp~cYB [font="Times] list.add("3rd");
(o6u^#6 [font="Times] list.add(new Integer(4));
W#b++}S [font="Times] list.add(new Float( 5.0F ));
mMhe,8E& [font="Times] list.add("second");
_;(QMeR [font="Times] list.add(new Integer(4));
AsZyPybq [font="Times] System.out.println(list);
a3Z()|t> [font="Times] }}
_["97>q @J@bD+Q+0 #lVSQZO~a [font="Times]Map举例
N/^[c+J[E [font="Times]import java.util.Map;
l%2B4d9"v [font="Times]import java.util.HashMap;
2(D&jL [font="Times]import java.util.Iterator;
|@-y+vbA* [font="Times]import java.io.FileReader;
Dhg/>@tw [font="Times]
; zy;M5l5. [font="Times]public class MapExample {
_x#r,1V+D [font="Times] public static void main(String[] args) throws java.io.FileNotFoundException {
i3Nt?FSN [font="Times] Map word_count_map = new HashMap();
+xmZK<{< [font="Times] FileReader reader = new FileReader(args[0]);
Git2Cet [font="Times] Iterator words = new WordStreamIterator(reader);
gAi}"}; [font="Times]
r:^`005 [font="Times] while ( words.hasNext() ) {
lgAE`Os [font="Times] String word = (String) words.next();
QQ,w:OjA0 [font="Times] String word_lowercase = word.toLowerCase();
A@k=Mk [font="Times] Integer frequency = (Integer)word_count_map.get(word_lowercase);
)^^}!U#|e [font="Times]if ( frequency == null ) {
~>$(5s2 [font="Times] frequency = new Integer(1);
ER$~kFE2yP [font="Times] } else {
kS7T'[d [font="Times] int value = frequency.intValue();
}>j1j^c1=' [font="Times] frequency = new Integer(value + 1);}
T5U(B3j_ [font="Times] word_count_map.put(word_lowercase, frequency);
H
@E-=Ly [font="Times] }
8J9o$Se [font="Times] System.out.println(word_count_map);
{24Pv#ZG#^ [font="Times] }}
'Uo:b< P#Ikj&l i%B$p0U< [font="Times]Queue举例:
tQ?}x#J [font="Times]import java.io.IOException;
e''Wm.>g(+ [font="Times]import java.io.PrintStream;
gwF@'Uu [font="Times]import java.util.LinkedList;
!lB,2_ [font="Times]import java.util.Queue;
q%^gG03. [font="Times]
)=D9L [font="Times]public class QueueTester {
Ipmr@%~ [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
==j39 [font="Times] public QueueTester() {q = new LinkedList();}
{la^useg[ [font="Times]public void testFIFO(PrintStream out) throws IOException {
:Ws3+OI'm3 [font="Times] q.add("First");
8c$IsvJg [font="Times] q.add("Second");
&l|B>{4v [font="Times] q.add("Third");
r>q`# ~ [font="Times] Object o;
8i"{GGVC [font="Times] while ((o = q.poll()) != null) {
{gi"ktgk [font="Times] out.println(o);}}
1Kebl [font="Times] public static void main(String[] args) {
veE8
N~0N. [font="Times] QueueTester tester = new QueueTester();
kp;MNRc [font="Times] try { tester.testFIFO(System.out);
Z#W`0G>' [font="Times] } catch (IOException e) {
L,X6L @Q [font="Times] e.printStackTrace(); } }}
9k"nx ," #wm)e)2@ bmddh2 上述例子和一些相关例子(共6个例子)打包下载[font="Times]
]X _& 总结:
j({L6</x 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。
Ap> n4~ 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。