全新java初学者实践教程10(java SE5.0版)
@d 7V@F0d jdk5的集合类
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pp< #m?)XB^_ 4E=v)C' J9]cs?`) 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。
_[Imwu} 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。
_]4p51r0 [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。
! {G0' -XV+F@`Md [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]
Q]N&^ E _\tv ${ [font="Times](1) Set
t $Rc
0 集([font="Times]set)是最简单的一种集合,它的对象不按特定方式排序,只是简单的把对象加入集合中,就像往口袋里放东西。对集中成员的访问和操作是通过集中对象的引用进行的,所以集中不能有重复对象。我们知道数学上的集合也是[font="Times]Set这个,集合里面一定是没有重复的元素的。
t4v'X}7q] zEW+1-=)+7 ([font="Times]2)[font="Times]List
[yQ%g;m 列表([font="Times]List)的主要特征是其对象以线性方式存储,没有特定顺序,只有一个开头和一个结尾,当然,它与根本没有顺序的[font="Times]Set是不同的。它是链表嘛,一条链肯定有顺序这个顺序就不一定了。
[NO4Wzc \#dacQ2E@ ^ `Ozw^~ [font="Times] ([font="Times]3)[font="Times]Map
C7NSmZ [font="Times] 映射([font="Times]Map),这个在[font="Times]java里不是地图的意思,其实地图也是映射哈。它里面的东西是键-值对([font="Times]key-value)出现的,键值对是什么呢?举个例子,比如我们查字典,用部首查字法。目录那个字就是键,这个字的解释就是值。键和值成对出现。这样说可以理解吧。这也是很常用的数据结构哦。
n^ fUKi*; I(pq3_9$ 2m yxwA5 [font="Times] ([font="Times]4)[font="Times]Queue
%ZWt 45A [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。
C2v_],] [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:
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Fl=H5HR +c<iVc| 实践:[font="Times] Set举例
'0q$qN [font="Times] import java.util.*;
b1 w@toc [font="Times]public class SetExample {
,jis@]: [font="Times] public static void main(String[] args) {
T&]-p:mg^ [font="Times] Set set = new HashSet(); //HashSet是[font="Times]Set的子接口
ur'<8pDb$ [font="Times] set.add("one");
0@cIj
] [font="Times] set.add("second");
b}0h()v [font="Times] set.add("3rd");
H2R3I<j [font="Times] set.add(new Integer(4));
"*t6KXVaM [font="Times] set.add(new Float( 5.0F ));
P%e7c, [font="Times] set.add("second");
EyPJ Jc8 [font="Times] set.add(new Integer(4));
/CsP@f_Gw [font="Times] System.out.println(set);
YQG
l8E' [font="Times] }}
H"AL@= >bZ-mX)j\0 [font="Times]List举例:
Dbn344s [font="Times] import java.util.*;
k"i3$^v8 [font="Times]public class ListExample {
!mUJ["# [font="Times] public static void main(String[] args) {
m-
<y|3 [font="Times] List list = new ArrayList();
Io3-\Ff [font="Times] list.add("one");
23^>#b7st [font="Times] list.add("second");
pPRX#3 [font="Times] list.add("3rd");
|@JTSz*Or [font="Times] list.add(new Integer(4));
uM[|>t [font="Times] list.add(new Float( 5.0F ));
vNs%e/~vj [font="Times] list.add("second");
Zc~7R`v7} [font="Times] list.add(new Integer(4));
WC~;t4 [font="Times] System.out.println(list);
*GE6zGdN [font="Times] }}
K)LoZ^x0) o1(;"5MM Y%"$v0D [font="Times]Map举例
6;'[v}O^^ [font="Times]import java.util.Map;
+-|""`I1I [font="Times]import java.util.HashMap;
rueaP [font="Times]import java.util.Iterator;
AEyD?^? [font="Times]import java.io.FileReader;
Zwc&4:5% [font="Times]
h76j|1gI [font="Times]public class MapExample {
%@a;q?/?Nd [font="Times] public static void main(String[] args) throws java.io.FileNotFoundException {
f~mwDkf?L [font="Times] Map word_count_map = new HashMap();
-6- sI [font="Times] FileReader reader = new FileReader(args[0]);
G=17]>U [font="Times] Iterator words = new WordStreamIterator(reader);
UDi(7c0. [font="Times]
,nteIR'?? [font="Times] while ( words.hasNext() ) {
@9 S :: [font="Times] String word = (String) words.next();
$mGzJ4& [font="Times] String word_lowercase = word.toLowerCase();
(]'wQ4iQ [font="Times] Integer frequency = (Integer)word_count_map.get(word_lowercase);
PM[W7gT [font="Times]if ( frequency == null ) {
QZvQ8 [font="Times] frequency = new Integer(1);
fW2NYQP$: [font="Times] } else {
ek]JzD~w$ [font="Times] int value = frequency.intValue();
]}g\te [font="Times] frequency = new Integer(value + 1);}
wL),/i&< [font="Times] word_count_map.put(word_lowercase, frequency);
S,:!H@~B [font="Times] }
i`F5 [font="Times] System.out.println(word_count_map);
[IOI&`?D [font="Times] }}
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<<d # d {U%q
d [font="Times]Queue举例:
S&UP;oc [font="Times]import java.io.IOException;
=_k [font="Times]import java.io.PrintStream;
3 twA5)v [font="Times]import java.util.LinkedList;
zy$jTqDH [font="Times]import java.util.Queue;
~>>_`;B [font="Times]
),N,!15j, [font="Times]public class QueueTester {
fphCQO^#vW [font="Times] public Queue q; //发现了一个奇怪的语法,这个尖括号是泛型声明
Iz+%wAZ|B6 [font="Times] public QueueTester() {q = new LinkedList();}
Wd;t(5Xl [font="Times]public void testFIFO(PrintStream out) throws IOException {
N:U}b1$L6 [font="Times] q.add("First");
Cty{ [font="Times] q.add("Second");
72~L ? [font="Times] q.add("Third");
c+3(|k-M [font="Times] Object o;
Fj<*!J$, [font="Times] while ((o = q.poll()) != null) {
.S,E= [font="Times] out.println(o);}}
*RS/`a;, [font="Times] public static void main(String[] args) {
ldTXW(^j [font="Times] QueueTester tester = new QueueTester();
f{VV U/$ [font="Times] try { tester.testFIFO(System.out);
%|H]T]s [font="Times] } catch (IOException e) {
y_L8i[ [font="Times] e.printStackTrace(); } }}
hev;M)t &Pme4IHtm YNV,
dKB 上述例子和一些相关例子(共6个例子)打包下载[font="Times]
BmpAH}%T 总结:
Mu/(Xp6 2 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。
Ld.9.d] 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。