全新java初学者实践教程10(java SE5.0版)
R|*Eg,1g - jdk5的集合类
g-/ }*ml , $cpm=1 %T}*DC$&S oC3W_vH.% 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。
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S,E 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。
2f rwU~y [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。
y (%y'xBP 4 *.
O% [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]
P_.AqEH 0R-W9qP [font="Times](1) Set
7H,)heA 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。
< 7*9b ;2gO( ([font="Times]2)[font="Times]List
"_+8z_ 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。
p$Floubh] +'[/eW F84<='K [font="Times] ([font="Times]3)[font="Times]Map
{?}^HW9{ [font="Times] 映射([font="Times]Map),这个在[font="Times]java里不是地图的意思,其实地图也是映射哈。它里面的东西是键-值对([font="Times]key-value)出现的,键值对是什么呢?举个例子,比如我们查字典,用部首查字法。目录那个字就是键,这个字的解释就是值。键和值成对出现。这样说可以理解吧。这也是很常用的数据结构哦。
5'|W(yR} ;[:IC^9fv .k,,PuP [font="Times] ([font="Times]4)[font="Times]Queue
/%bnG(4 [font="Times] 在[font="Times]jdk5.0以前,通常的实现方式是使用[font="Times]java.util.List集合来模仿[font="Times]Queue。[font="Times]Queue的概念通过把对象添加(称为[font="Times]enqueuing的操作)到[font="Times]List的尾部(即[font="Times]Queue的后部)并通过从[font="Times]List的头部(即[font="Times]Queue的前部)提取对象而从[font="Times] List中移除(称为[font="Times]dequeuing的操作)来模拟。你需要执行先进先出的动作时可以直接使用[font="Times]Queue接口就可以了。
B~YOU3 [font="Times] 这[font="Times]4个东西,有时候功能还不太完善,需要有些子类继承它的特性。[font="Times]Set的子接口有[font="Times]TreeSet,SortedSet,[font="Times]List的有[font="Times]ArrayList等,[font="Times]Map里有[font="Times]HashMap,HashTable等,[font="Times]Queue里面有[font="Times]BlockingQueue等。我们来看看例子吧:
8/"C0I (G qtz~Y~h|> q0nIJ( 实践:[font="Times] Set举例
}Am5b@g"$Y [font="Times] import java.util.*;
'sa>G [font="Times]public class SetExample {
c?Mbyay [font="Times] public static void main(String[] args) {
+u`4@~D# [font="Times] Set set = new HashSet(); //HashSet是[font="Times]Set的子接口
NBw{ [font="Times] set.add("one");
n8z++T& [font="Times] set.add("second");
?*~sx=mC [font="Times] set.add("3rd");
IJ7wUZp" [font="Times] set.add(new Integer(4));
{}H/N [font="Times] set.add(new Float( 5.0F ));
>H,E3Z [font="Times] set.add("second");
^h2!u'IQ [font="Times] set.add(new Integer(4));
c1
j@*6B [font="Times] System.out.println(set);
G4\|bwh [font="Times] }}
`r %lB _9<Mo;C [font="Times]List举例:
ehZ/J5 [font="Times] import java.util.*;
vPrlRG6 [font="Times]public class ListExample {
BxaGBK<k [font="Times] public static void main(String[] args) {
4K|O?MUNS [font="Times] List list = new ArrayList();
\GZ|fmYn [font="Times] list.add("one");
xiqeKoAD [font="Times] list.add("second");
T sdgg?# [font="Times] list.add("3rd");
Dnd [font="Times] list.add(new Integer(4));
MieO1l [font="Times] list.add(new Float( 5.0F ));
D&OskM60 [font="Times] list.add("second");
({cWb:+r [font="Times] list.add(new Integer(4));
D"IxQ2}k [font="Times] System.out.println(list);
)OK"H^}f [font="Times] }}
TQQh:y _SMi`ie# ^-"tK:{ [font="Times]Map举例
r,:acK [font="Times]import java.util.Map;
ONFx -U] [font="Times]import java.util.HashMap;
*qFl&*h} [font="Times]import java.util.Iterator;
#S[Y}-]T [font="Times]import java.io.FileReader;
UQbk%K2 [font="Times]
x4v&%d=M [font="Times]public class MapExample {
!iK{q0 [font="Times] public static void main(String[] args) throws java.io.FileNotFoundException {
CXTtN9N9 [font="Times] Map word_count_map = new HashMap();
('o}EoXS [font="Times] FileReader reader = new FileReader(args[0]);
jI9#OEH_g [font="Times] Iterator words = new WordStreamIterator(reader);
&%u m#XE [font="Times]
C)QKodI [font="Times] while ( words.hasNext() ) {
&
s:\tL [font="Times] String word = (String) words.next();
28 zZ3|Z3 [font="Times] String word_lowercase = word.toLowerCase();
~JP3C5q [font="Times] Integer frequency = (Integer)word_count_map.get(word_lowercase);
*]!rT&E [font="Times]if ( frequency == null ) {
.fS{j$ [font="Times] frequency = new Integer(1);
q(v|@l|)yO [font="Times] } else {
bEmzigN[ [font="Times] int value = frequency.intValue();
zT93Sb [font="Times] frequency = new Integer(value + 1);}
U(%6ny [font="Times] word_count_map.put(word_lowercase, frequency);
J'yCVb)V [font="Times] }
0:c3aq&u [font="Times] System.out.println(word_count_map);
gLK0L%"5 [font="Times] }}
0-/@-qV\ B[t>T>~ #+$PD`j [font="Times]Queue举例:
46~nwi$,^ [font="Times]import java.io.IOException;
-C* 6>$A [font="Times]import java.io.PrintStream;
uavyms^ [font="Times]import java.util.LinkedList;
{`(MK6D8 c [font="Times]import java.util.Queue;
S>jOVWB [font="Times]
E%a&6W [font="Times]public class QueueTester {
Z/ L%?zH [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
K#VGG,h7Y [font="Times] public QueueTester() {q = new LinkedList();}
OLoo#HW [font="Times]public void testFIFO(PrintStream out) throws IOException {
p[)yn%uh [font="Times] q.add("First");
:SY,;..3e [font="Times] q.add("Second");
^)h&s* [font="Times] q.add("Third");
+{#Z^y6& [font="Times] Object o;
i`?yi-R& [font="Times] while ((o = q.poll()) != null) {
\[%_ :9eq [font="Times] out.println(o);}}
_joW%`T8 [font="Times] public static void main(String[] args) {
Y=y
0`?K [font="Times] QueueTester tester = new QueueTester();
.:e#!~Ki [font="Times] try { tester.testFIFO(System.out);
hf;S#.k [font="Times] } catch (IOException e) {
+RnWeBXAT [font="Times] e.printStackTrace(); } }}
XJk~bgO* _,igN> Xe(]4Ux 上述例子和一些相关例子(共6个例子)打包下载[font="Times]
nswhYSX 总结:
Bj\Us$cZ 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。
b`f6(6 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。