全新java初学者实践教程10(java SE5.0版)
u!K1K3T6k jdk5的集合类
8* A%k1+
;2C 5GM-*Ak @ ,>-j Ztm 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。
K83'`W^ 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。
D6L+mTN [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。
Rn?JMM] FaeKDbLJr [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]
9vV==A# 3&y-xZ u] [font="Times](1) Set
AXlVH%' 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。
(u &x.J Rge>20uTl$ ([font="Times]2)[font="Times]List
wOf8\s1 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。
;dzL9P9IU KUJ Lx R,BJr y [font="Times] ([font="Times]3)[font="Times]Map
Z[nHo' [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。
p}QDX*/sSu
WwB_L.{ [OCjYC` [font="Times] ([font="Times]4)[font="Times]Queue
e{E\YEc
[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。
%D4)Bqr [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:
dL$ iTSfz" ;z4J)qw 8'*x88+ 实践:[font="Times] Set举例
z,aMbgt [font="Times] import java.util.*;
"SMJ:g", [font="Times]public class SetExample {
t$$YiO [font="Times] public static void main(String[] args) {
bny5e:= d [font="Times] Set set = new HashSet(); //HashSet是[font="Times]Set的子接口
*\XOQWrF [font="Times] set.add("one");
I;w! [font="Times] set.add("second");
B$g\;$G [font="Times] set.add("3rd");
-FJ3;fP& [font="Times] set.add(new Integer(4));
_{jC?rzb [font="Times] set.add(new Float( 5.0F ));
h^ Cm\V [font="Times] set.add("second");
{IgH0+z [font="Times] set.add(new Integer(4));
$eFMn$o [font="Times] System.out.println(set);
5REFz [font="Times] }}
j,.M!q] i M !`4 [font="Times]List举例:
#uU(G\^T [font="Times] import java.util.*;
IB;yL/T [font="Times]public class ListExample {
dy_Uh)$$|g [font="Times] public static void main(String[] args) {
;O}%SCF7 [font="Times] List list = new ArrayList();
v^JzbO~|gj [font="Times] list.add("one");
e6taQz@} [font="Times] list.add("second");
"B{3q`( [font="Times] list.add("3rd");
$BdwKk
!k [font="Times] list.add(new Integer(4));
uA#K59E+ [font="Times] list.add(new Float( 5.0F ));
^t})T*hM0 [font="Times] list.add("second");
Oo
:Dt~Ib [font="Times] list.add(new Integer(4));
d3c.lD)L9 [font="Times] System.out.println(list);
or*{P=m+R [font="Times] }}
gHPJiiCv @mCe{r*` MSmr7%g3D [font="Times]Map举例
.z gh,#= [font="Times]import java.util.Map;
)7
Mss/2T [font="Times]import java.util.HashMap;
vqN/ crJ@ [font="Times]import java.util.Iterator;
DP@1to@ [font="Times]import java.io.FileReader;
J!l/.:`6 [font="Times]
<W#G)c0 [font="Times]public class MapExample {
:Dty([ [font="Times] public static void main(String[] args) throws java.io.FileNotFoundException {
n0lOq [font="Times] Map word_count_map = new HashMap();
Ca'BE#q [font="Times] FileReader reader = new FileReader(args[0]);
44u)F@) [font="Times] Iterator words = new WordStreamIterator(reader);
Yk|6?e{+) [font="Times]
+g
g_C'" [font="Times] while ( words.hasNext() ) {
!CU-5bpu [font="Times] String word = (String) words.next();
{g.YGO [font="Times] String word_lowercase = word.toLowerCase();
YIRe__7-NU [font="Times] Integer frequency = (Integer)word_count_map.get(word_lowercase);
n}UJ-\$ [font="Times]if ( frequency == null ) {
G*4I;'6 [font="Times] frequency = new Integer(1);
c
K\
[font="Times] } else {
xeFx!$3 [font="Times] int value = frequency.intValue();
ee?
d?:L [font="Times] frequency = new Integer(value + 1);}
>8"(go+02
[font="Times] word_count_map.put(word_lowercase, frequency);
684& H8 [font="Times] }
_]zX W [font="Times] System.out.println(word_count_map);
tM]Gu?6 [font="Times] }}
0;l~B h}a}HabA s;I
@En [font="Times]Queue举例:
"<=4]Z [font="Times]import java.io.IOException;
59zWB,y(P [font="Times]import java.io.PrintStream;
a=}1`Q [font="Times]import java.util.LinkedList;
uLzE'ZmV [font="Times]import java.util.Queue;
JPZp*5c6A [font="Times]
iHhdoY[] [font="Times]public class QueueTester {
nook/ 7] [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_&Zd j,B [font="Times] public QueueTester() {q = new LinkedList();}
JH\:9B+:L [font="Times]public void testFIFO(PrintStream out) throws IOException {
Hl}lxK,] [font="Times] q.add("First");
:f[ w [font="Times] q.add("Second");
eE'P)^KV [font="Times] q.add("Third");
TBt5Nqks- [font="Times] Object o;
GM2}]9 [font="Times] while ((o = q.poll()) != null) {
![%wM Pp [font="Times] out.println(o);}}
c[ZrQJ [font="Times] public static void main(String[] args) {
[e` |< [font="Times] QueueTester tester = new QueueTester();
8n5~K.;< [font="Times] try { tester.testFIFO(System.out);
R:f!ywj% [font="Times] } catch (IOException e) {
6]iU-k0b [font="Times] e.printStackTrace(); } }}
W+a/>U #HgNwM "Vq=
Ph 上述例子和一些相关例子(共6个例子)打包下载[font="Times]
y-26\eY^P 总结:
l+6c|([ 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。
8e-nzc,] 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。