全新java初学者实践教程10(java SE5.0版)
j>70AE3[8 jdk5的集合类
=(x W7Pt~ z sZP\ $stBB u(!@6%?- 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。
J^R# 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。
L,B#%t [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。
-:|1>og &b#O=LF [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]
))qOsphN 4x'N#m{p [font="Times](1) Set
U%~L){<V[ 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。
~A5MzrvIO2 ATb[/=hP<R ([font="Times]2)[font="Times]List
lB0: 4cIj 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。
UvtSNP&/2d 9Xv>FVG! Jn>6y:s [font="Times] ([font="Times]3)[font="Times]Map
Jt3]'Nr04@ [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。
c88I"5@[bD $O/@bh1@p %;Dp~T`0 [font="Times] ([font="Times]4)[font="Times]Queue
7Q(5Nlfcz [font="Times] 在[font="Times]jdk5.0以前,通常的实现方式是使用[font="Times]java.util.List集合来模仿[font="Times]Queue。[font="Times]Queue的概念通过把对象添加(称为[font="Times]enqueuing的操作)到[font="Times]List的尾部(即[font="Times]Queue的后部)并通过从[font="Times]List的头部(即[font="Times]Queue的前部)提取对象而从[font="Times] List中移除(称为[font="Times]dequeuing的操作)来模拟。你需要执行先进先出的动作时可以直接使用[font="Times]Queue接口就可以了。
,%FBELqOW [font="Times] 这[font="Times]4个东西,有时候功能还不太完善,需要有些子类继承它的特性。[font="Times]Set的子接口有[font="Times]TreeSet,SortedSet,[font="Times]List的有[font="Times]ArrayList等,[font="Times]Map里有[font="Times]HashMap,HashTable等,[font="Times]Queue里面有[font="Times]BlockingQueue等。我们来看看例子吧:
oq<n5 &Jr~)o `2M`;$~ 5 实践:[font="Times] Set举例
)OAd[u< [font="Times] import java.util.*;
M@n9i@UsO [font="Times]public class SetExample {
AJ*FQo.U [font="Times] public static void main(String[] args) {
AIR\>.~"i* [font="Times] Set set = new HashSet(); //HashSet是[font="Times]Set的子接口
Q'ok%9q!p [font="Times] set.add("one");
xgi/,Nk ' [font="Times] set.add("second");
fA]b'8 [font="Times] set.add("3rd");
W\tSXM-Hg [font="Times] set.add(new Integer(4));
$1h , <$5H [font="Times] set.add(new Float( 5.0F ));
Y!8Ik(/~i [font="Times] set.add("second");
-2dk8]KB] [font="Times] set.add(new Integer(4));
<3;Sq~^ [font="Times] System.out.println(set);
) DzbJ} [font="Times] }}
Fj`6v"h (>E70|T [font="Times]List举例:
=psX2?%L [font="Times] import java.util.*;
Zljj [font="Times]public class ListExample {
`nxm<~-\ [font="Times] public static void main(String[] args) {
kAEm#oz=g [font="Times] List list = new ArrayList();
=3Y:DPMB [font="Times] list.add("one");
yX:*TK4 [font="Times] list.add("second");
U2DE" [font="Times] list.add("3rd");
.5',w"R [font="Times] list.add(new Integer(4));
GJL lMi [font="Times] list.add(new Float( 5.0F ));
_IA@X. )? [font="Times] list.add("second");
XL/?v"
/ [font="Times] list.add(new Integer(4));
` R;6]/I? [font="Times] System.out.println(list);
/GK1}h [font="Times] }}
*)V1Sd#m d8|bO#a%9 RE72%w(oM [font="Times]Map举例
26c,hPIeXY [font="Times]import java.util.Map;
V0,%g+.^ [font="Times]import java.util.HashMap;
, 8NY<sFh [font="Times]import java.util.Iterator;
Q.q'pJ- [font="Times]import java.io.FileReader;
ccUq!1 [font="Times]
?3Ytn+Py [font="Times]public class MapExample {
=+T$1 [font="Times] public static void main(String[] args) throws java.io.FileNotFoundException {
Qz+hS\yx [font="Times] Map word_count_map = new HashMap();
pV>M,f [font="Times] FileReader reader = new FileReader(args[0]);
s/,wyxKd [font="Times] Iterator words = new WordStreamIterator(reader);
kAF[K,GG [font="Times]
e%(,)WlTaU [font="Times] while ( words.hasNext() ) {
|z!Y,zaX [font="Times] String word = (String) words.next();
3J2j5N:g [font="Times] String word_lowercase = word.toLowerCase();
j0p'_|)( [font="Times] Integer frequency = (Integer)word_count_map.get(word_lowercase);
] Fx9!S [font="Times]if ( frequency == null ) {
1]L 0r [font="Times] frequency = new Integer(1);
C0xjM0 [font="Times] } else {
X
8V^ [font="Times] int value = frequency.intValue();
t,*hxzD" [font="Times] frequency = new Integer(value + 1);}
jXBAo [font="Times] word_count_map.put(word_lowercase, frequency);
r>=)Y32Q [font="Times] }
\;z*j|;B [font="Times] System.out.println(word_count_map);
{ XN"L3A [font="Times] }}
[>IAS> m'))prl IpX>G]"-C [font="Times]Queue举例:
VpDNp
(2 [font="Times]import java.io.IOException;
JsfX&dX0 [font="Times]import java.io.PrintStream;
<p<J;@ [font="Times]import java.util.LinkedList;
87Sqs1>cw [font="Times]import java.util.Queue;
cr{;gP [font="Times]
+ht -Bl [font="Times]public class QueueTester {
<<zYF.9L] [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
KaJCfu yp [font="Times] public QueueTester() {q = new LinkedList();}
w`kn!k8 [font="Times]public void testFIFO(PrintStream out) throws IOException {
e12.suv [font="Times] q.add("First");
yG)zrRU [font="Times] q.add("Second");
S}q6CG7 u [font="Times] q.add("Third");
^Z:oCTOP [font="Times] Object o;
28KS*5S [font="Times] while ((o = q.poll()) != null) {
a=<l}`* [font="Times] out.println(o);}}
"v
@h [font="Times] public static void main(String[] args) {
3d qj:4[f [font="Times] QueueTester tester = new QueueTester();
,k*g`OTW [font="Times] try { tester.testFIFO(System.out);
l2)) StEm [font="Times] } catch (IOException e) {
WUQlAsme [font="Times] e.printStackTrace(); } }}
[NE:$@ ~kdxJP" 5]/i[T_ 上述例子和一些相关例子(共6个例子)打包下载[font="Times]
j Y>BU& 总结:
sx ;7 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。
G@Z,Hbgm 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。