全新java初学者实践教程10(java SE5.0版)
':4<[Vk jdk5的集合类
Pw<?Dw]m ]%6%rq%9C x *I'Ar 0(y*EJA$ 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。
U7x 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。
d%P2V>P [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。
FSQB{9,H \|Af26 [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]
.z,-ThTH@\ ElW\;C:K* [font="Times](1) Set
L>14=Pr^( 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。
Z2]0brV mKe6rEUs| ([font="Times]2)[font="Times]List
=T[P 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。
daKZ*B| K]m#~J3d> s=jmvvs_V} [font="Times] ([font="Times]3)[font="Times]Map
[}4zqY{ [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。
#g6 _)B=S H2jypVs$2 X
<xM ' [font="Times] ([font="Times]4)[font="Times]Queue
%0-oZL [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。
;
pBLmm*F [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:
u;t<rEC2 1Gr^,Ry -KGJr 实践:[font="Times] Set举例
F `:Q [font="Times] import java.util.*;
bra2xHK@ [font="Times]public class SetExample {
wMCMrv: [font="Times] public static void main(String[] args) {
t`JT [font="Times] Set set = new HashSet(); //HashSet是[font="Times]Set的子接口
$JOz7j( [font="Times] set.add("one");
3 ~P$p< [font="Times] set.add("second");
.@&FJYkLYi [font="Times] set.add("3rd");
Wmd@%K [font="Times] set.add(new Integer(4));
nr]=O`Mvh [font="Times] set.add(new Float( 5.0F ));
%_E5B6xi{ [font="Times] set.add("second");
66?`7j X [font="Times] set.add(new Integer(4));
ELwXp|L [font="Times] System.out.println(set);
_K#7#qp2 [font="Times] }}
K7&]|^M9 HHx:s2G [font="Times]List举例:
6h/!,j0:t_ [font="Times] import java.util.*;
^ZsIQ4 @` [font="Times]public class ListExample {
F[\T'{ [font="Times] public static void main(String[] args) {
t_Eivm-,B [font="Times] List list = new ArrayList();
js"Yh [font="Times] list.add("one");
J0IKI,X. [font="Times] list.add("second");
_W(xO
|,M [font="Times] list.add("3rd");
Nt8"6k_ [font="Times] list.add(new Integer(4));
\*CXXp` [font="Times] list.add(new Float( 5.0F ));
c_qox [font="Times] list.add("second");
)$^xbC#j`3 [font="Times] list.add(new Integer(4));
3/vtx9D [font="Times] System.out.println(list);
\/1~5mQ+ [font="Times] }}
2tK~]0x l^R:W#*+U &;ddnxFI
[font="Times]Map举例
-J63'bb7oi [font="Times]import java.util.Map;
'n7|fjX?Y [font="Times]import java.util.HashMap;
BPkMw'a: [font="Times]import java.util.Iterator;
s&ox%L4 [font="Times]import java.io.FileReader;
&G%AQpDW5 [font="Times]
i}LQ}35@ [font="Times]public class MapExample {
qE2<vjRg [font="Times] public static void main(String[] args) throws java.io.FileNotFoundException {
&k) +]r [font="Times] Map word_count_map = new HashMap();
3)VO{Cj! [font="Times] FileReader reader = new FileReader(args[0]);
-aJ(-Np$f [font="Times] Iterator words = new WordStreamIterator(reader);
49E|
f
^q [font="Times]
{@KLN< [font="Times] while ( words.hasNext() ) {
ruagJS)+ [font="Times] String word = (String) words.next();
kVtP~ [font="Times] String word_lowercase = word.toLowerCase();
*P
*.'XM [font="Times] Integer frequency = (Integer)word_count_map.get(word_lowercase);
:c]y/lQmV [font="Times]if ( frequency == null ) {
g[i;>XyP [font="Times] frequency = new Integer(1);
3\ajnd| [font="Times] } else {
%rs2{Q2k [font="Times] int value = frequency.intValue();
uvl91~&G [font="Times] frequency = new Integer(value + 1);}
fAStM: [font="Times] word_count_map.put(word_lowercase, frequency);
S3x^#83 [font="Times] }
*}:P [font="Times] System.out.println(word_count_map);
PYQ [font="Times] }}
VT>-* d
>L8SL i/!{k2 [font="Times]Queue举例:
){GJgk|P [font="Times]import java.io.IOException;
51s\)d%l [font="Times]import java.io.PrintStream;
rs4:jS$) [font="Times]import java.util.LinkedList;
>%6j -:S [font="Times]import java.util.Queue;
# d"M(nt [font="Times]
0 F8xS8vK+ [font="Times]public class QueueTester {
o7we'1(O [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
im<!JMI [font="Times] public QueueTester() {q = new LinkedList();}
C|H`.|Q [font="Times]public void testFIFO(PrintStream out) throws IOException {
a. u{b&+9 [font="Times] q.add("First");
~jKIuO/ [font="Times] q.add("Second");
TH4f"h+B3" [font="Times] q.add("Third");
B_Wig2xH0 [font="Times] Object o;
ShRMzU [font="Times] while ((o = q.poll()) != null) {
OtL~NTY [font="Times] out.println(o);}}
=:T"naY( [font="Times] public static void main(String[] args) {
P `<TO [font="Times] QueueTester tester = new QueueTester();
u@Gum|_=N [font="Times] try { tester.testFIFO(System.out);
J8FzQ2 [font="Times] } catch (IOException e) {
,%m~OB# [font="Times] e.printStackTrace(); } }}
dT1UYG}>j XH0{|#hwN d+P<ce2G 上述例子和一些相关例子(共6个例子)打包下载[font="Times]
G8F;fG N 总结:
Nc6y]eGz 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。
0F!Uai1 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。