全新java初学者实践教程10(java SE5.0版)
""1^k2fj jdk5的集合类
{FIXc^m' %QKRFPYhS k-HCeZ :)_~w4& 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。
l*kPOyB 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。
LX@/RAd vz [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。
AQ+]|XYo_ _-9@qe [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]
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;M_o)OS3 [font="Times](1) Set
S`"LV $8 集([font="Times]set)是最简单的一种集合,它的对象不按特定方式排序,只是简单的把对象加入集合中,就像往口袋里放东西。对集中成员的访问和操作是通过集中对象的引用进行的,所以集中不能有重复对象。我们知道数学上的集合也是[font="Times]Set这个,集合里面一定是没有重复的元素的。
M\Z6$<H?U bV8!"{ ([font="Times]2)[font="Times]List
z 6?)3' 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。
YR>B_,Gl B,K>rCZ/ FcRW;e8- [font="Times] ([font="Times]3)[font="Times]Map
_jNj-)RB_ [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。
v}tag#f5>? ZP;j9T! _=NwQu\_F [font="Times] ([font="Times]4)[font="Times]Queue
}p!HT6 tZ [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。
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6V [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:
5fm?Lxr&? kIGbG;"_ 9P~\Mpk 实践:[font="Times] Set举例
`xywho%/Y [font="Times] import java.util.*;
gOr%!QaF [font="Times]public class SetExample {
`S2[5i [font="Times] public static void main(String[] args) {
0qo)."V{ [font="Times] Set set = new HashSet(); //HashSet是[font="Times]Set的子接口
BVr0Gk [font="Times] set.add("one");
GW$.lo1|) [font="Times] set.add("second");
+[R/=$ [font="Times] set.add("3rd");
3$m4q`J [font="Times] set.add(new Integer(4));
VA9Gb9 [font="Times] set.add(new Float( 5.0F ));
%_(H{y_! [font="Times] set.add("second");
m^H21P"z [font="Times] set.add(new Integer(4));
F6K4#t+9 [font="Times] System.out.println(set);
qnoNT%xazo [font="Times] }}
s_>
f5/i2 CMCO}# [font="Times]List举例:
|R56ho5C [font="Times] import java.util.*;
B';6r4I- [font="Times]public class ListExample {
Rhe Re [font="Times] public static void main(String[] args) {
8:#rA*Y [font="Times] List list = new ArrayList();
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?\ [font="Times] list.add("one");
-,+q#F [font="Times] list.add("second");
,Vh{gm1 [font="Times] list.add("3rd");
^ mS
o1?< [font="Times] list.add(new Integer(4));
|6(ZD^w [font="Times] list.add(new Float( 5.0F ));
B"v.*
%"&/ [font="Times] list.add("second");
KGWyJ [font="Times] list.add(new Integer(4));
9(L)&S{4K [font="Times] System.out.println(list);
s.x&LG [font="Times] }}
L
W;heO" {O,{c\ Uv?|G%cD- [font="Times]Map举例
sL@U [font="Times]import java.util.Map;
sPps q [font="Times]import java.util.HashMap;
Wa1,
p [font="Times]import java.util.Iterator;
dpFVN[\oK [font="Times]import java.io.FileReader;
u&9 r2R959 [font="Times]
]\xy\\b/` [font="Times]public class MapExample {
]_8qn'7 [font="Times] public static void main(String[] args) throws java.io.FileNotFoundException {
<]T` 3W9 [font="Times] Map word_count_map = new HashMap();
gCN$} [font="Times] FileReader reader = new FileReader(args[0]);
Ai/b\:V9S [font="Times] Iterator words = new WordStreamIterator(reader);
gt ";2,;X [font="Times]
hTEx]# ( [font="Times] while ( words.hasNext() ) {
UH"#2< |b [font="Times] String word = (String) words.next();
-CR?<A4mud [font="Times] String word_lowercase = word.toLowerCase();
/MF!GM [font="Times] Integer frequency = (Integer)word_count_map.get(word_lowercase);
hTM[8 ~<^ [font="Times]if ( frequency == null ) {
~O]]N;>72" [font="Times] frequency = new Integer(1);
!Mu|mz= [font="Times] } else {
PZm:T+5H [font="Times] int value = frequency.intValue();
PNA\ TXT [font="Times] frequency = new Integer(value + 1);}
\T\b NbPn [font="Times] word_count_map.put(word_lowercase, frequency);
2{Chu85 [font="Times] }
IZm(`b;t^ [font="Times] System.out.println(word_count_map);
^m/oDB- [font="Times] }}
>(<ytn t= Hsihytdj :UbM ! [font="Times]Queue举例:
v0kqu [font="Times]import java.io.IOException;
UTSL [font="Times]import java.io.PrintStream;
}?@rO`:EF+ [font="Times]import java.util.LinkedList;
^<:sdv>Y5 [font="Times]import java.util.Queue;
GV^i`r^" [font="Times]
C-?%uF [font="Times]public class QueueTester {
Q3 eM2i8Y [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
(^5 7UmFv] [font="Times] public QueueTester() {q = new LinkedList();}
=1u@7Bh [font="Times]public void testFIFO(PrintStream out) throws IOException {
m "M("% [font="Times] q.add("First");
ncX/L[L [font="Times] q.add("Second");
<d<mvXbw_@ [font="Times] q.add("Third");
3VUWX5K? [font="Times] Object o;
^47PLLRP [font="Times] while ((o = q.poll()) != null) {
u- o--q [font="Times] out.println(o);}}
RC^9HuR& [font="Times] public static void main(String[] args) {
5|I[>Su [font="Times] QueueTester tester = new QueueTester();
q\q=PB6r [font="Times] try { tester.testFIFO(System.out);
Bcjx>#3?L [font="Times] } catch (IOException e) {
`xc^_781\ [font="Times] e.printStackTrace(); } }}
D!oc>K$B %&Fk4Z}M Lj"A4i_ 上述例子和一些相关例子(共6个例子)打包下载[font="Times]
e.*%K!( 总结:
cDoo* 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。
$%%os6y2v 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。