全新java初学者实践教程10(java SE5.0版)
_b0S jdk5的集合类
d<^_w!4X} QJ a4R hGed/Yr B:O+*3j 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。
'!wPnYT@D 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。
^V<J69ny|9 [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。
GB<R7J zP:~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]
V<i<0E px w{ [font="Times](1) Set
(|g").L 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。
>`hSye{ Gva}J6{ ([font="Times]2)[font="Times]List
?eL='>Ne 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。
DC|xilP1O 9 m\)\/V S9G8aea/ [font="Times] ([font="Times]3)[font="Times]Map
BgJkrv7~ [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。
2q"_^deI5* =MTj4VXh" <#xrrRhm} [font="Times] ([font="Times]4)[font="Times]Queue
R=\v3m [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。
2f3=?YqD [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:
"H5&3sF2 a3O nW\N fDU+3b 实践:[font="Times] Set举例
cP*c(k~N [font="Times] import java.util.*;
x*unye7 [font="Times]public class SetExample {
Z $!C= [font="Times] public static void main(String[] args) {
@+?+6sS [font="Times] Set set = new HashSet(); //HashSet是[font="Times]Set的子接口
AA))KBXq [font="Times] set.add("one");
DUZQO{V [font="Times] set.add("second");
!Z
U_,[ [font="Times] set.add("3rd");
"?i>p z [font="Times] set.add(new Integer(4));
Az
U|p [font="Times] set.add(new Float( 5.0F ));
MxY50^}( [font="Times] set.add("second");
4id3P{aU [font="Times] set.add(new Integer(4));
M'\pkzx [font="Times] System.out.println(set);
CxJfrI_W [font="Times] }}
RRx`}E9, #mgA/q?A [font="Times]List举例:
n&_YYEHx [font="Times] import java.util.*;
@<vF]\Ce [font="Times]public class ListExample {
4JGE2ArR [font="Times] public static void main(String[] args) {
xJvLuzUD [font="Times] List list = new ArrayList();
UskZ%J [font="Times] list.add("one");
/GsSrP_?] [font="Times] list.add("second");
^pJ0nY#c [font="Times] list.add("3rd");
{B@*DQv [font="Times] list.add(new Integer(4));
.=Pm>o/, [font="Times] list.add(new Float( 5.0F ));
$!a?i@ [font="Times] list.add("second");
>W8bWQ^fK [font="Times] list.add(new Integer(4));
myD{sE2A [font="Times] System.out.println(list);
1 h<fJzh [font="Times] }}
jo_o`j mYX56,b}5 j: <t [font="Times]Map举例
q^u1z|'Z [font="Times]import java.util.Map;
b![t6-f^z [font="Times]import java.util.HashMap;
hgj CXl [font="Times]import java.util.Iterator;
HKpD2M [font="Times]import java.io.FileReader;
PdR >;$1 [font="Times]
Qqp)@uM^ [font="Times]public class MapExample {
PT mf [font="Times] public static void main(String[] args) throws java.io.FileNotFoundException {
> P(eW7RL [font="Times] Map word_count_map = new HashMap();
:OHSxb>[ [font="Times] FileReader reader = new FileReader(args[0]);
- dl}_ [font="Times] Iterator words = new WordStreamIterator(reader);
0[lS(K [font="Times]
?^U c= [font="Times] while ( words.hasNext() ) {
BApa^j\? [font="Times] String word = (String) words.next();
]X*YAPv [font="Times] String word_lowercase = word.toLowerCase();
9^oo-,Su_ [font="Times] Integer frequency = (Integer)word_count_map.get(word_lowercase);
y0;,dv] [font="Times]if ( frequency == null ) {
8,=G1c [font="Times] frequency = new Integer(1);
=Wj{]&` [font="Times] } else {
7k>zuzRyF [font="Times] int value = frequency.intValue();
K~USK?Q% [font="Times] frequency = new Integer(value + 1);}
Mz93 [font="Times] word_count_map.put(word_lowercase, frequency);
?;Un#6b [font="Times] }
s}jlS [font="Times] System.out.println(word_count_map);
}gCG&7C [font="Times] }}
-=}b;Kf- rWJ*e Y \kxh#{$z? [font="Times]Queue举例:
@Eo4U]- [font="Times]import java.io.IOException;
EVG"._I@ [font="Times]import java.io.PrintStream;
`%uK0qw" [font="Times]import java.util.LinkedList;
S:#e8H_7m] [font="Times]import java.util.Queue;
0k%hY{ [font="Times]
'X54dXS?l [font="Times]public class QueueTester {
}0Y`|H\v [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
^ZR8s^X [font="Times] public QueueTester() {q = new LinkedList();}
O"qR }W [font="Times]public void testFIFO(PrintStream out) throws IOException {
gNTh% e [font="Times] q.add("First");
1f<RyAE?5 [font="Times] q.add("Second");
+1x)z~q= [font="Times] q.add("Third");
zFOL(s.h|0 [font="Times] Object o;
!Pw$48cg [font="Times] while ((o = q.poll()) != null) {
1ADv?+j)A/ [font="Times] out.println(o);}}
^L ]B5,}- [font="Times] public static void main(String[] args) {
N^lAG"Jao[ [font="Times] QueueTester tester = new QueueTester();
k9l^6#<? [font="Times] try { tester.testFIFO(System.out);
*=TYVM9 [font="Times] } catch (IOException e) {
7g5@vYS+ [font="Times] e.printStackTrace(); } }}
zb>;?et;) yu=piP # J]~ 上述例子和一些相关例子(共6个例子)打包下载[font="Times]
;t|,nz4kJ 总结:
%w$mSG 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。
?;_H{/)m 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。