全新java初学者实践教程10(java SE5.0版)
/ 3A6xPOg jdk5的集合类
DaQ"Df_X 5]xuU.w' #c"eff d,<ni" 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。
NBikYxa 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。
.~z'm$s1o [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。
9shfy4?k ]WT@&F [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]
la!]Y-s)'4 8@3K, [Mo [font="Times](1) Set
h2l;xt 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。
~9X^3.nI @AyteHK
([font="Times]2)[font="Times]List
\Mf>X\} 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。
PEMkx"h + 9 {4yC9Oz> \kADh?phV [font="Times] ([font="Times]3)[font="Times]Map
sNf& "C!; [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。
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>Po%r: [font="Times] ([font="Times]4)[font="Times]Queue
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[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。
GES}o9?# [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:
f/Gx}x= 53Adic &L o TO+ 实践:[font="Times] Set举例
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TcoCN [font="Times] import java.util.*;
@s5=6z]=H [font="Times]public class SetExample {
eP{srP3 9 [font="Times] public static void main(String[] args) {
J-W9B amx [font="Times] Set set = new HashSet(); //HashSet是[font="Times]Set的子接口
^-o{3Q(w [font="Times] set.add("one");
G0FzXtu)q [font="Times] set.add("second");
%mI0*YRma [font="Times] set.add("3rd");
'yo@5*x7 [font="Times] set.add(new Integer(4));
FX:`7c]:9 [font="Times] set.add(new Float( 5.0F ));
/dnwN7Gf [font="Times] set.add("second");
&kb`)F3nU [font="Times] set.add(new Integer(4));
FD=%
4#| [font="Times] System.out.println(set);
c*USA
eP [font="Times] }}
k"wQ9=HP7 :]3X Ez [font="Times]List举例:
Vl^(K_`( [font="Times] import java.util.*;
~!S3J2kG{ [font="Times]public class ListExample {
)^(*B6;z5 [font="Times] public static void main(String[] args) {
Zxk~X}K\P [font="Times] List list = new ArrayList();
ffKgVQux [font="Times] list.add("one");
FU`(mQ*Yd [font="Times] list.add("second");
*$p*'vR [font="Times] list.add("3rd");
hmy%X`%j [font="Times] list.add(new Integer(4));
r
)|3MUj [font="Times] list.add(new Float( 5.0F ));
i~B?p[ [font="Times] list.add("second");
2y[Q [font="Times] list.add(new Integer(4));
=8FvkNr [font="Times] System.out.println(list);
W4$o\yA] [font="Times] }}
X>(1fra4 ,67Q!/O A40DbD\^ad [font="Times]Map举例
>e]g T [font="Times]import java.util.Map;
(;NJ<x [font="Times]import java.util.HashMap;
''17(% [font="Times]import java.util.Iterator;
Im]@#X [font="Times]import java.io.FileReader;
]8G 'R-8} [font="Times]
}\_.Mg^y [font="Times]public class MapExample {
yOM/UdWq [font="Times] public static void main(String[] args) throws java.io.FileNotFoundException {
[8V;Q [font="Times] Map word_count_map = new HashMap();
_3IT3mb2n [font="Times] FileReader reader = new FileReader(args[0]);
"be\%W+< [font="Times] Iterator words = new WordStreamIterator(reader);
'nmGHorp [font="Times]
4.A^5J'W [font="Times] while ( words.hasNext() ) {
q^X7x_ [font="Times] String word = (String) words.next();
w,|@e_|J [font="Times] String word_lowercase = word.toLowerCase();
ns[/M~_r [font="Times] Integer frequency = (Integer)word_count_map.get(word_lowercase);
5eAZfe%H [font="Times]if ( frequency == null ) {
UmKE]1Yw4r [font="Times] frequency = new Integer(1);
I}$`gUXX8x [font="Times] } else {
'|yx B') [font="Times] int value = frequency.intValue();
Bk8}K=%w [font="Times] frequency = new Integer(value + 1);}
<JPN<
Kv [font="Times] word_count_map.put(word_lowercase, frequency);
iFIGJS [font="Times] }
j?T'N:Qd [font="Times] System.out.println(word_count_map);
7UTfafOGX [font="Times] }}
`IHP_IfR )W\)37=. I| TNo-!$ [font="Times]Queue举例:
$<*) 5|6 [font="Times]import java.io.IOException;
B4s$| i{D [font="Times]import java.io.PrintStream;
2- iY:r [font="Times]import java.util.LinkedList;
!$)reaS [font="Times]import java.util.Queue;
HZrA}|:h [font="Times]
J+D|/^ [font="Times]public class QueueTester {
:UwBs [font="Times] public Queue q; //发现了一个奇怪的语法,这个尖括号是泛型声明
KQ~y;{h?b [font="Times] public QueueTester() {q = new LinkedList();}
oZ{,IZ45 [font="Times]public void testFIFO(PrintStream out) throws IOException {
ss^a=?~ [font="Times] q.add("First");
RhYe=Qh4{p [font="Times] q.add("Second");
~DH9iB [font="Times] q.add("Third");
J,$xQ?,wE [font="Times] Object o;
:s)cTq| 3 [font="Times] while ((o = q.poll()) != null) {
If'q8G3]- [font="Times] out.println(o);}}
1UQ,V`y [font="Times] public static void main(String[] args) {
xU'z>y4V$ [font="Times] QueueTester tester = new QueueTester();
2H%9l@}u [font="Times] try { tester.testFIFO(System.out);
`
w;Wud'*< [font="Times] } catch (IOException e) {
14$%v;Su4 [font="Times] e.printStackTrace(); } }}
xd?=#d NKY|Z\ i0M6;W1T 上述例子和一些相关例子(共6个例子)打包下载[font="Times]
B>{%$@4 总结:
(l5p_x 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。
Q0A4} 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。