全新java初学者实践教程10(java SE5.0版)
_u#r;h[ jdk5的集合类
G~f|Sx .6iJ:A6T P#,g5 80LN(0?x 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。
2KNs,4X@ 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。
Et;Ubj"+ [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。
2=n,{rkmj% $N4i)>&T2 [font="Times] 集合类型主要有[font="Times]3种:[font="Times]set(集)、[font="Times]list(列表)、[font="Times]map(映射[font="Times])和[font="Times]Queue(队列)。[font="Times]//队列为[font="Times]jdk5中的加上的[font="Times]
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af=lzKt* [font="Times](1) Set
|u[@g`Z 集([font="Times]set)是最简单的一种集合,它的对象不按特定方式排序,只是简单的把对象加入集合中,就像往口袋里放东西。对集中成员的访问和操作是通过集中对象的引用进行的,所以集中不能有重复对象。我们知道数学上的集合也是[font="Times]Set这个,集合里面一定是没有重复的元素的。
"l(<<Ha/ LiJ. / ([font="Times]2)[font="Times]List
*nHkK!d<N 列表([font="Times]List)的主要特征是其对象以线性方式存储,没有特定顺序,只有一个开头和一个结尾,当然,它与根本没有顺序的[font="Times]Set是不同的。它是链表嘛,一条链肯定有顺序这个顺序就不一定了。
R`%O=S*] 0BP=SCi Co:Rg@i(F [font="Times] ([font="Times]3)[font="Times]Map
r<$"T [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。
[4mIww% Ro#O{ &M#}?@!C [font="Times] ([font="Times]4)[font="Times]Queue
yY80E[v [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。
oK2j PP [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:
J+qcA} Nbt.y 'd M{X; H'2 实践:[font="Times] Set举例
4` :Eiik&p [font="Times] import java.util.*;
#D%l;Ae [font="Times]public class SetExample {
is{H >#+" [font="Times] public static void main(String[] args) {
YF)c.Q0 [font="Times] Set set = new HashSet(); //HashSet是[font="Times]Set的子接口
cXt]55" [font="Times] set.add("one");
TcH7!fUj [font="Times] set.add("second");
YS>VQl [font="Times] set.add("3rd");
&[[Hfs2:-] [font="Times] set.add(new Integer(4));
r@G34QC+ [font="Times] set.add(new Float( 5.0F ));
4z^VwKH\ j [font="Times] set.add("second");
&C6*"JZ4 [font="Times] set.add(new Integer(4));
S|_"~Nd= [font="Times] System.out.println(set);
c,5yH [font="Times] }}
L ?S#3@Pa -'j|U[&N\ [font="Times]List举例:
*,Sa*-7( [font="Times] import java.util.*;
`m-7L [font="Times]public class ListExample {
E~`<n]{G-C [font="Times] public static void main(String[] args) {
(5)DQ1LaF [font="Times] List list = new ArrayList();
9@YhAj [font="Times] list.add("one");
xepp."O [font="Times] list.add("second");
SB^xq [font="Times] list.add("3rd");
v<gve<] [font="Times] list.add(new Integer(4));
BBj>ML\X [font="Times] list.add(new Float( 5.0F ));
G@,XUP [font="Times] list.add("second");
=u.hHkx [font="Times] list.add(new Integer(4));
Wtp;se@# [font="Times] System.out.println(list);
W<Asr@ [font="Times] }}
!eD
f}~ =gO4B-[ 1*OZu.NdK [font="Times]Map举例
;sY n=r [font="Times]import java.util.Map;
#6 M3BF [font="Times]import java.util.HashMap;
Vd A!tL [font="Times]import java.util.Iterator;
CD)JCv [font="Times]import java.io.FileReader;
{br6* [font="Times]
~L9I@(/S [font="Times]public class MapExample {
le~p2l#e [font="Times] public static void main(String[] args) throws java.io.FileNotFoundException {
17!<8vIV$C [font="Times] Map word_count_map = new HashMap();
pUeok+k_ [font="Times] FileReader reader = new FileReader(args[0]);
gO_d!x* [font="Times] Iterator words = new WordStreamIterator(reader);
rC6{-42bb [font="Times]
GNM+sdy+ [font="Times] while ( words.hasNext() ) {
US]I[Y6V [font="Times] String word = (String) words.next();
yzyK$WN\[3 [font="Times] String word_lowercase = word.toLowerCase();
U;FJSy [font="Times] Integer frequency = (Integer)word_count_map.get(word_lowercase);
b4>1UZGW- [font="Times]if ( frequency == null ) {
jJe?pT]o [font="Times] frequency = new Integer(1);
)Vpt.4IBd [font="Times] } else {
LDj*~\vsq [font="Times] int value = frequency.intValue();
BSyS
DM [font="Times] frequency = new Integer(value + 1);}
1oL3y;>iL [font="Times] word_count_map.put(word_lowercase, frequency);
h&:XO9dY [font="Times] }
?GeMD
/] [font="Times] System.out.println(word_count_map);
Z4D[nPm$ [font="Times] }}
X=%e'P*X t+A9nvj) 4&G
#Bi [font="Times]Queue举例:
*m[[>wE [font="Times]import java.io.IOException;
o|y1 m7X [font="Times]import java.io.PrintStream;
jL:GP}I= [font="Times]import java.util.LinkedList;
9QEK|x`8 [font="Times]import java.util.Queue;
;~( yv|f6 [font="Times]
]eo%eaA [font="Times]public class QueueTester {
>4nQ&b.u [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
B;J8^esypD [font="Times] public QueueTester() {q = new LinkedList();}
b}Xh|0`b+ [font="Times]public void testFIFO(PrintStream out) throws IOException {
nc.:Wm6Mj [font="Times] q.add("First");
Z^#u n [font="Times] q.add("Second");
uMK8V_p*? [font="Times] q.add("Third");
75H;6(7 [font="Times] Object o;
1abQoe [font="Times] while ((o = q.poll()) != null) {
B$_-1^L
e [font="Times] out.println(o);}}
!qug^F [font="Times] public static void main(String[] args) {
#? 7g_ [font="Times] QueueTester tester = new QueueTester();
?~tx@k$;Es [font="Times] try { tester.testFIFO(System.out);
f<3lxu [font="Times] } catch (IOException e) {
af}JS2=$ [font="Times] e.printStackTrace(); } }}
E[c6*I Dh)(?"^9A REJHh\:.77 上述例子和一些相关例子(共6个例子)打包下载[font="Times]
#bGYd}BfD 总结:
WUGFo$xA 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。
%8?XOkH) 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。