全新java初学者实践教程10(java SE5.0版)
IYCKF/2o jdk5的集合类
G0(c@FBK 5W{>5.Arx) ;!~;05^iD +tV(8h4 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。
I8Zp#'|U 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。
yh0|f94m [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。
4
^+hw; BO9Z"|" [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]
N0qC/da1 ?&GMp[ [font="Times](1) Set
.y\HQ^j 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。
,F-tvSc\Q Qs?+vk?*h ([font="Times]2)[font="Times]List
.e"jnP~ 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。
Sm Ei _u]' h?ia4t 9=kTTF s [font="Times] ([font="Times]3)[font="Times]Map
/ P:Hfq [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。
H]!y |p eY(usK KOmP-q=6 [font="Times] ([font="Times]4)[font="Times]Queue
4KM$QHS5{ [font="Times] 在[font="Times]jdk5.0以前,通常的实现方式是使用[font="Times]java.util.List集合来模仿[font="Times]Queue。[font="Times]Queue的概念通过把对象添加(称为[font="Times]enqueuing的操作)到[font="Times]List的尾部(即[font="Times]Queue的后部)并通过从[font="Times]List的头部(即[font="Times]Queue的前部)提取对象而从[font="Times] List中移除(称为[font="Times]dequeuing的操作)来模拟。你需要执行先进先出的动作时可以直接使用[font="Times]Queue接口就可以了。
&
9]KkY= [font="Times] 这[font="Times]4个东西,有时候功能还不太完善,需要有些子类继承它的特性。[font="Times]Set的子接口有[font="Times]TreeSet,SortedSet,[font="Times]List的有[font="Times]ArrayList等,[font="Times]Map里有[font="Times]HashMap,HashTable等,[font="Times]Queue里面有[font="Times]BlockingQueue等。我们来看看例子吧:
HJ'93, kK1qFe?] #P4dx'vm 实践:[font="Times] Set举例
,m"ztu- [font="Times] import java.util.*;
@LE?XlhD [font="Times]public class SetExample {
)R +o8C [font="Times] public static void main(String[] args) {
2s4=%l [font="Times] Set set = new HashSet(); //HashSet是[font="Times]Set的子接口
!:n),sFv45 [font="Times] set.add("one");
jo |q,t [font="Times] set.add("second");
L$Leo6<3a [font="Times] set.add("3rd");
6>L. )V [font="Times] set.add(new Integer(4));
TcP1"wc [font="Times] set.add(new Float( 5.0F ));
/-hF<oNQ [font="Times] set.add("second");
)>Q 2G/@ [font="Times] set.add(new Integer(4));
H4m6H)KOG [font="Times] System.out.println(set);
(k5DbP[ [font="Times] }}
"~(&5M\8` 3N
bn|_`( [font="Times]List举例:
V-cuG. [font="Times] import java.util.*;
.#:,j1L"53 [font="Times]public class ListExample {
KJ)nGoP> [font="Times] public static void main(String[] args) {
J@GfO\
o [font="Times] List list = new ArrayList();
9V66~Bf5 [font="Times] list.add("one");
fD~!t 8J [font="Times] list.add("second");
!U#++Zig% [font="Times] list.add("3rd");
\i,cL)HM [font="Times] list.add(new Integer(4));
NI1HUUZz [font="Times] list.add(new Float( 5.0F ));
osd^SnL1/5 [font="Times] list.add("second");
q%k+x) [font="Times] list.add(new Integer(4));
@|GeR [font="Times] System.out.println(list);
p\{+l;` [font="Times] }}
|tg?b&QR -/-6Td1JY> Z8&C-yCC [font="Times]Map举例
V{h@nhq [font="Times]import java.util.Map;
+r0eTP=zf [font="Times]import java.util.HashMap;
^c\ IZ5 [font="Times]import java.util.Iterator;
jOb[h=B" [font="Times]import java.io.FileReader;
H{f_:z{{ [font="Times]
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[font="Times]public class MapExample {
)KqR8UO [font="Times] public static void main(String[] args) throws java.io.FileNotFoundException {
$CmX
&%L= [font="Times] Map word_count_map = new HashMap();
p0rwiBC=q [font="Times] FileReader reader = new FileReader(args[0]);
O&,O:b:@ [font="Times] Iterator words = new WordStreamIterator(reader);
nF'xV44" [font="Times]
hf<$vRti> [font="Times] while ( words.hasNext() ) {
\u]CD}/ [font="Times] String word = (String) words.next();
t%U[\\ic [font="Times] String word_lowercase = word.toLowerCase();
;-?ZI$ [font="Times] Integer frequency = (Integer)word_count_map.get(word_lowercase);
DVhTb [font="Times]if ( frequency == null ) {
?nZ <? [font="Times] frequency = new Integer(1);
'%EZoc/U [font="Times] } else {
?*'$(}r3 [font="Times] int value = frequency.intValue();
z9ShP&^4[ [font="Times] frequency = new Integer(value + 1);}
|)qK
g [font="Times] word_count_map.put(word_lowercase, frequency);
f%{Tu` [font="Times] }
p{a]pG+3 [font="Times] System.out.println(word_count_map);
-WUYE [font="Times] }}
B/=q_.1F> 16QbB; _g 4/% [font="Times]Queue举例:
r.W,-%=bL [font="Times]import java.io.IOException;
=$8@JF' [font="Times]import java.io.PrintStream;
mY`]33??v [font="Times]import java.util.LinkedList;
|2@en=EYk [font="Times]import java.util.Queue;
&^IcL!t[ [font="Times]
z"K(
bw6 [font="Times]public class QueueTester {
^
6.lb\ [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
>C3NtGvy [font="Times] public QueueTester() {q = new LinkedList();}
ZCBPO~&hO' [font="Times]public void testFIFO(PrintStream out) throws IOException {
)_v\{N [font="Times] q.add("First");
naIv= [font="Times] q.add("Second");
HB7( [font="Times] q.add("Third");
ChTXvkdH [font="Times] Object o;
d>, V [font="Times] while ((o = q.poll()) != null) {
]1?=jlUl [font="Times] out.println(o);}}
yIIETE [font="Times] public static void main(String[] args) {
(.1 rtj [font="Times] QueueTester tester = new QueueTester();
14D7U/zer [font="Times] try { tester.testFIFO(System.out);
V-_/(xt* [font="Times] } catch (IOException e) {
ufCqvv>' [font="Times] e.printStackTrace(); } }}
v'@b. R, L"foL
gt_XAH 上述例子和一些相关例子(共6个例子)打包下载[font="Times]
XocsSs 总结:
S9p?* 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。
'e]HP-Y< 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。