全新java初学者实践教程10(java SE5.0版)
FX:`7c]:9 jdk5的集合类
w.qtSW6M+ BN/4O?jD9 C]^Ep w)btv{* 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。
k"wQ9=HP7 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。
Gnc`CyN:H [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。
Q|y }mC/ Psb !Z( [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]
Pt]>AW;i K<JzIuf& [font="Times](1) Set
ts]e M1; 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。
FU`(mQ*Yd *$p*'vR ([font="Times]2)[font="Times]List
hmy%X`%j 列表([font="Times]List)的主要特征是其对象以线性方式存储,没有特定顺序,只有一个开头和一个结尾,当然,它与根本没有顺序的[font="Times]Set是不同的。它是链表嘛,一条链肯定有顺序这个顺序就不一定了。
r
)|3MUj i~B?p[ {UiSa'TR1b [font="Times] ([font="Times]3)[font="Times]Map
r(,U{bU< [font="Times] 映射([font="Times]Map),这个在[font="Times]java里不是地图的意思,其实地图也是映射哈。它里面的东西是键-值对([font="Times]key-value)出现的,键值对是什么呢?举个例子,比如我们查字典,用部首查字法。目录那个字就是键,这个字的解释就是值。键和值成对出现。这样说可以理解吧。这也是很常用的数据结构哦。
HC`0Ni1 5Xy(za ;(Yb9Mr)z [font="Times] ([font="Times]4)[font="Times]Queue
"ra$x2|=} [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。
9QZaa(vN [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:
WMbkKC.{J qF)J#$4;6 u?').c4 实践:[font="Times] Set举例
awLvLkQb{ [font="Times] import java.util.*;
a ~o<>H [font="Times]public class SetExample {
XF`2*:7 [font="Times] public static void main(String[] args) {
)f8>kz( [font="Times] Set set = new HashSet(); //HashSet是[font="Times]Set的子接口
h]7_
N, [font="Times] set.add("one");
c:Ua\$)u3, [font="Times] set.add("second");
h>Kx [font="Times] set.add("3rd");
1"
'3/MFQ8 [font="Times] set.add(new Integer(4));
Ple.fKu [font="Times] set.add(new Float( 5.0F ));
n ]%2Kx [font="Times] set.add("second");
!$I~3_c [font="Times] set.add(new Integer(4));
5epI'D [font="Times] System.out.println(set);
a@}.96lStD [font="Times] }}
iTxWXij I}$`gUXX8x [font="Times]List举例:
0TN28:hcD [font="Times] import java.util.*;
so))J`ca) [font="Times]public class ListExample {
u=`H n-( [font="Times] public static void main(String[] args) {
.1QGNW [font="Times] List list = new ArrayList();
,0'GHQWz$ [font="Times] list.add("one");
%G?@Hye3 [font="Times] list.add("second");
*)^6'4= [font="Times] list.add("3rd");
manw;`Q [font="Times] list.add(new Integer(4));
RB>=#03 [font="Times] list.add(new Float( 5.0F ));
K)SWM3r [font="Times] list.add("second");
#*A'<Zm
[font="Times] list.add(new Integer(4));
]:%DDlRb [font="Times] System.out.println(list);
?G{0{c2 [font="Times] }}
>t+ ENYb &61U1"&$ R lZzW-
%K [font="Times]Map举例
)@]%:m!ER [font="Times]import java.util.Map;
7w
)?s@CD [font="Times]import java.util.HashMap;
d<c 29Y [font="Times]import java.util.Iterator;
Omd; [font="Times]import java.io.FileReader;
ss^a=?~ [font="Times]
RhYe=Qh4{p [font="Times]public class MapExample {
~DH9iB [font="Times] public static void main(String[] args) throws java.io.FileNotFoundException {
J,$xQ?,wE [font="Times] Map word_count_map = new HashMap();
:s)cTq| 3 [font="Times] FileReader reader = new FileReader(args[0]);
If'q8G3]- [font="Times] Iterator words = new WordStreamIterator(reader);
}:$cK(| [font="Times]
?;~!C2Zs [font="Times] while ( words.hasNext() ) {
N2:Hdu: [font="Times] String word = (String) words.next();
XJul~"
[font="Times] String word_lowercase = word.toLowerCase();
T!/o^0w [font="Times] Integer frequency = (Integer)word_count_map.get(word_lowercase);
"LlpZtw [font="Times]if ( frequency == null ) {
>Eh U{@Y [font="Times] frequency = new Integer(1);
s.M39W? [font="Times] } else {
p.:651b [font="Times] int value = frequency.intValue();
wm@m(ArE= [font="Times] frequency = new Integer(value + 1);}
5Fy dh0. [font="Times] word_count_map.put(word_lowercase, frequency);
@ZEBtM%.O [font="Times] }
=DwLNyjU4 [font="Times] System.out.println(word_count_map);
YNr5*P1 [font="Times] }}
N:G]wsh ?mMM{{%(. _\AQJ?<M [font="Times]Queue举例:
*QK)
1Y1W [font="Times]import java.io.IOException;
r3V1l8MV [font="Times]import java.io.PrintStream;
5(~Lr3v0 [font="Times]import java.util.LinkedList;
kBP?_ O [font="Times]import java.util.Queue;
i)l0[FNI} [font="Times]
iXWzIb}CJ- [font="Times]public class QueueTester {
Om.%K>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
/gAT@Vx [font="Times] public QueueTester() {q = new LinkedList();}
^f[6NYS? [font="Times]public void testFIFO(PrintStream out) throws IOException {
P9!awLM- [font="Times] q.add("First");
he|Q(? [font="Times] q.add("Second");
" {<X! ^u> [font="Times] q.add("Third");
qrMED_(D [font="Times] Object o;
~+.= [font="Times] while ((o = q.poll()) != null) {
z ]f(lwo{ [font="Times] out.println(o);}}
#-|fdcb [font="Times] public static void main(String[] args) {
1dvP2E [font="Times] QueueTester tester = new QueueTester();
`wa;@p+j8 [font="Times] try { tester.testFIFO(System.out);
MlTC?Rp# [font="Times] } catch (IOException e) {
XPhP1 ^>\ [font="Times] e.printStackTrace(); } }}
Dgz,Uad8f nbxY'`8F 81nD:]7 上述例子和一些相关例子(共6个例子)打包下载[font="Times]
E PE_2a} 总结:
j_C"O,WS 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&-3`< 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。