全新java初学者实践教程10(java SE5.0版)
Z/6B[,V jdk5的集合类
8%>
Ls O=u.PRNT8 69TQHJ[ \oLRNr[F 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。
b78'yM& 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。
L:%;
Fx2 [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。
$kvF]|<bu Vb|DNl@ [font="Times] 集合类型主要有[font="Times]3种:[font="Times]set(集)、[font="Times]list(列表)、[font="Times]map(映射[font="Times])和[font="Times]Queue(队列)。[font="Times]//队列为[font="Times]jdk5中的加上的[font="Times]
ld$LG6[PA Quc9lL [font="Times](1) Set
,8cw jS2E 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。
fG2\p&z R0,
Q` ([font="Times]2)[font="Times]List
8yA: C 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。
Tg)Fr) 1E=%:? d 3RZP 12x [font="Times] ([font="Times]3)[font="Times]Map
s>76?Q:i [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。
Qte=<Z) \y"!`.E7\d TOeJnk [font="Times] ([font="Times]4)[font="Times]Queue
c+Ejah+ [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。
-Q<3Q_ [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:
#rF|X6P G!L=W#{ #/MUiV 实践:[font="Times] Set举例
8s6[?=nM [font="Times] import java.util.*;
&5)Kg%r [font="Times]public class SetExample {
srw5&s(3X [font="Times] public static void main(String[] args) {
<dLdSEw [font="Times] Set set = new HashSet(); //HashSet是[font="Times]Set的子接口
U[u9RB [font="Times] set.add("one");
n$F&gx'^ [font="Times] set.add("second");
J`Q#p%W [font="Times] set.add("3rd");
JyvXNV, [font="Times] set.add(new Integer(4));
l;b5 v]~ [font="Times] set.add(new Float( 5.0F ));
,3!l'|0jJ [font="Times] set.add("second");
#]q<fhJhr$ [font="Times] set.add(new Integer(4));
F!tn|!~ [font="Times] System.out.println(set);
b6'%nR*f [font="Times] }}
+8]}'6m -A[iTI" [font="Times]List举例:
#x"4tI [font="Times] import java.util.*;
ijw'7d|, [font="Times]public class ListExample {
0jro0f' [font="Times] public static void main(String[] args) {
yOxJx7uD [font="Times] List list = new ArrayList();
]}<wS]1 [font="Times] list.add("one");
?tQUZO [font="Times] list.add("second");
/Uz2.Ua= [font="Times] list.add("3rd");
S/"-x{Gc2v [font="Times] list.add(new Integer(4));
IwiR2K [font="Times] list.add(new Float( 5.0F ));
B!jT@b{ [font="Times] list.add("second");
+D&W!m [font="Times] list.add(new Integer(4));
s,\!@[N [font="Times] System.out.println(list);
9\0 [font="Times] }}
6(f[<V!r UW8b(b[-6b {()8 Wr [font="Times]Map举例
lGwX.cA!' [font="Times]import java.util.Map;
LBk1Qw}- [font="Times]import java.util.HashMap;
6-{QU] # [font="Times]import java.util.Iterator;
#f5-f [font="Times]import java.io.FileReader;
-e3m!h [font="Times]
_1VtVfiZ{ [font="Times]public class MapExample {
fpwge/w [font="Times] public static void main(String[] args) throws java.io.FileNotFoundException {
rgWGe6;! [font="Times] Map word_count_map = new HashMap();
CD:@OI [font="Times] FileReader reader = new FileReader(args[0]);
5#p [Q _ [font="Times] Iterator words = new WordStreamIterator(reader);
.36z [font="Times]
rg]eSP3W [font="Times] while ( words.hasNext() ) {
r77?s? [font="Times] String word = (String) words.next();
9?:SxI;v [font="Times] String word_lowercase = word.toLowerCase();
-4mUGh1dy [font="Times] Integer frequency = (Integer)word_count_map.get(word_lowercase);
ff**) Xdh [font="Times]if ( frequency == null ) {
S^]i [font="Times] frequency = new Integer(1);
H5j~<@STC [font="Times] } else {
.Vj;[p8 [font="Times] int value = frequency.intValue();
3+;]dqZ [font="Times] frequency = new Integer(value + 1);}
v<,?%(g)7 [font="Times] word_count_map.put(word_lowercase, frequency);
qY]IX9'kV [font="Times] }
cxFfAk\,en [font="Times] System.out.println(word_count_map);
{a- p/\U [font="Times] }}
S^HuQe!# I
$!Y 4E}]> [font="Times]Queue举例:
w^sM,c5d [font="Times]import java.io.IOException;
@@9#odO [font="Times]import java.io.PrintStream;
)f>s\T [font="Times]import java.util.LinkedList;
zjs@7LN [font="Times]import java.util.Queue;
Ev|2bk \ [font="Times]
mWZoo/xtT [font="Times]public class QueueTester {
Fyrr,# [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
V
lN&Lz [font="Times] public QueueTester() {q = new LinkedList();}
RcitW;{|Kg [font="Times]public void testFIFO(PrintStream out) throws IOException {
;]3Tuq [font="Times] q.add("First");
,YX[6eZr [font="Times] q.add("Second");
N9 3
ZI|T [font="Times] q.add("Third");
44B)=p7
[font="Times] Object o;
):E4qlB [font="Times] while ((o = q.poll()) != null) {
#>g]CRN [font="Times] out.println(o);}}
i9[=x(-@ [font="Times] public static void main(String[] args) {
:(VD<"X [font="Times] QueueTester tester = new QueueTester();
5 5>^H1M [font="Times] try { tester.testFIFO(System.out);
@[D-2s [font="Times] } catch (IOException e) {
eVL'Ao&Ho [font="Times] e.printStackTrace(); } }}
M]oO1GM 3de<H=H' +]*4!4MK6 上述例子和一些相关例子(共6个例子)打包下载[font="Times]
WUkx v* 总结:
5K|1Y#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。
Q7zg i 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。