全新java初学者实践教程10(java SE5.0版)
Q>71uM%e` jdk5的集合类
=2}V=E/85 zRbY]dW z#1"0Ks&P 9ENI%Jz 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。
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PB% 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。
}9&dY!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。
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*eX/ZCn [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]
ci/qm\JI<< D$@2H>.- [font="Times](1) Set
D c;k)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。
.(3ec/i4CF 4c[/%e:\- ([font="Times]2)[font="Times]List
Y6Ux*vhK 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。
Cy)N hgz i<):%[Q)> "YWZ&_n** [font="Times] ([font="Times]3)[font="Times]Map
tzv4uD] [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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) jw`05rw: [font="Times] ([font="Times]4)[font="Times]Queue
PQa0m)H@ [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。
^bP`Iv [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:
y#th&YC_b 1z4_QZZ.NG @b,6W
wc 实践:[font="Times] Set举例
WdlGnFAWh [font="Times] import java.util.*;
`oH4"9&]k3 [font="Times]public class SetExample {
SN]g4}K- [font="Times] public static void main(String[] args) {
Sc/$2gSG [font="Times] Set set = new HashSet(); //HashSet是[font="Times]Set的子接口
-e_op'` [font="Times] set.add("one");
(m6V)y [font="Times] set.add("second");
[cco/=c [font="Times] set.add("3rd");
lcy<taNu) [font="Times] set.add(new Integer(4));
j9l32<h7] [font="Times] set.add(new Float( 5.0F ));
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^K#\*P [font="Times] set.add("second");
Ga-cto1Y [font="Times] set.add(new Integer(4));
,II3b(l [font="Times] System.out.println(set);
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j [font="Times] }}
\P")Eh =d V)l:fUm2 [font="Times]List举例:
`*B V@ [font="Times] import java.util.*;
l1KgPRmEP [font="Times]public class ListExample {
+cSc0: [font="Times] public static void main(String[] args) {
{dm>]@"S [font="Times] List list = new ArrayList();
~KYzEqy [font="Times] list.add("one");
wc.=`Me [font="Times] list.add("second");
iy_Y!wZ{ [font="Times] list.add("3rd");
Pq8oK'z- [font="Times] list.add(new Integer(4));
z;F HZb9t, [font="Times] list.add(new Float( 5.0F ));
O"Nr$bS(Y [font="Times] list.add("second");
RRV%g! [font="Times] list.add(new Integer(4));
k!}(a0h [font="Times] System.out.println(list);
8A.7q [font="Times] }}
EmR82^_: d~QM@<SV w;j<$<4=7 [font="Times]Map举例
>TY;l3ew [font="Times]import java.util.Map;
_U-`/r o [font="Times]import java.util.HashMap;
9}m?E<6& [font="Times]import java.util.Iterator;
GBT|1c'i [font="Times]import java.io.FileReader;
!|UX4 [font="Times]
X^K^az&L [font="Times]public class MapExample {
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[ [font="Times] public static void main(String[] args) throws java.io.FileNotFoundException {
cz{`'VN}` [font="Times] Map word_count_map = new HashMap();
{\CWoFht> [font="Times] FileReader reader = new FileReader(args[0]);
0c`nk\vUy [font="Times] Iterator words = new WordStreamIterator(reader);
c)B3g.C4m [font="Times]
6h2keyod [font="Times] while ( words.hasNext() ) {
Dmr*Lh~ [font="Times] String word = (String) words.next();
y_}vVHT, [font="Times] String word_lowercase = word.toLowerCase();
1[8^JVC>6 [font="Times] Integer frequency = (Integer)word_count_map.get(word_lowercase);
u!5q)>Wt( [font="Times]if ( frequency == null ) {
`[g$EXX [font="Times] frequency = new Integer(1);
ES AX}uF [font="Times] } else {
2xf lRks [font="Times] int value = frequency.intValue();
ybw\^t [font="Times] frequency = new Integer(value + 1);}
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P5K [font="Times] word_count_map.put(word_lowercase, frequency);
Pd\4hy [font="Times] }
Fa[^D~$l* [font="Times] System.out.println(word_count_map);
)Uy%iE* [font="Times] }}
Z,e|L4& R54ae:8 I;%1xdPt [font="Times]Queue举例:
\X _}\_c,d [font="Times]import java.io.IOException;
_uLpU4# ? [font="Times]import java.io.PrintStream;
BDvkY [font="Times]import java.util.LinkedList;
,]7ouH$H} [font="Times]import java.util.Queue;
HI 1T [font="Times]
/51$o\4S [font="Times]public class QueueTester {
]oVP_ &E [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
#}+H [font="Times] public QueueTester() {q = new LinkedList();}
+pwTM]bV [font="Times]public void testFIFO(PrintStream out) throws IOException {
"nCK%w= [font="Times] q.add("First");
5WJ ~%"O [font="Times] q.add("Second");
#IvKI+" [font="Times] q.add("Third");
GdI,&|/ [font="Times] Object o;
ye9GBAj
/ [font="Times] while ((o = q.poll()) != null) {
2[ofz}k]r) [font="Times] out.println(o);}}
G3io!XM)D [font="Times] public static void main(String[] args) {
/MY's&D( [font="Times] QueueTester tester = new QueueTester();
vj%"x/TP [font="Times] try { tester.testFIFO(System.out);
<)wLxWalF [font="Times] } catch (IOException e) {
dGm%If9P [font="Times] e.printStackTrace(); } }}
$f0u 19qHWU^0V Pz{MYw 上述例子和一些相关例子(共6个例子)打包下载[font="Times]
m~AAO{\:b 总结:
V [g^R*b 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。
j8p<HE51 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。