全新java初学者实践教程10(java SE5.0版)
<7M-?g:vj jdk5的集合类
8NWo)y49H pFv u,Q" X H-_tvB HeOdCr-PN 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。
D5TDg\E 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。
gcU*rml [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。
2yZr!Rb~* "f,{d}u [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]
"2l`XH @1MnJP [font="Times](1) Set
"9wD|wsz 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。
Dwp,d~z m^k0j/ ([font="Times]2)[font="Times]List
98>GHl'lM 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。
T$I_nxh[)L Mfj82rHg ,%M[$S' [font="Times] ([font="Times]3)[font="Times]Map
A*EOn1hN [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。
Rff F:,b wDJ`#"5p{ v $Iw?y [font="Times] ([font="Times]4)[font="Times]Queue
''y.4dvX [font="Times] 在[font="Times]jdk5.0以前,通常的实现方式是使用[font="Times]java.util.List集合来模仿[font="Times]Queue。[font="Times]Queue的概念通过把对象添加(称为[font="Times]enqueuing的操作)到[font="Times]List的尾部(即[font="Times]Queue的后部)并通过从[font="Times]List的头部(即[font="Times]Queue的前部)提取对象而从[font="Times] List中移除(称为[font="Times]dequeuing的操作)来模拟。你需要执行先进先出的动作时可以直接使用[font="Times]Queue接口就可以了。
Xw-[Sf]p [font="Times] 这[font="Times]4个东西,有时候功能还不太完善,需要有些子类继承它的特性。[font="Times]Set的子接口有[font="Times]TreeSet,SortedSet,[font="Times]List的有[font="Times]ArrayList等,[font="Times]Map里有[font="Times]HashMap,HashTable等,[font="Times]Queue里面有[font="Times]BlockingQueue等。我们来看看例子吧:
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e(a 实践:[font="Times] Set举例
X\AH^I6S [font="Times] import java.util.*;
xu2KEwgb [font="Times]public class SetExample {
S/nPK,^d2 [font="Times] public static void main(String[] args) {
Zh=arlk [font="Times] Set set = new HashSet(); //HashSet是[font="Times]Set的子接口
2
T!Tiu [font="Times] set.add("one");
c0oHE8@ [font="Times] set.add("second");
TSlB.pw%v [font="Times] set.add("3rd");
9a}9cMJ^" [font="Times] set.add(new Integer(4));
M|WBJ'#x0 [font="Times] set.add(new Float( 5.0F ));
Y%pab/Y [font="Times] set.add("second");
-8Jw_ [font="Times] set.add(new Integer(4));
CM;b_E)9)f [font="Times] System.out.println(set);
=p+y$ [font="Times] }}
7>FXsUt_
=<HDek [font="Times]List举例:
Ld4U [font="Times] import java.util.*;
UB/> Ro [font="Times]public class ListExample {
ZJYn[\] [font="Times] public static void main(String[] args) {
1(
pHC [font="Times] List list = new ArrayList();
Wg']a/m [font="Times] list.add("one");
J ^'El^F [font="Times] list.add("second");
Zxa.x?:?n [font="Times] list.add("3rd");
t`Kbm''d[ [font="Times] list.add(new Integer(4));
6b2UPI7m~ [font="Times] list.add(new Float( 5.0F ));
szI7I$Qb [font="Times] list.add("second");
M/zO|-j& [font="Times] list.add(new Integer(4));
,_2-Op [font="Times] System.out.println(list);
T5S4,.o9W [font="Times] }}
Yj%]|E- a.Ho>(V/4 d*gAL<M7E [font="Times]Map举例
qJhsMo2IH [font="Times]import java.util.Map;
1Kg0y71" [font="Times]import java.util.HashMap;
L:z0cvn" [font="Times]import java.util.Iterator;
ag-A}k>v [font="Times]import java.io.FileReader;
X8nos [font="Times]
G]^[i6PQs [font="Times]public class MapExample {
w!.@64- [font="Times] public static void main(String[] args) throws java.io.FileNotFoundException {
yvAO"43 [font="Times] Map word_count_map = new HashMap();
"mlQ z4D)5 [font="Times] FileReader reader = new FileReader(args[0]);
@60D@Y [font="Times] Iterator words = new WordStreamIterator(reader);
2w 2Bc+#o [font="Times]
}E'0vf/ [font="Times] while ( words.hasNext() ) {
uDf<D.+5Ze [font="Times] String word = (String) words.next();
#Y'eS'lv4 [font="Times] String word_lowercase = word.toLowerCase();
+P/kfY" [font="Times] Integer frequency = (Integer)word_count_map.get(word_lowercase);
#36QO [font="Times]if ( frequency == null ) {
g^ AQBF [font="Times] frequency = new Integer(1);
N[%u>! [font="Times] } else {
mk-{@$Q Jb [font="Times] int value = frequency.intValue();
XzUGlrp:Y# [font="Times] frequency = new Integer(value + 1);}
'xwCeZcg [font="Times] word_count_map.put(word_lowercase, frequency);
}A#IBqf5 [font="Times] }
g@.$P>Bh [font="Times] System.out.println(word_count_map);
y.r N( [font="Times] }}
Y'v[2s ]lB zp D 5xQ-f [font="Times]Queue举例:
>=~\b [font="Times]import java.io.IOException;
18X@0e [font="Times]import java.io.PrintStream;
g3R(,IH [font="Times]import java.util.LinkedList;
Syk)S< [font="Times]import java.util.Queue;
\Wbmmd}8 [font="Times]
TT$Ao [font="Times]public class QueueTester {
ys[Li.s: [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
Q;h.}N8W [font="Times] public QueueTester() {q = new LinkedList();}
_Nx
/<isdL [font="Times]public void testFIFO(PrintStream out) throws IOException {
e#"h@kZP [font="Times] q.add("First");
^D yw(>9 [font="Times] q.add("Second");
{ e|qQ4~h [font="Times] q.add("Third");
|VfEp [font="Times] Object o;
A1WUK=P [font="Times] while ((o = q.poll()) != null) {
F3tps
jQ [font="Times] out.println(o);}}
gQ1obT"| [font="Times] public static void main(String[] args) {
.tBlGMcN [font="Times] QueueTester tester = new QueueTester();
0-.
d{P [font="Times] try { tester.testFIFO(System.out);
Ndmt$(b [font="Times] } catch (IOException e) {
Fn4v/)*H [font="Times] e.printStackTrace(); } }}
f:c'j` 8|u4xf< Z;BS@e 上述例子和一些相关例子(共6个例子)打包下载[font="Times]
Up9{aX 总结:
s#2t\}/ 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。
g:
i5%1 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。