全新java初学者实践教程10(java SE5.0版)
X`:(-3T jdk5的集合类
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7l7eUy/z 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。
TpZ) wC 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。
*2 4P T7 [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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Vs8os+ [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]
I\Y/*u ~0|~Fg [font="Times](1) Set
&EXql'] 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。
_xAdvr' W fbwo2qe@K ([font="Times]2)[font="Times]List
{.H}+ @0 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。
3l`yy])t 6H VS0 h}z^NX [font="Times] ([font="Times]3)[font="Times]Map
88pz<$ [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。
6VQ*z8wLw d~S.PRg= QCa$<~c [font="Times] ([font="Times]4)[font="Times]Queue
]Vl*!,(i [font="Times] 在[font="Times]jdk5.0以前,通常的实现方式是使用[font="Times]java.util.List集合来模仿[font="Times]Queue。[font="Times]Queue的概念通过把对象添加(称为[font="Times]enqueuing的操作)到[font="Times]List的尾部(即[font="Times]Queue的后部)并通过从[font="Times]List的头部(即[font="Times]Queue的前部)提取对象而从[font="Times] List中移除(称为[font="Times]dequeuing的操作)来模拟。你需要执行先进先出的动作时可以直接使用[font="Times]Queue接口就可以了。
-YA1Uk [font="Times] 这[font="Times]4个东西,有时候功能还不太完善,需要有些子类继承它的特性。[font="Times]Set的子接口有[font="Times]TreeSet,SortedSet,[font="Times]List的有[font="Times]ArrayList等,[font="Times]Map里有[font="Times]HashMap,HashTable等,[font="Times]Queue里面有[font="Times]BlockingQueue等。我们来看看例子吧:
O<iE,PN) 5p94b*l A<9ZX=DAjw 实践:[font="Times] Set举例
z6|P]u [font="Times] import java.util.*;
Si6%6rAhj [font="Times]public class SetExample {
eAu3,qoM [font="Times] public static void main(String[] args) {
6g@j,iFy [font="Times] Set set = new HashSet(); //HashSet是[font="Times]Set的子接口
*FK!^Y [font="Times] set.add("one");
vay_QxB5 [font="Times] set.add("second");
@ L% 3} [font="Times] set.add("3rd");
26o68U8&y [font="Times] set.add(new Integer(4));
S=krF yFw [font="Times] set.add(new Float( 5.0F ));
~e~4S~{ [font="Times] set.add("second");
aMZ6C <N [font="Times] set.add(new Integer(4));
K}`.?6O [font="Times] System.out.println(set);
Tp)-L0kD_k [font="Times] }}
hP`3Ao t \DS}3pv [font="Times]List举例:
RQ9fA1YP [font="Times] import java.util.*;
yFl@z [font="Times]public class ListExample {
UR1U; k [font="Times] public static void main(String[] args) {
tE3!; [font="Times] List list = new ArrayList();
Xy_+L_h^ [font="Times] list.add("one");
iW-w?!>|m [font="Times] list.add("second");
C[& \Xq [font="Times] list.add("3rd");
1W$ @ V! [font="Times] list.add(new Integer(4));
b0PF7PEEQ [font="Times] list.add(new Float( 5.0F ));
~BrERUk [font="Times] list.add("second");
V. =! ^0'A [font="Times] list.add(new Integer(4));
iKLN !QR [font="Times] System.out.println(list);
(gvaYKvr [font="Times] }}
FGzB7w# !RFlv ~U~KUL| [font="Times]Map举例
3"6lPUS [font="Times]import java.util.Map;
*S:^3{.m= [font="Times]import java.util.HashMap;
Dx 4?6 [font="Times]import java.util.Iterator;
Zh{Pzyp [font="Times]import java.io.FileReader;
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B r`Xw^S [font="Times]public class MapExample {
n0>#?ek12 [font="Times] public static void main(String[] args) throws java.io.FileNotFoundException {
&Jk0SUk MP [font="Times] Map word_count_map = new HashMap();
uPr@xff [font="Times] FileReader reader = new FileReader(args[0]);
Gi6sl_"q [font="Times] Iterator words = new WordStreamIterator(reader);
QyJ}zwD [font="Times]
9=ns.r [font="Times] while ( words.hasNext() ) {
t}c ymX~ [font="Times] String word = (String) words.next();
Eu l,1yR [font="Times] String word_lowercase = word.toLowerCase();
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[font="Times] Integer frequency = (Integer)word_count_map.get(word_lowercase);
*6df|q [font="Times]if ( frequency == null ) {
}NMA($@A [font="Times] frequency = new Integer(1);
c] $X+ [font="Times] } else {
8jMw7ti [font="Times] int value = frequency.intValue();
=A$5~op% [font="Times] frequency = new Integer(value + 1);}
PL@hsZty~c [font="Times] word_count_map.put(word_lowercase, frequency);
i'H{cN6 [font="Times] }
8uP,#D<wZ [font="Times] System.out.println(word_count_map);
z{]$WVs:^ [font="Times] }}
4;<?ec(dc lr=? &>MXj eY-W5TgU [font="Times]Queue举例:
g1@zk$ [font="Times]import java.io.IOException;
/a[i:Oa# [font="Times]import java.io.PrintStream;
5@%$M$E [font="Times]import java.util.LinkedList;
T4]/w|?G [font="Times]import java.util.Queue;
AA6_D?)vv [font="Times]
WldlN?[j [font="Times]public class QueueTester {
ZNuz%VO [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
Pv17wUB [font="Times] public QueueTester() {q = new LinkedList();}
?T3zA2 [font="Times]public void testFIFO(PrintStream out) throws IOException {
Ynl^Z [font="Times] q.add("First");
!trt]?*- [font="Times] q.add("Second");
64%P}On [font="Times] q.add("Third");
f*,jhJ_I [font="Times] Object o;
GbUw:I [font="Times] while ((o = q.poll()) != null) {
v*Ds:1"H-I [font="Times] out.println(o);}}
t3|If@T [font="Times] public static void main(String[] args) {
-*~= 4m< [font="Times] QueueTester tester = new QueueTester();
Se.\wkl#Y [font="Times] try { tester.testFIFO(System.out);
cY8XA6 [font="Times] } catch (IOException e) {
{F6>XuS=u [font="Times] e.printStackTrace(); } }}
Jz&a9 =iW!Mq ##+|zka!U 上述例子和一些相关例子(共6个例子)打包下载[font="Times]
cXU8}>qY7 总结:
L.ScC 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。
"(zvI>A 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。