全新java初学者实践教程10(java SE5.0版)
{f06Ki jdk5的集合类
:{u`qi |q`NJ VL%. maj =<]`'15"V 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。
7_~ A*LM 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。
d$IROZK-D [font="Times] 集合类是放在[font="Times]java.util.*;这个包里。集合类存放的都是对象的引用,而非对象本身,为了说起来方便些,我们称集合中的对象就是指集合中对象的引用([font="Times]reference)。引用的概念大家不会忘了吧,在前边我们讲
数据类型时讲的。
H'AN osv Ft5A(P > [font="Times] 集合类型主要有[font="Times]3种:[font="Times]set(集)、[font="Times]list(列表)、[font="Times]map(映射[font="Times])和[font="Times]Queue(队列)。[font="Times]//队列为[font="Times]jdk5中的加上的[font="Times]
*%xbn8 Y ^^4n$ [font="Times](1) Set
4m*)("H 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。
XkI'm\W Q)75?mn ([font="Times]2)[font="Times]List
+p 6Ty2rz 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。
xHgC':l(0 (p]FI# y ?Y"%BS+pt [font="Times] ([font="Times]3)[font="Times]Map
161P%sGx2 [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。
,Ckcc la[pA TY8gB!^ [font="Times] ([font="Times]4)[font="Times]Queue
_a09;C [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。
AVT% AS [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:
ETZf U]hqRL [@@{z9c 实践:[font="Times] Set举例
U4XW
Kwq [font="Times] import java.util.*;
*p/,Z2f [font="Times]public class SetExample {
^h?fr` [font="Times] public static void main(String[] args) {
@O"7@%nu [font="Times] Set set = new HashSet(); //HashSet是[font="Times]Set的子接口
zgD?e?yPO [font="Times] set.add("one");
Q68~D.V%r [font="Times] set.add("second");
L0w6K0J4 [font="Times] set.add("3rd");
1UP
{j`-K| [font="Times] set.add(new Integer(4));
6_mi9_w [font="Times] set.add(new Float( 5.0F ));
h<9vm[ . [font="Times] set.add("second");
7FH(C`uKi [font="Times] set.add(new Integer(4));
_k:8ib2TQ [font="Times] System.out.println(set);
!}Xoqamm [font="Times] }}
APu$t$dmm |5u~L#P [font="Times]List举例:
KL \>-
[font="Times] import java.util.*;
t6Iy5)=zY [font="Times]public class ListExample {
BU -;P [font="Times] public static void main(String[] args) {
t/|0"\ p [font="Times] List list = new ArrayList();
gIo\^ktW [font="Times] list.add("one");
aM5]cc% [font="Times] list.add("second");
=3-=p&* [font="Times] list.add("3rd");
3IYFvq~ [font="Times] list.add(new Integer(4));
kf@JEcKV [font="Times] list.add(new Float( 5.0F ));
1PY]Q{r [font="Times] list.add("second");
"Enb [font="Times] list.add(new Integer(4));
4cQP+ n [font="Times] System.out.println(list);
KV0*dB; [font="Times] }}
e!6yxL*[@[ Bx45yaT A]c'TT@6 [font="Times]Map举例
bM?gAY]mB8 [font="Times]import java.util.Map;
dN5{W0_ [font="Times]import java.util.HashMap;
8N&'n [font="Times]import java.util.Iterator;
oAO{4xP [font="Times]import java.io.FileReader;
XG|N$~N+ 2 [font="Times]
}
=OE.cf@ [font="Times]public class MapExample {
Kx9u|fp5 [font="Times] public static void main(String[] args) throws java.io.FileNotFoundException {
E2DfG^sGV [font="Times] Map word_count_map = new HashMap();
YR'F]FI [font="Times] FileReader reader = new FileReader(args[0]);
l'I:0a
4T [font="Times] Iterator words = new WordStreamIterator(reader);
)<5k+O~ [font="Times]
)j;^3LiV3 [font="Times] while ( words.hasNext() ) {
=p+n(C/ [font="Times] String word = (String) words.next();
W&5/1``u\ [font="Times] String word_lowercase = word.toLowerCase();
_X#R v2a [font="Times] Integer frequency = (Integer)word_count_map.get(word_lowercase);
L[<#>/NPy [font="Times]if ( frequency == null ) {
;6/WjUDw<| [font="Times] frequency = new Integer(1);
m>=DJ{KQ [font="Times] } else {
SKC;@? [font="Times] int value = frequency.intValue();
DS?.'"n[u [font="Times] frequency = new Integer(value + 1);}
Pn!~U] A$% [font="Times] word_count_map.put(word_lowercase, frequency);
!.P||$x`& [font="Times] }
!E$$FvL [font="Times] System.out.println(word_count_map);
n])#<0 [font="Times] }}
Wt/;iq" _ [k
\S|iY z~Q=OPCnY [font="Times]Queue举例:
aL1%BGlmZ< [font="Times]import java.io.IOException;
-
lX4; [font="Times]import java.io.PrintStream;
1$b@C-B@g [font="Times]import java.util.LinkedList;
exq5Z c% [font="Times]import java.util.Queue;
L-+g` [font="Times]
6R45+<. [font="Times]public class QueueTester {
}AS?q?4? [font="Times] public Queue q; //发现了一个奇怪的语法,这个尖括号是泛型声明
{+9RJmZg [font="Times] public QueueTester() {q = new LinkedList();}
)Qb,zS6 [font="Times]public void testFIFO(PrintStream out) throws IOException {
i~h@}0WR" [font="Times] q.add("First");
z}E_wg [font="Times] q.add("Second");
\%<M[r= [font="Times] q.add("Third");
[wQ48\^ [font="Times] Object o;
4r(0+SO [font="Times] while ((o = q.poll()) != null) {
o2
ng [font="Times] out.println(o);}}
vM/*S
6[ [font="Times] public static void main(String[] args) {
y,5qY}P+ [font="Times] QueueTester tester = new QueueTester();
wPg/.N9H [font="Times] try { tester.testFIFO(System.out);
/\%<VBx ?q [font="Times] } catch (IOException e) {
rZ?:$],U! [font="Times] e.printStackTrace(); } }}
JpS}X\]i JP4DV=}L 6]v} 上述例子和一些相关例子(共6个例子)打包下载[font="Times]
ET.jjV 总结:
MZGhN
brd 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。
l5-[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。