全新java初学者实践教程10(java SE5.0版)
eO<:X|9T jdk5的集合类
Q)pm3Wi Gp6|0:2,L~ NUB 3L yj]\%3o<Z7 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。
c o}o$} 我们上节课讲到了,数组的长度在创建时已经确定了,但是有时候我们事先根本不知道长度是多少啊,比如我们做电子商务网站时,有个购物车程序。你总不能用数组规定,人家只能买[font="Times]5样东西吧。你就是把长度定为[font="Times]10000也不行,万一遇上个特别有钱的呢!呵呵,这只是开玩笑的。我们会使用集合类解决这个问题。
4.@gV/U(| [font="Times] 集合类是放在[font="Times]java.util.*;这个包里。集合类存放的都是对象的引用,而非对象本身,为了说起来方便些,我们称集合中的对象就是指集合中对象的引用([font="Times]reference)。引用的概念大家不会忘了吧,在前边我们讲
数据类型时讲的。
I^'U_"vB >we/#C"x [font="Times] 集合类型主要有[font="Times]3种:[font="Times]set(集)、[font="Times]list(列表)、[font="Times]map(映射[font="Times])和[font="Times]Queue(队列)。[font="Times]//队列为[font="Times]jdk5中的加上的[font="Times]
[ Tv!Pc 6wV{}K^0 [font="Times](1) Set
3)SO-Bz\ 集([font="Times]set)是最简单的一种集合,它的对象不按特定方式排序,只是简单的把对象加入集合中,就像往口袋里放东西。对集中成员的访问和操作是通过集中对象的引用进行的,所以集中不能有重复对象。我们知道数学上的集合也是[font="Times]Set这个,集合里面一定是没有重复的元素的。
JStT"*4j E2f9J{Ki= ([font="Times]2)[font="Times]List
?<@yo&) 列表([font="Times]List)的主要特征是其对象以线性方式存储,没有特定顺序,只有一个开头和一个结尾,当然,它与根本没有顺序的[font="Times]Set是不同的。它是链表嘛,一条链肯定有顺序这个顺序就不一定了。
bY6y)l 5~WMb6/ t-#Y6U}b+ [font="Times] ([font="Times]3)[font="Times]Map
\W73W_P&g [font="Times] 映射([font="Times]Map),这个在[font="Times]java里不是地图的意思,其实地图也是映射哈。它里面的东西是键-值对([font="Times]key-value)出现的,键值对是什么呢?举个例子,比如我们查字典,用部首查字法。目录那个字就是键,这个字的解释就是值。键和值成对出现。这样说可以理解吧。这也是很常用的数据结构哦。
H}KJd5A7 !wl3}]q (bP\_F5D [font="Times] ([font="Times]4)[font="Times]Queue
Gx75EQ2 [font="Times] 在[font="Times]jdk5.0以前,通常的实现方式是使用[font="Times]java.util.List集合来模仿[font="Times]Queue。[font="Times]Queue的概念通过把对象添加(称为[font="Times]enqueuing的操作)到[font="Times]List的尾部(即[font="Times]Queue的后部)并通过从[font="Times]List的头部(即[font="Times]Queue的前部)提取对象而从[font="Times] List中移除(称为[font="Times]dequeuing的操作)来模拟。你需要执行先进先出的动作时可以直接使用[font="Times]Queue接口就可以了。
;dq AmBG{8 [font="Times] 这[font="Times]4个东西,有时候功能还不太完善,需要有些子类继承它的特性。[font="Times]Set的子接口有[font="Times]TreeSet,SortedSet,[font="Times]List的有[font="Times]ArrayList等,[font="Times]Map里有[font="Times]HashMap,HashTable等,[font="Times]Queue里面有[font="Times]BlockingQueue等。我们来看看例子吧:
q#W7.8 Z@ cB5|%@$I iRwqt-WZ 实践:[font="Times] Set举例
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dvs [font="Times] import java.util.*;
U4hsbraz [font="Times]public class SetExample {
enu",wC3 [font="Times] public static void main(String[] args) {
[&mYW.O< [font="Times] Set set = new HashSet(); //HashSet是[font="Times]Set的子接口
<{\UE~ [font="Times] set.add("one");
UYu 54`'kg [font="Times] set.add("second");
-:txmMT [font="Times] set.add("3rd");
nU Oy-c [font="Times] set.add(new Integer(4));
eit>4xMu [font="Times] set.add(new Float( 5.0F ));
MYqxkhcLH1 [font="Times] set.add("second");
*.ffyBI*~ [font="Times] set.add(new Integer(4));
^FLuhLS\* [font="Times] System.out.println(set);
7 R1;'/; [font="Times] }}
Z4#lZS`'A /uSEG<D [font="Times]List举例:
,"/<N*vh [font="Times] import java.util.*;
oL' :07_ [font="Times]public class ListExample {
gd9ZlHo'Id [font="Times] public static void main(String[] args) {
pH&Q]u;O [font="Times] List list = new ArrayList();
pf.T{/% [font="Times] list.add("one");
G6X [font="Times] list.add("second");
m9^?p [font="Times] list.add("3rd");
5" U8| [font="Times] list.add(new Integer(4));
^0 t81,` [font="Times] list.add(new Float( 5.0F ));
E.Hw|y0_(| [font="Times] list.add("second");
Q}!U4!{i|p [font="Times] list.add(new Integer(4));
-Kt36:| [font="Times] System.out.println(list);
_tE$a3` [font="Times] }}
mea]m)P Q$iGpTL >M7e'}0; [font="Times]Map举例
u(KeS` [font="Times]import java.util.Map;
i,/|H]Mzr [font="Times]import java.util.HashMap;
KZV$rJ%G [font="Times]import java.util.Iterator;
cm]D"GFLY [font="Times]import java.io.FileReader;
l7 D/]& [font="Times]
?9q{b\=l [font="Times]public class MapExample {
luZqW`?Bt [font="Times] public static void main(String[] args) throws java.io.FileNotFoundException {
Yyl2J#$! [font="Times] Map word_count_map = new HashMap();
k|l"Rh<\~ [font="Times] FileReader reader = new FileReader(args[0]);
8xUmg& [font="Times] Iterator words = new WordStreamIterator(reader);
;8sEE?C$g [font="Times]
(bo{vX [font="Times] while ( words.hasNext() ) {
hB:R8Y^?H [font="Times] String word = (String) words.next();
Fs:l"5~>1 [font="Times] String word_lowercase = word.toLowerCase();
Jrlc%,pZ [font="Times] Integer frequency = (Integer)word_count_map.get(word_lowercase);
BY:
cSqAW [font="Times]if ( frequency == null ) {
whP>'9t.w [font="Times] frequency = new Integer(1);
(E)/' sEb [font="Times] } else {
Xmy(pV!PF [font="Times] int value = frequency.intValue();
]4@z.1Mr [font="Times] frequency = new Integer(value + 1);}
Dbr(Wg [font="Times] word_count_map.put(word_lowercase, frequency);
st36xS [font="Times] }
/IVw}:G [font="Times] System.out.println(word_count_map);
,)+O.Lf7&. [font="Times] }}
FK!9to> g#=^U`y R{.wAH( [font="Times]Queue举例:
Ki-CJy [font="Times]import java.io.IOException;
z$p+l] [font="Times]import java.io.PrintStream;
=Feavyx [font="Times]import java.util.LinkedList;
nM8aC&Rd\ [font="Times]import java.util.Queue;
De|@}@ [font="Times]
PpN+q:( [font="Times]public class QueueTester {
WT(R =bLw [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
ox {Cm [font="Times] public QueueTester() {q = new LinkedList();}
O*oL(dk*8L [font="Times]public void testFIFO(PrintStream out) throws IOException {
3 Yl[J;i [font="Times] q.add("First");
9!V<=0b/ [font="Times] q.add("Second");
]\P [font="Times] q.add("Third");
iZ[o2Tre [font="Times] Object o;
,%dn)gt7 [font="Times] while ((o = q.poll()) != null) {
V&KH{j/P [font="Times] out.println(o);}}
M:?eK
[h [font="Times] public static void main(String[] args) {
M 0-> [font="Times] QueueTester tester = new QueueTester();
|6\ ?"# [font="Times] try { tester.testFIFO(System.out);
_}Jz_RS2` [font="Times] } catch (IOException e) {
B7t#H? [font="Times] e.printStackTrace(); } }}
%{/0K<M Uy ;oJY I}Q3B3Byg 上述例子和一些相关例子(共6个例子)打包下载[font="Times]
Fg4eIE-/M 总结:
n<yV]i$ 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。
TO[5h Y\ 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。