全新java初学者实践教程10(java SE5.0版)
.7Ys@;>B jdk5的集合类
ev#d1s|<S VE6T&fz` _d]w)YMO 0;3;Rs 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。
91%+Bf()J6 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。
7|T5N[3?l, [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?'HVx ]<4Yor}t{; [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]
naXo <B bu%@1:l [font="Times](1) Set
>;HbDp 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。
Cr4shdN34 HHCsWe- ([font="Times]2)[font="Times]List
{j,bV6X 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。
^Cyx"s't u-pE
;| RXt`y62yK [font="Times] ([font="Times]3)[font="Times]Map
S]9:3~ [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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nG ~0|~Fg [font="Times] ([font="Times]4)[font="Times]Queue
E[nW B"pxE [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。
"a5?cX; [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:
`wB(J%w A]BG* .+ai
dWd 实践:[font="Times] Set举例
idPkJf/ [font="Times] import java.util.*;
B HoZ}1_ [font="Times]public class SetExample {
E^1uZI\z [font="Times] public static void main(String[] args) {
VV?+q) [font="Times] Set set = new HashSet(); //HashSet是[font="Times]Set的子接口
mmTc.xh [font="Times] set.add("one");
W5_t/_EWD [font="Times] set.add("second");
6N~~:Gt [font="Times] set.add("3rd");
%nF6n:| : [font="Times] set.add(new Integer(4));
}/(fe`7: [font="Times] set.add(new Float( 5.0F ));
#R305 [font="Times] set.add("second");
HHWB_QaL [font="Times] set.add(new Integer(4));
Qgo0uuM [font="Times] System.out.println(set);
wR nt$1 [font="Times] }}
0RYh4'=F H `),PY2 [font="Times]List举例:
1U@qRU [font="Times] import java.util.*;
S<88>|&n] [font="Times]public class ListExample {
G:c)e,pD [font="Times] public static void main(String[] args) {
7I^(vQ [font="Times] List list = new ArrayList();
,Ztj [font="Times] list.add("one");
?%;7k'0" [font="Times] list.add("second");
Cj_cu [font="Times] list.add("3rd");
, YTuZS [font="Times] list.add(new Integer(4));
`-]*Qb+ [font="Times] list.add(new Float( 5.0F ));
*;Hvx32I [font="Times] list.add("second");
'Xxt[Jy [font="Times] list.add(new Integer(4));
3)atqM)i [font="Times] System.out.println(list);
8g(%6 ET [font="Times] }}
|"5NI'X? s$&:F4=? _+ oX9 [font="Times]Map举例
zK k;&y|{ [font="Times]import java.util.Map;
1 c3gHc7{t [font="Times]import java.util.HashMap;
.N5}JUj [font="Times]import java.util.Iterator;
$;@^coz9U [font="Times]import java.io.FileReader;
w%S\)wjS [font="Times]
9p+DAs{i [font="Times]public class MapExample {
(pREo/ T [font="Times] public static void main(String[] args) throws java.io.FileNotFoundException {
ou6yi;
l% [font="Times] Map word_count_map = new HashMap();
)f*&}SV [font="Times] FileReader reader = new FileReader(args[0]);
Gi6sl_"q [font="Times] Iterator words = new WordStreamIterator(reader);
Wf-i)oc4I [font="Times]
['tGc{4 [font="Times] while ( words.hasNext() ) {
^0pd- n@pn [font="Times] String word = (String) words.next();
(6^v`SZ [font="Times] String word_lowercase = word.toLowerCase();
Ldf< [font="Times] Integer frequency = (Integer)word_count_map.get(word_lowercase);
\WQ\q
\ [font="Times]if ( frequency == null ) {
=n
cu#T] [font="Times] frequency = new Integer(1);
#i%it [font="Times] } else {
-,uTAk0+@ [font="Times] int value = frequency.intValue();
.Quu_S_vH [font="Times] frequency = new Integer(value + 1);}
UP58Cln* [font="Times] word_count_map.put(word_lowercase, frequency);
5 qt]~v%y [font="Times] }
z{]$WVs:^ [font="Times] System.out.println(word_count_map);
3 i Id> [font="Times] }}
z4HIDb *l)_&p t:eZ`6o$T\ [font="Times]Queue举例:
f<=<:+ [font="Times]import java.io.IOException;
T4]/w|?G [font="Times]import java.io.PrintStream;
p^3]Q [font="Times]import java.util.LinkedList;
qg7.E+ [font="Times]import java.util.Queue;
f-F+Y`P [font="Times]
lG I1LUo [font="Times]public class QueueTester {
+0{m(%i [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
zflq|d W [font="Times] public QueueTester() {q = new LinkedList();}
%YkJA: [font="Times]public void testFIFO(PrintStream out) throws IOException {
6}6;%{p"Gu [font="Times] q.add("First");
LBq~?Q.e [font="Times] q.add("Second");
]i(tou-[i [font="Times] q.add("Third");
k@L},Td [font="Times] Object o;
q_bE?j{ [font="Times] while ((o = q.poll()) != null) {
|_"JyGR2 [font="Times] out.println(o);}}
(OwAhjHE [font="Times] public static void main(String[] args) {
A'vQtlvKA [font="Times] QueueTester tester = new QueueTester();
![h+R@_( [font="Times] try { tester.testFIFO(System.out);
IppzQ0'=y1 [font="Times] } catch (IOException e) {
Uy2NZ%rnt [font="Times] e.printStackTrace(); } }}
<o@&I "
o dw#K!,g M<s16 上述例子和一些相关例子(共6个例子)打包下载[font="Times]
jd8`D6|Z 总结:
,V1/(|[h 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。
m?'H7cFR 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。