全新java初学者实践教程10(java SE5.0版)
PzJ(Q jdk5的集合类
[N,+mX 7$*E0 Tvv>9gS r_+Vb*|Y 上次课我们学过了数组,知道它只是一组数(或是对象),但是有些自己的特性。在[font="Times]java里还有一类东西与数组类似,也是有着特性的一组数[font="Times](或是对象[font="Times]),叫做集合类。
=%U&$d|@G 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。
"51/,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。
h8/tKyr8( B-
@bU@H [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]
"Q@m7j)( klKUX/g [font="Times](1) Set
)Xdq+$w. 集([font="Times]set)是最简单的一种集合,它的对象不按特定方式排序,只是简单的把对象加入集合中,就像往口袋里放东西。对集中成员的访问和操作是通过集中对象的引用进行的,所以集中不能有重复对象。我们知道数学上的集合也是[font="Times]Set这个,集合里面一定是没有重复的元素的。
v!I z&M:z )@!fLAT ([font="Times]2)[font="Times]List
dA<%4_WZty 列表([font="Times]List)的主要特征是其对象以线性方式存储,没有特定顺序,只有一个开头和一个结尾,当然,它与根本没有顺序的[font="Times]Set是不同的。它是链表嘛,一条链肯定有顺序这个顺序就不一定了。
6 IvAs-%W .$\-{) 2J=`"6c [font="Times] ([font="Times]3)[font="Times]Map
=%` s-[5b [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。
-r*|N.5c #$UwJ B]_D onuG [font="Times] ([font="Times]4)[font="Times]Queue
d/
Lz" [font="Times] 在[font="Times]jdk5.0以前,通常的实现方式是使用[font="Times]java.util.List集合来模仿[font="Times]Queue。[font="Times]Queue的概念通过把对象添加(称为[font="Times]enqueuing的操作)到[font="Times]List的尾部(即[font="Times]Queue的后部)并通过从[font="Times]List的头部(即[font="Times]Queue的前部)提取对象而从[font="Times] List中移除(称为[font="Times]dequeuing的操作)来模拟。你需要执行先进先出的动作时可以直接使用[font="Times]Queue接口就可以了。
5(<O?#P [font="Times] 这[font="Times]4个东西,有时候功能还不太完善,需要有些子类继承它的特性。[font="Times]Set的子接口有[font="Times]TreeSet,SortedSet,[font="Times]List的有[font="Times]ArrayList等,[font="Times]Map里有[font="Times]HashMap,HashTable等,[font="Times]Queue里面有[font="Times]BlockingQueue等。我们来看看例子吧:
';R]`vWFe QGN+f) =-^A;AO( 实践:[font="Times] Set举例
x-i,v"8 [font="Times] import java.util.*;
S(.J [font="Times]public class SetExample {
vjX,7NY? [font="Times] public static void main(String[] args) {
P5my]4|x [font="Times] Set set = new HashSet(); //HashSet是[font="Times]Set的子接口
"G%S
m") [font="Times] set.add("one");
,$`}Rf< [font="Times] set.add("second");
t?9J'.p [font="Times] set.add("3rd");
?)9L($VVD [font="Times] set.add(new Integer(4));
<y7{bk~i [font="Times] set.add(new Float( 5.0F ));
db 99S [font="Times] set.add("second");
>_j(uw?u [font="Times] set.add(new Integer(4));
SY
Bp-o [font="Times] System.out.println(set);
+pYgh8w@ [font="Times] }}
e6p3!)@P1 7c\W&ZEmb- [font="Times]List举例:
A.*e8a/6X [font="Times] import java.util.*;
Rxdj}xy [font="Times]public class ListExample {
g=mKTk [font="Times] public static void main(String[] args) {
4}C
\N [font="Times] List list = new ArrayList();
L9) gN.# [font="Times] list.add("one");
y],opG6 [font="Times] list.add("second");
"6C
a{n1hk [font="Times] list.add("3rd");
q:kGJxfaW [font="Times] list.add(new Integer(4));
5&%M L [font="Times] list.add(new Float( 5.0F ));
d5-Q}D,P [font="Times] list.add("second");
PxYK)n9& [font="Times] list.add(new Integer(4));
h GA2.{ [font="Times] System.out.println(list);
G^{~'TZv% [font="Times] }}
"d<ucj 6"iNh) #pZeGI|'J [font="Times]Map举例
_1)n_P4 [font="Times]import java.util.Map;
~Bl,_?CBr [font="Times]import java.util.HashMap;
\JPMGcL [font="Times]import java.util.Iterator;
a=$ZM4Bn [font="Times]import java.io.FileReader;
xDeM7L' [font="Times]
aNry> 2: [font="Times]public class MapExample {
-`8@ [font="Times] public static void main(String[] args) throws java.io.FileNotFoundException {
f\5w@nX [font="Times] Map word_count_map = new HashMap();
2<*"@Vj [font="Times] FileReader reader = new FileReader(args[0]);
8(j]=n6r [font="Times] Iterator words = new WordStreamIterator(reader);
:.=:N%3[ [font="Times]
dWzf C@] [font="Times] while ( words.hasNext() ) {
}t#|+T2f [font="Times] String word = (String) words.next();
!84Lvg0& [font="Times] String word_lowercase = word.toLowerCase();
yl?LXc[) [font="Times] Integer frequency = (Integer)word_count_map.get(word_lowercase);
MP}-7UA#K [font="Times]if ( frequency == null ) {
P,ZQ*Ju [font="Times] frequency = new Integer(1);
oaha5aWH [font="Times] } else {
hqeknTGsIn [font="Times] int value = frequency.intValue();
+6>2= ,?Z [font="Times] frequency = new Integer(value + 1);}
r1F5'?NZ(0 [font="Times] word_count_map.put(word_lowercase, frequency);
G\tN(%.f [font="Times] }
Pz*BuL< [font="Times] System.out.println(word_count_map);
l\0PwD [font="Times] }}
[;hkT rXmrT%7k 0#GnmH [font="Times]Queue举例:
L[PqEN\i [font="Times]import java.io.IOException;
)'jGf;du [font="Times]import java.io.PrintStream;
M#Z^8( [font="Times]import java.util.LinkedList;
E
1`g8Hk' [font="Times]import java.util.Queue;
KT<i%)t2 [font="Times]
,X|FyO(p [font="Times]public class QueueTester {
@[joM*U [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
w}6~t\9D [font="Times] public QueueTester() {q = new LinkedList();}
\>4>sCC [font="Times]public void testFIFO(PrintStream out) throws IOException {
Ng<ic [font="Times] q.add("First");
o_\vudXK [font="Times] q.add("Second");
=oXlJ[)h [font="Times] q.add("Third");
AHr^G' [font="Times] Object o;
/V0Put [font="Times] while ((o = q.poll()) != null) {
]u<U[l-w [font="Times] out.println(o);}}
D(Z#um8n [font="Times] public static void main(String[] args) {
y}FG5'5$13 [font="Times] QueueTester tester = new QueueTester();
xN$V(ZX4 [font="Times] try { tester.testFIFO(System.out);
fFVQu\ [font="Times] } catch (IOException e) {
hQ>$"0K
[font="Times] e.printStackTrace(); } }}
N6kMl O<wH+k[ xK0;saG# 上述例子和一些相关例子(共6个例子)打包下载[font="Times]
KUPQ6v } 总结:
|H=5Am 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。
RF8,qz 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。