代码与编程题
135、写一个Singleton出来
Singleton模式主要作用是保证在Java应用程序中,一个类Class只有一个实例存在。
一般Singleton模式通常有几种种形式:
第一种形式: 定义一个类,它的构造函数为private的,它有一个static的private的该类变量,在类初始化时实例话,通过一个public的getInstance方法获取对它的引用,继而调用其中的方法。
public class Singleton {
private Singleton(){}
//在自己内部定义自己一个实例,是不是很奇怪?
//注意这是private 只供内部调用
private static Singleton instance = new Singleton();
//这里提供了一个供外部访问本class的静态方法,可以直接访问
public static Singleton getInstance() {
return instance;
}
}
第二种形式:
public class Singleton {
private static Singleton instance = null;
public static synchronized Singleton getInstance() {
//这个方法比上面有所改进,不用每次都进行生成对象,只是第一次
//使用时生成实例,提高了效率!
if (instance==null)
instance=new Singleton();
return instance; }
}
其他形式:
定义一个类,它的构造函数为private的,所有方法为static的。
一般认为第一种形式要更加安全些
136、继承时候类的执行顺序问题,一般都是选择题,问你将会打印出什么?
答:父类:
package test;
public class FatherClass
{
public FatherClass()
{
System.out.println(“FatherClass Create”);
}
}
子类:
package test;
import test.FatherClass;
public class ChildClass extends FatherClass
{
public ChildClass()
{
System.out.println(“ChildClass Create”);
}
public static void main(String[] args)
{
FatherClass fc = new FatherClass();
ChildClass cc = new ChildClass();
}
}
输出结果:
C:>java test.ChildClass
FatherClass Create
FatherClass Create
ChildClass Create
137、内部类的实现方式?
答:示例代码如下:
package test;
public class OuterClass
{
private class InterClass
{
public InterClass()
{
System.out.println(“InterClass Create”);
}
}
public OuterClass()
{
InterClass ic = new InterClass();
System.out.println(“OuterClass Create”);
}
public static void main(String[] args)
{
OuterClass oc = new OuterClass();
}
}
输出结果:
C:>java test/OuterClass
InterClass Create
OuterClass Create
再一个例题:
public class OuterClass {
private double d1 = 1.0;
//insert code here
}
You need to insert an inner class declaration at line 3. Which two inner class declarations are
valid?(Choose two.)
A. class InnerOne{
public static double methoda() {return d1;}
}
B. public class InnerOne{
static double methoda() {return d1;}
}
C. private class InnerOne{
double methoda() {return d1;}
}
D. static class InnerOne{
protected double methoda() {return d1;}
}
E. abstract class InnerOne{
public abstract double methoda();
}
说明如下:
一.静态内部类可以有静态成员,而非静态内部类则不能有静态成员。 故 A、B 错
二.静态内部类的非静态成员可以访问外部类的静态变量,而不可访问外部类的非静态变量;return d1 出错。
故 D 错
三.非静态内部类的非静态成员可以访问外部类的非静态变量。 故 C 正确
四.答案为C、E
138、Java 的通信编程,编程题(或问答),用JAVA SOCKET编程,读服务器几个字符,再写入本地显示?
答:Server端程序:
package test;
import java.net.;
import java.io.;
public class Server
{
private ServerSocket ss;
private Socket socket;
private BufferedReader in;
private PrintWriter out;
public Server()
{
try
{
ss=new ServerSocket(10000);
while(true)
{
socket = ss.accept();
String RemoteIP = socket.getInetAddress().getHostAddress();
String RemotePort = “:”+socket.getLocalPort();
System.out.println(“A client come in!IP:”+RemoteIP+RemotePort);
in = new BufferedReader(new
InputStreamReader(socket.getInputStream()));
String line = in.readLine();
System.out.println(“Cleint send is :” + line);
out = new PrintWriter(socket.getOutputStream(),true);
out.println(“Your Message Received!”);
out.close();
in.close();
socket.close();
}
}catch (IOException e)
{
out.println(“wrong”);
}
}
public static void main(String[] args)
{
new Server();
}
};
Client端程序:
package test;
import java.io.;
import java.net.;
public class Client
{
Socket socket;
BufferedReader in;
PrintWriter out;
public Client()
{
try
{
System.out.println(“Try to Connect to 127.0.0.1:10000”);
socket = new Socket(“127.0.0.1”,10000);
System.out.println(“The Server Connected!”);
System.out.println(“Please enter some Character:”);
BufferedReader line = new BufferedReader(new
InputStreamReader(System.in));
out = new PrintWriter(socket.getOutputStream(),true);
out.println(line.readLine());
in = new BufferedReader(new InputStreamReader(socket.getInputStream()));
System.out.println(in.readLine());
out.close();
in.close();
socket.close();
}catch(IOException e)
{
out.println(“Wrong”);
}
}
public static void main(String[] args)
{
new Client();
}
};
139、用JAVA实现一种排序,JAVA类实现序列化的方法(二种)? 如在COLLECTION框架中,实现比较要实现什么样的接口?
答:用插入法进行排序代码如下
package test;
import java.util.;
class InsertSort
{
ArrayList al;
public InsertSort(int num,int mod)
{
al = new ArrayList(num);
Random rand = new Random();
System.out.println(“The ArrayList Sort Before:”);
for (int i=0;i
al.add(new Integer(Math.abs(rand.nextInt()) % mod + 1));
System.out.println(“al[”+i+"]="+al.get(i));
}
}
public void SortIt()
{
Integer tempInt;
int MaxSize=1;
for(int i=1;i
tempInt = (Integer)al.remove(i);
if(tempInt.intValue()>=((Integer)al.get(MaxSize-1)).intValue())
{
al.add(MaxSize,tempInt);
MaxSize++;
System.out.println(al.toString());
} else {
for (int j=0;j
if
(((Integer)al.get(j)).intValue()>=tempInt.intValue())
{
al.add(j,tempInt);
MaxSize++;
System.out.println(al.toString());
break;
}
}
}
}
System.out.println(“The ArrayList Sort After:”);
for(int i=0;i
System.out.println(“al[”+i+"]="+al.get(i));
}
}
public static void main(String[] args)
{
InsertSort is = new InsertSort(10,100);
is.SortIt();
}
}
140、编程:编写一个截取字符串的函数,输入为一个字符串和字节数,输出为按字节截取的字符串。 但是要保证汉字不被截半个,如“我ABC”4,应该截为“我AB”,输入“我ABC汉DEF”,6,应该输出为“我ABC”而不是“我ABC+汉的半个”。
答:代码如下:
package test;
class SplitString
{
String SplitStr;
int SplitByte;
public SplitString(String str,int bytes)
{
SplitStr=str;
SplitByte=bytes;
System.out.println(“The String is:’”+SplitStr+"’;SplitBytes="+SplitByte);
}
public void SplitIt()
{
int loopCount;
loopCount=(SplitStr.length()%SplitByte0)?(SplitStr.length()/SplitByte):(SplitStr.length()/Split
Byte+1);
System.out.println("Will Split into "+loopCount);
for (int i=1;i<=loopCount ;i++ )
{
if (iloopCount){
System.out.println(SplitStr.substring((i-1)*SplitByte,SplitStr.length()));
} else {
System.out.println(SplitStr.substring((i-1)SplitByte,(iSplitByte)));
}
}
}
public static void main(String[] args)
{
SplitString ss = new SplitString(“test中dd文dsaf中男大3443n中国43中国人
0ewldfls=103”,4);
ss.SplitIt();
}
}
141、JAVA多线程编程。 用JAVA写一个多线程程序,如写四个线程,二个加1,二个对一个变量减一,输出。
希望大家补上,谢谢
142、可能会让你写一段Jdbc连Oracle的程序,并实现数据查询.
答:程序如下:
package hello.ant;
import java.sql.;
public class jdbc
{
String dbUrl=“jdbc:oracle:thin:@127.0.0.1:1521:orcl”;
String theUser=“admin”;
String thePw=“manager”;
Connection c=null;
Statement conn;
ResultSet rs=null;
public jdbc()
{
try{
Class.forName(“oracle.jdbc.driver.OracleDriver”).newInstance();
c = DriverManager.getConnection(dbUrl,theUser,thePw);
conn=c.createStatement();
}catch(Exception e){
e.printStackTrace();
}
}
public boolean executeUpdate(String sql)
{
try
{
conn.executeUpdate(sql);
return true;
}
catch (SQLException e)
{
e.printStackTrace();
return false;
}
}
public ResultSet executeQuery(String sql)
{
rs=null;
try
{
rs=conn.executeQuery(sql);
}
catch (SQLException e)
{
e.printStackTrace();
}
return rs;
}
public void close()
{
try
{
conn.close();
c.close();
}
catch (Exception e)
{
e.printStackTrace();
}
}
public static void main(String[] args)
{
ResultSet rs;
jdbc conn = new jdbc();
rs=conn.executeQuery(“select * from test”);
try{
while (rs.next())
{
System.out.println(rs.getString(“id”));
System.out.println(rs.getString(“name”));
}
}catch(Exception e)
{
e.printStackTrace();
}
}
}
143、ORACLE大数据量下的分页解决方法。一般用截取ID方法,还有是三层嵌套方法。
答:一种分页方法
<%
int i=1;
int numPages=14;
String pages = request.getParameter(“page”) ;
int currentPage = 1;
currentPage=(pages==null)?(1):{Integer.parseInt(pages)}
sql = "select count() from tables";*
ResultSet rs = DBLink.executeQuery(sql) ;
while(rs.next()) i = rs.getInt(1) ;
int intPageCount=1;
intPageCount=(i%numPages==0)?(i/numPages):(i/numPages+1);
int nextPage ;
int upPage;
nextPage = currentPage+1;
if (nextPage>=intPageCount) nextPage=intPageCount;
upPage = currentPage-1;
if (upPage<=1) upPage=1;
rs.close();
sql=“select * from tables”;
rs=DBLink.executeQuery(sql);
i=0;
while((i
//输出内容
//输出翻页连接
合计:<%=currentPage%>/<%=intPageCount%>第一页上一页
<%
for(int j=1;j<=intPageCount;j++){
if(currentPage!=j){
%>
[<%=j%>]
<%
}else{
out.println(j);
}
}
%>
下一页最后页
144、用jdom解析xml文件时如何解决中文问题?如何解析?
答:看如下代码,用编码方式加以解决
package test;
import java.io.*;
public class DOMTest
{
private String inFile = “c:\people.xml”;
private String outFile = “c:\people.xml”;
public static void main(String args[])
{
new DOMTest();
}
public DOMTest()
{
try
{
javax.xml.parsers.DocumentBuilder builder =
javax.xml.parsers.DocumentBuilderFactory.newInstance().newDocumentBuilder();
org.w3c.dom.Document doc = builder.newDocument();
org.w3c.dom.Element root = doc.createElement(“老师”);
org.w3c.dom.Element wang = doc.createElement(“王”);
org.w3c.dom.Element liu = doc.createElement(“刘”);
wang.appendChild(doc.createTextNode(“我是王老师”));
root.appendChild(wang);
doc.appendChild(root);
javax.xml.transform.Transformer transformer =
javax.xml.transform.TransformerFactory.newInstance().newTransformer();
transformer.setOutputProperty(javax.xml.transform.OutputKeys.ENCODING, “gb2312”);
transformer.setOutputProperty(javax.xml.transform.OutputKeys.INDENT, “yes”);
transformer.transform(new javax.xml.transform.dom.DOMSource(doc),
new
javax.xml.transform.stream.StreamResult(outFile));
}
catch (Exception e)
{
System.out.println (e.getMessage());
}
}
}
145、编程用JAVA解析XML的方式.
答:用SAX方式解析XML,XML文件如下:
姓名 | " + (String)hashTable.get(new String("name")) + " |
学院 | " + (String)hashTable.get(new String("college"))+" |
电话 | " + (String)hashTable.get(new String("telephone")) + " |
备注 | " + (String)hashTable.get(new String("notes")) + " |
if (strNum.substring(0,1).equals("0")){
this.strNum = strNum.substring(1);
}
try {
new Double(strNum);
valid = true;
}
catch (NumberFormatException ex) {
System.out.println("Bad number format!");
}
return valid;
}
public void init() {
strNumChFormat = “”;
intNumLen = strNum.length();
strNumTemp = strNum;
strNumTemp = strNumTemp.replace(‘1’, ‘一’);
strNumTemp = strNumTemp.replace(‘2’, ‘二’);
strNumTemp = strNumTemp.replace(‘3’, ‘三’);
strNumTemp = strNumTemp.replace(‘4’, ‘四’);
strNumTemp = strNumTemp.replace(‘5’, ‘五’);
strNumTemp = strNumTemp.replace(‘6’, ‘六’);
strNumTemp = strNumTemp.replace(‘7’, ‘七’);
strNumTemp = strNumTemp.replace(‘8’, ‘八’);
strNumTemp = strNumTemp.replace(‘9’, ‘九’);
strNumTemp = strNumTemp.replace(‘0’, ‘零’);
strNumTemp = strNumTemp.replace(’.’, ‘点’);
strBegin = strNumTemp.substring(0, 1);
}
public String readNum() {
if (check(strNum)) {
init();
try {
for (int i = 1, j = 1, k = 1; i < intNumLen; i++) {
if (strNumTemp.charAt(intNumLen - 1) == ‘零’ && i == 1) {
strNumChFormat = “位”;
}
else if (strNumTemp.charAt(intNumLen - i) == ‘零’ && j == 1) {
strNumChFormat = “位” + strNumChFormat;
}
else if (strNumTemp.charAt(intNumLen - i) == ‘点’) {
j = 1;
k = 1;
strNumChFormat = strNumTemp.charAt(intNumLen - i) + strNumChFormat;
continue;
}
else {
strNumChFormat = strNumTemp.charAt(intNumLen - i) + strNumChFormat;
}
if (strNumTemp.charAt(intNumLen - i - 1) != ‘位’ &&
strNumTemp.charAt(intNumLen - i - 1) != ‘零’) {
if (j == 1 && i < intNumLen) {
strNumChFormat = ‘拾’ + strNumChFormat;
}
else if (j == 2 && i < intNumLen) {
strNumChFormat = ‘百’ + strNumChFormat;
}
else if (j == 3 && i < intNumLen) {
strNumChFormat = ‘千’ + strNumChFormat;
}
}
if (j == 4 && i < intNumLen) {
j = 0;
}
if (k == 4 && i < intNumLen) {
strNumChFormat = ‘万’ + strNumChFormat;
}
else if (k == 8 && i < intNumLen) {
k = 0;
strNumChFormat = ‘亿’ + strNumChFormat;
}
j++;
k++;
}
while (strNumChFormat.indexOf(“位”) != -1) {
strNumChFormat = strNumChFormat.replaceAll(“位”, " ");
}
if (strNumChFormat.substring(0, 2) == “一拾”) {
strNumChFormat = strNumChFormat.substring(1, strNumChFormat.length());
}
if (strNumChFormat.indexOf(“点”) >= 0) {
String rebegin = strNumChFormat.substring(0,
strNumChFormat.indexOf(“点”));
String relast = strNumChFormat.substring(strNumChFormat.indexOf(“点”),
strNumChFormat.length());
for (int i = 1; i <= relast.length(); i++) {
relast = relast.replaceAll(“拾”, “”);
relast = relast.replaceAll(“百”, “”);
relast = relast.replaceAll(“千”, “”);
relast = relast.replaceAll(“万”, “”);
relast = relast.replaceAll(“亿”, “”);
}
strNumChFormat = rebegin + relast;
}
}
catch (ArrayIndexOutOfBoundsException ex) {
ex.printStackTrace();
}
catch (Exception ex) {
ex.printStackTrace();
}
int off = strNumChFormat.indexOf(“点”);
strNumChFormat = strBegin + strNumChFormat.substring(0);
}
else {
strNumChFormat = “”;
}
return strNumChFormat;
}
public static void main(String args[]) {
try {
String number = args[0].toString();
System.out.println("The number is: " + number);
Reader reader = new Reader(number);
System.out.println("Output String: " + reader.readNum());
}
catch (Exception ex) {
System.out.println("Please input like that: javac Reader ");
}
}
}
149、JAVA代码查错
1.
abstract class Name {
private String name;
public abstract boolean isStupidName(String name) {}
}
大侠们,这有何错误?
答案: 错。abstract method必须以分号结尾,且不带花括号。
2.
public class Something {
void doSomething () {
private String s = “”;
int l = s.length();
}
}
有错吗?
答案: 错。局部变量前不能放置任何访问修饰符 (private,public,和protected)。final可以用来修饰局部变量
(final如同abstract和strictfp,都是非访问修饰符,strictfp只能修饰class和method而非variable)。
3.
abstract class Something {
private abstract String doSomething ();
}
这好像没什么错吧?
答案: 错。abstract的methods不能以private修饰。abstract的methods就是让子类implement(实现)具体细节的,怎么可以用private把abstract
method封锁起来呢? (同理,abstract method前不能加final)。
4.
public class Something {
public int addOne(final int x) {
return ++x;
}
}
这个比较明显。
答案: 错。int x被修饰成final,意味着x不能在addOne method中被修改。
5.
public class Something {
public static void main(String[] args) {
Other o = new Other();
new Something().addOne(o);
}
public void addOne(final Other o) {
o.i++;
}
}
class Other {
public int i;
}
和上面的很相似,都是关于final的问题,这有错吗?
答案: 正确。在addOne method中,参数o被修饰成final。如果在addOne method里我们修改了o的reference
(比如: o = new Other()?,那么如同上例这题也是错的。但这里修改的是o的member vairable
(成员变量),而o的reference并没有改变。
6.
class Something {
int i;
public void doSomething() {
System.out.println(“i = " + i);
}
}
有什么错呢? 看不出来啊。
答案: 正确。输出的是"i = 0”。int i属於instant variable (实例变量,或叫成员变量)。instant variable有default value。int的default value是0。
7.
class Something {
final int i;
public void doSomething() {
System.out.println(“i = " + i);
}
}
和上面一题只有一个地方不同,就是多了一个final。这难道就错了吗?
答案: 错。final int i是个final的instant variable (实例变量,或叫成员变量)。final的instant variable没有default value,必须在constructor (构造器)结束之前被赋予一个明确的值。可以修改为"final int i = 0;”。
8.
public class Something {
public static void main(String[] args) {
Something s = new Something();
System.out.println(“s.doSomething() returns " + doSomething());
}
public String doSomething() {
return “Do something …”;
}
}
看上去很完美。
答案: 错。看上去在main里call doSomething没有什么问题,毕竟两个methods都在同一个class里。但仔细看,main是static的。static method不能直接call non-static methods。可改成"System.out.println(“s.doSomething() returns " + s.doSomething());”。同理,static method不能访问non-static instant variable。
9.
此处,Something类的文件名叫OtherThing.java
class Something {
private static void main(String[] something_to_do) {
System.out.println(“Do something …”);
}
}
这个好像很明显。
答案: 正确。从来没有人说过Java的Class名字必须和其文件名相同。但public class的名字必须和文件名相同。
10.
interface A{
int x = 0;
}
class B{
int x =1;
}
class C extends B implements A {
public void pX(){
System.out.println(x);
}
public static void main(String[] args) {
new C().pX();
}
}
答案:错误。在编译时会发生错误(错误描述不同的JVM有不同的信息,意思就是未明确的x调用,两个x都匹配(就象在同时import java.util和java.sql两个包时直接声明Date一样)。对于父类的变量,可以用super.x来明确,而接口的属性默认隐含为 public static final.所以可以通过A.x来明确。
11.
interface Playable {
void play();
}
interface Bounceable {
void play();
}
interface Rollable extends Playable, Bounceable {
Ball ball = new Ball(“PingPang”);
}
class Ball implements Rollable {
private String name;
public String getName() {
return name;
}
public Ball(String name) {
this.name = name;
}
public void play() {
ball = new Ball(“Football”);
System.out.println(ball.getName());
}
}
这个错误不容易发现。
答案: 错。“interface Rollable extends Playable, Bounceable"没有问题。interface可继承多个interfaces,所以这里没错。问题出在interface Rollable里的"Ball ball = new Ball(“PingPang”);”。任何在interface里声明的interface variable (接口变量,也可称成员变量),默认为public static final。也就是说"Ball ball = new Ball(“PingPang”);“实际上是"public static final Ball ball = new Ball(“PingPang”);”。在Ball类的Play()方法中,“ball = new Ball(“Football”);“改变了ball的reference,而这里的ball来自Rollable interface,Rollable interface里的ball是public static final的,final的object是不能被改变reference的。因此编译器将在"ball = new Ball(“Football”);“这里显示有错。
28、设计4个线程,其中两个线程每次对j增加1,另外两个线程对j每次减少1。写出程序。
以下程序使用内部类实现线程,对j增减的时候没有考虑顺序问题。
public class ThreadTest1{
private int j;
public static void main(String args[]){
ThreadTest1 tt=new ThreadTest1();
Inc inc=tt.new Inc();
Dec dec=tt.new Dec();
for(int i=0;i<2;i++){
Thread t=new Thread(inc);
t.start();
t=new Thread(dec);
t.start();
}
}
private synchronized void inc(){
j++;
System.out.println(Thread.currentThread().getName()+”-inc:”+j);
}
private synchronized void dec(){
j–;
System.out.println(Thread.currentThread().getName()+”-dec:”+j);
}
class Inc implements Runnable{
public void run(){
for(int i=0;i<100;i++){
inc();
}
}
}
class Dec implements Runnable{
public void run(){
for(int i=0;i<100;i++){
dec();
}
}
}
}