代码与编程题
一个整数，大于0，不用循环和本地变量，按照n，2n，4n，8n的顺序递增，当值大于5000时，把值按照指定顺序输出来。例：n=1237则输出为：1237，2474，4948，9896，9896，4948，2474，1237，
public class S{public static void main(String[] args){printOne(1237);}public static void printOne(int n){System.out.println(n);if(n<5000)printOne(2*n);else{System.out.println(n);printTwo(n/2);}}public static void printTwo(int n){if(n>=1237){System.out.println(n);printTwo(n/2);}}}
1、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(); } } }
15、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()%SplitByte==0)?(SplitStr.length()/SplitByte):(SplitStr.length()/Split Byte+1); System.out.println("Will Split into "+loopCount); for (int i=1;i<=loopCount ;i++ ) { if (i==loopCount){ System.out.println(SplitStr.substring((i-1)*SplitByte,SplitStr.length())); } else { System.out.println(SplitStr.substring((i-1)*SplitByte,(i*SplitByte))); } } } 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(); } }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.ChildClassFatherClass CreateFatherClass CreateChildClass 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/OuterClassInterClass CreateOuterClass 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<num ;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<al.size();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<MaxSize ;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<al.size();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：
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()%SplitByte==0)?(SplitStr.length()/SplitByte):(SplitStr.length()/Split
Byte+1); System.out.println("Will Split into "+loopCount); for (int i=1;i<=loopCount ;i++ ) { if (i==loopCount){
System.out.println(SplitStr.substring((i-1)*SplitByte,SplitStr.length())); } else {
System.out.println(SplitStr.substring((i-1)*SplitByte,(i*SplitByte))); } } } 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。 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();}}}}
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<numPages*(currentPage-1))&&rs.next()){i++;}%>//输出内容//输出翻页连接合计:<%=currentPage%>/<%=intPageCount%><a href="List.jsp?page=1">第一页</a><a
href="List.jsp?page=<%=upPage%>">上一页</a><% for(int j=1;j<=intPageCount;j++){ if(currentPage!=j){%> <a href="list.jsp?page=<%=j%>">[<%=j%>]</a> <% }else{ out.println(j); } }%><a href="List.jsp?page=<%=nextPage%>">下一页</a><a href="List.jsp?page=<%=intPageCount%>">最后页
</a>
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：<?xml version="1.0" encoding="gb2312"?> <person> <name>王小明</name> <college>信息学院</college> <telephone>6258113</telephone> <notes>男,1955年生,博士，95年调入海南大学</notes> </person> 事件回调类SAXHandler.java import java.io.*; import java.util.Hashtable; import org.xml.sax.*; public class SAXHandler extends HandlerBase { private Hashtable table = new Hashtable(); private String currentElement = null; private String currentValue = null; public void setTable(Hashtable table) { this.table = table; } public Hashtable getTable() { return table; } public void startElement(String tag, AttributeList attrs) throws SAXException { currentElement = tag; } public void characters(char[] ch, int start, int length) throws SAXException { currentValue = new String(ch, start, length); } public void endElement(String name) throws SAXException { if (currentElement.equals(name)) table.put(currentElement, currentValue); } } JSP内容显示源码,SaxXml.jsp:<HTML> <HEAD> <TITLE>剖析XML文件people.xml</TITLE> </HEAD> <BODY> <%@ page errorPage="ErrPage.jsp" contentType="text/html;charset=GB2312" %> <%@ page import="java.io.*" %> <%@ page import="java.util.Hashtable" %> <%@ page import="org.w3c.dom.*" %> <%@ page import="org.xml.sax.*" %> <%@ page import="javax.xml.parsers.SAXParserFactory" %> <%@ page import="javax.xml.parsers.SAXParser" %> <%@ page import="SAXHandler" %> <% File file = new File("c:\\people.xml"); FileReader reader = new FileReader(file); Parser parser; SAXParserFactory spf = SAXParserFactory.newInstance(); SAXParser sp = spf.newSAXParser(); SAXHandler handler = new SAXHandler(); sp.parse(new InputSource(reader), handler); Hashtable hashTable = handler.getTable(); out.println("<TABLE BORDER=2><CAPTION>教师信息表</CAPTION>"); out.println("<TR><TD>姓名</TD>" + "<TD>" + (String)hashTable.get(new String("name")) + "</TD></TR>"); out.println("<TR><TD>学院</TD>" + "<TD>" + (String)hashTable.get(new String("college"))+"</TD></TR>"); out.println("<TR><TD>电话</TD>" + "<TD>" + (String)hashTable.get(new String("telephone")) + "</TD></TR>"); out.println("<TR><TD>备注</TD>" + "<TD>" + (String)hashTable.get(new String("notes")) + "</TD></TR>"); out.println("</TABLE>"); %> </BODY> </HTML>
146、EJB的基本架构答:一个EJB包括三个部分: Remote Interface 接口的代码 package Beans; import javax.ejb.EJBObject; import java.rmi.RemoteException; public interface Add extends EJBObject { //some method declare } Home Interface 接口的代码 package Beans; import java.rmi.RemoteException; import jaax.ejb.CreateException; import javax.ejb.EJBHome; public interface AddHome extends EJBHome { //some method declare } EJB类的代码 package Beans; import java.rmi.RemoteException; import javax.ejb.SessionBean; import javx.ejb.SessionContext; public class AddBean Implements SessionBean { //some method declare }
147、如何校验数字型?var re=/^\d{1,8}$|\.\d{1,2}$/;var str=document.form1.all(i).value;var r=str.match(re);if (r==null){ sign=-4; break;}else{ document.form1.all(i).value=parseFloat(str);}
148、将一个键盘输入的数字转化成中文输出(例如:输入:1234567 输出:一百二拾三万四千五百六拾七)用java语言实现,,请编一段程序实现! public class Reader { private String strNum; private String strNumChFormat; private String strNumTemp; private int intNumLen; private String strBegin; public Reader(String strNum) { this.strNum = strNum; } public boolean check(String strNum) { boolean valid = false; 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 <number>"); } }}
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abstractmethod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.javaclass 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();} } }}