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疯狂软件Java知识点大汇总之七


 

     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。

  JAVA编程题

  1.现在输入n个数字,以逗号,分开;然后可选择升或者降序排序;按提交键就在另一页面显示按什么排序,结果为,提供reset

  import java.util.*;

  public class bycomma{

  public static String[] splitStringByComma(String source){

  if(source==null||source.trim().equals(""))

  return null;

  StringTokenizer commaToker = new StringTokenizer(source,",");

  String[] result = new String[commaToker.countTokens()];

  int i=0;

  while(commaToker.hasMoreTokens()){

  result[i] = commaToker.nextToken();

  i++;

  }

  return result;

  }

  public static void main(String args[]){

  String[] s = splitStringByComma("5,8,7,4,3,9,1");

  int[] ii = new int[s.length];

  for(int i = 0;i

  ii[i] =Integer.parseInt(s[i]);

  }

  Arrays.sort(ii);

  //asc

  for(int i=0;i

  System.out.println(ii[i]);

  }

  //desc

  for(int i=(s.length-1);i>=0;i--){

  System.out.println(ii[i]);

  }

  }

  }

  2.金额转换,阿拉伯数字的金额转换成中国传统的形式如:(¥1011)->(一千零一拾一元整)输出。

  package test.format;

  import java.text.NumberFormat;

  import java.util.HashMap;

  public class SimpleMoneyFormat {

  public static final String EMPTY = "";

  public static final String ZERO = "零";

  public static final String ONE = "壹";

  public static final String TWO = "贰";

  public static final String THREE = "叁";

  public static final String FOUR = "肆";

  public static final String FIVE = "伍";

  public static final String SIX = "陆";

  public static final String SEVEN = "柒";

  public static final String EIGHT = "捌";

  public static final String NINE = "玖";

  public static final String TEN = "拾";

  public static final String HUNDRED = "佰";

  public static final String THOUSAND = "仟";

  public static final String TEN_THOUSAND = "万";

  public static final String HUNDRED_MILLION = "亿";

  public static final String YUAN = "元";

  public static final String JIAO = "角";

  public static final String FEN = "分";

  public static final String DOT = ".";

  private static SimpleMoneyFormat formatter = null;

  private HashMap chineseNumberMap = new HashMap();

  private HashMap chineseMoneyPattern = new HashMap();

  private NumberFormat numberFormat = NumberFormat.getInstance();

  private SimpleMoneyFormat() {

  numberFormat.setMaximumFractionDigits(4);

  numberFormat.setMinimumFractionDigits(2);

  numberFormat.setGroupingUsed(false);

  chineseNumberMap.put("0", ZERO);

  chineseNumberMap.put("1", ONE);

  chineseNumberMap.put("2", TWO);

  chineseNumberMap.put("3", THREE);

  chineseNumberMap.put("4", FOUR);

  chineseNumberMap.put("5", FIVE);

  chineseNumberMap.put("6", SIX);

  chineseNumberMap.put("7", SEVEN);

  chineseNumberMap.put("8", EIGHT);

  chineseNumberMap.put("9", NINE);

  chineseNumberMap.put(DOT, DOT);

  chineseMoneyPattern.put("1", TEN);

  chineseMoneyPattern.put("2", HUNDRED);

  chineseMoneyPattern.put("3", THOUSAND);

  chineseMoneyPattern.put("4", TEN_THOUSAND);

  chineseMoneyPattern.put("5", TEN);

  chineseMoneyPattern.put("6", HUNDRED);

  chineseMoneyPattern.put("7", THOUSAND);

  chineseMoneyPattern.put("8", HUNDRED_MILLION);

  }

  public static SimpleMoneyFormat getInstance() {

  if (formatter == null)

  formatter = new SimpleMoneyFormat();

  return formatter;

  }

  public String format(String moneyStr) {

  checkPrecision(moneyStr);

  String result;

  result = convertToChineseNumber(moneyStr);

  result = addUnitsToChineseMoneyString(result);

  return result;

  }

  public String format(double moneyDouble) {

  return format(numberFormat.format(moneyDouble));

  }

  public String format(int moneyInt) {

  return format(numberFormat.format(moneyInt));

  }

  public String format(long moneyLong) {

  return format(numberFormat.format(moneyLong));

  }

  public String format(Number moneyNum) {

  return format(numberFormat.format(moneyNum));

  }

  private String convertToChineseNumber(String moneyStr) {

  String result;

  StringBuffer cMoneyStringBuffer = new StringBuffer();

  for (int i = 0; i < moneyStr.length(); i++) {

  cMoneyStringBuffer.append(chineseNumberMap.get(moneyStr.substring(i, i + 1)));

  }

  //拾佰仟万亿等都是汉字里面才有的单位,加上它们

  int indexOfDot = cMoneyStringBuffer.indexOf(DOT);

  int moneyPatternCursor = 1;

  for (int i = indexOfDot - 1; i > 0; i--) {

  cMoneyStringBuffer.insert(i, chineseMoneyPattern.get(EMPTY + moneyPatternCursor));

  moneyPatternCursor = moneyPatternCursor == 8 ? 1 : moneyPatternCursor + 1;

  }

  String fractionPart = cMoneyStringBuffer.substring(cMoneyStringBuffer.indexOf("."));

  cMoneyStringBuffer.delete(cMoneyStringBuffer.indexOf("."), cMoneyStringBuffer.length());

  while (cMoneyStringBuffer.indexOf("零拾") != -1) {

  cMoneyStringBuffer.replace(cMoneyStringBuffer.indexOf("零拾"), cMoneyStringBuffer.indexOf("零拾") + 2, ZERO);

  }

  while (cMoneyStringBuffer.indexOf("零佰") != -1) {

  cMoneyStringBuffer.replace(cMoneyStringBuffer.indexOf("零佰"), cMoneyStringBuffer.indexOf("零佰") + 2, ZERO);

  }

  while (cMoneyStringBuffer.indexOf("零仟") != -1) {

  cMoneyStringBuffer.replace(cMoneyStringBuffer.indexOf("零仟"), cMoneyStringBuffer.indexOf("零仟") + 2, ZERO);

  }

  while (cMoneyStringBuffer.indexOf("零万") != -1) {

  cMoneyStringBuffer.replace(cMoneyStringBuffer.indexOf("零万"), cMoneyStringBuffer.indexOf("零万") + 2, TEN_THOUSAND);

  }

  while (cMoneyStringBuffer.indexOf("零亿") != -1) {

  cMoneyStringBuffer.replace(cMoneyStringBuffer.indexOf("零亿"), cMoneyStringBuffer.indexOf("零亿") + 2, HUNDRED_MILLION);

  }

  while (cMoneyStringBuffer.indexOf("零零") != -1) {

  cMoneyStringBuffer.replace(cMoneyStringBuffer.indexOf("零零"), cMoneyStringBuffer.indexOf("零零") + 2, ZERO);

  }

  if (cMoneyStringBuffer.lastIndexOf(ZERO) == cMoneyStringBuffer.length() - 1)

  cMoneyStringBuffer.delete(cMoneyStringBuffer.length() - 1, cMoneyStringBuffer.length());

  cMoneyStringBuffer.append(fractionPart);

  result = cMoneyStringBuffer.toString();

  return result;

  }

  private String addUnitsToChineseMoneyString(String moneyStr) {

  String result;

  StringBuffer cMoneyStringBuffer = new StringBuffer(moneyStr);

  int indexOfDot = cMoneyStringBuffer.indexOf(DOT);

  cMoneyStringBuffer.replace(indexOfDot, indexOfDot + 1, YUAN);