2016年关于Java编程与程序运行结果笔试题

发布时间:2016-10-24 00:00:00 编辑:嘉辉 手机版

  1.Java编程,打印昨天的当前时刻

  public class YesterdayCurrent{

  public void main(String[] args){

  Calendar cal = Calendar.getInstance();

  cal.add(Calendar.DATE, -1);

  System.out.println(cal.getTime());

  }

  }

  2.文件读写,实现一个计数器

  public int getNum(){

  int i = -1;

  try{

  String stri="";

  BufferedReader in = new BufferedReader(new FileReader(f));

  while((stri=in.readLine())!=null){

  i = Integer.parseInt(stri.trim());

  }

  in.close();

  }catch(Exception e){

  e.printStackTrace();

  }

  return i;

  }

  public void setNum(){

  int i = getNum();

  i++;

  try{

  PrintWriter out=new PrintWriter(new BufferedWriter(new FileWriter(f,false)));

  out.write(String.valueOf(i)); //可能是编码的原因,如果直接写入int的话,将出现java编码和windows编码的混乱,因此此处写入的是String

  out.close() ;

  }catch(Exception e){

  e.printStackTrace();

  }

  }

  3. 指出下面程序的运行结果:

  class A{

  static{

  System.out.print("1");

  }

  public A(){

  System.out.print("2");

  }

  }

  class B extends A{

  static{

  System.out.print("a");

  }

  public B(){

  System.out.print("b");

  }

  }

  public class Hello{

  public static void main(String[] ars){

  A ab = new B(); //执行到此处,结果: 1a2b

  ab = new B(); //执行到此处,结果: 1a2bab

  }

  }

  注:类的static 代码段,可以看作是类首次加载(被虚拟机加载)执行的代码,而对于类的加载,首先要执行其基类的构造,再执行其本身的构造

  4.写一个Singleton模式的例子

  public class Singleton{

  private static Singleton single = new Singleton();

  private Singleton(){}

  public Singleton getInstance(){

  return single;

  }

  }

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