java面试题系列1

package syn;

import java.util.ArrayList;
import java.util.Iterator;
import java.util.concurrent.CopyOnWriteArrayList;

 //不能改动此Test类 
 public class Test extends Thread{
  
  private TestDo testDo;
  private String key;
  private String value;
  
  public Test(String key,String key2,String value){
   this.testDo = TestDo.getInstance();
   /*常量"1"和"1"是同一个对象，下面这行代码就是要用"1"+""的方式产生新的对象，
   以实现内容没有改变，仍然相等（都还为"1"），但对象却不再是同一个的效果*/
   this.key = key+key2;   //变量,a = "1" + ""与b = "1" + ""不同
//   a = "1" + "";
//   b = "1" + "";  a,b此时是同一个对象，因为在编译时将常量a,b优化
   this.value = value;
  }

  
  public static void main(String[] args) throws InterruptedException{
   Test a = new Test("1","","1");
   Test b = new Test("1","","2");
   Test c = new Test("1","","3");
   Test d = new Test("1","","4");
   System.out.println("begin:"+(System.currentTimeMillis()/1000));
   a.start();
   b.start();
   c.start();
   d.start();

  }
  
  public void run(){
   testDo.doSome(key, value);
  }
 }

 class TestDo {

  private TestDo() {}
  private static TestDo _instance = new TestDo(); 
  public static TestDo getInstance() {
   return _instance;
  }

//  private ArrayList keys = new ArrayList();
  private CopyOnWriteArrayList keys = new CopyOnWriteArrayList();
  public void doSome(Object key, String value) {
 
   Object o = key;
   if(!keys.contains(o)){
    keys.add(o);
   }else{
    for(Iterator iter = keys.iterator(); iter.hasNext();){
     /*try {
      Thread.sleep(20);
     } catch (InterruptedException e) {
      // TODO Auto-generated catch block
      e.printStackTrace();
     }*/
     Object oo = iter.next();
     if(oo.equals(o)){
      o = oo;
     }
    }
   }
   synchronized(o)
   // 以大括号内的是需要局部同步的代码，不能改动!
   {
    try {
     Thread.sleep(1000);
     System.out.println(key+":"+value + ":"
       + (System.currentTimeMillis() / 1000));
    } catch (InterruptedException e) {
     e.printStackTrace();
    }
   }
  }

 }