空中网的一个用线程操作集合遇到的问题

package com.test;

/**
 * 当每个线程中指定的KEY相同时，
 * 这些相同KEY的线程应每隔一秒依次输出时间值（互斥），
 * 如果KEY不同，则并行执行（相互之间不互斥）
 */
public class Test extends Thread
{

	private TestDo testDo;
	private String key = "1";
	private String value;
	
	public Test(String key,String key2,String value){
		testDo = TestDo.getInstance();
		this.key = new String(key+key2);
		this.value = value;
	}
	
	public static void main(String[] args)
	{
		Test a1 = new Test("1","","1");
		Test a2 = new Test("1","","2");
		Test a3 = new Test("1","","3");
		Test a4 = new Test("1","","4");
		
		Test b1 = new Test("2","","1");
		
		Test c1 = new Test("3","","1");
		
		Test d1 = new Test("4","","1");
		
		System.out.println("begin:"+(System.currentTimeMillis()/1000));
		
		a1.start();
		a2.start();
		a3.start();
		a4.start();
		
		b1.start();
		
		c1.start();
		
		d1.start();
	}
	
	@Override
	public void run()
	{
		try{
			testDo.doSome(key, value);
		}catch(Exception e){
			e.printStackTrace();
		}
	}

}

package com.test;

import java.util.Iterator;
import java.util.Random;
import java.util.Set;
import java.util.concurrent.CopyOnWriteArraySet;

public class TestDo
{
	private Set set = new CopyOnWriteArraySet();

	private TestDo()
	{
	};

	private static TestDo testDo = new TestDo();

	public static TestDo getInstance()
	{
		return testDo;
	}

	public void doSome(String key, String value) throws Exception
	{
		Object o = null;
		if (!set.contains(key))
		{
			set.add(key);
			o = key;
		}
		else
		{
			for(Iterator it = set.iterator();it.hasNext();){
				Object oo = it.next();
				Thread.sleep(new Random().nextInt(100));
				if(key.equals(oo)){
					o = oo;
					break;
				}
			}
		}
		
		synchronized(o){
			try{
				System.out.println(key + ":" + value + ":"
						+ (System.currentTimeMillis()/1000));
			}catch(Exception e){
				e.printStackTrace();
			}
			Thread.sleep(1000);
		}
	}

}