NoSuchMethodError 错误

用序列化方法写深克隆时出现这个错误。求高手解答

Exception in thread "main" java.lang.NoSuchMethodError: Country.setP(LPopulation;)V
at Client2_3.main(Client2_3.java:14)

import java.io.ByteArrayInputStream;

import java.io.ByteArrayOutputStream;

import java.io.IOException;

import java.io.ObjectInputStream;

import java.io.ObjectOutputStream;

import java.io.Serializable;





public class Client2_3 {

	public static void main(String[] args) {

		Country america = new Country();

		Population p1 = new Population();

		p1.setPpl(10000);

		america.setP(p1);

		int america_ppl = america.getP().getPpl();

		

		Country china = (Country) Country.deepClone(new Country());

		china.getP().setPpl(20000);

		int china_ppl = china.getP().getPpl();

		

		System.out.println("the population of America is "+america_ppl);

		System.out.println("the population of China is "+china_ppl);

	}



}



class Population implements Serializable {

	private int ppl;



	public int getPpl() {

		return ppl;

	}



	public void setPpl(int ppl) {

		this.ppl = ppl;

	}

	

}



class Country implements Serializable{

    private Population p;

    

    public Population getP() {

        return p;

    }

    public void setP(Population p) {

        this.p = p;

    }

    public static Object deepClone(Object src) {

        Object dst = null;

        try {

            ByteArrayOutputStream out = new ByteArrayOutputStream();

            ObjectOutputStream oo = new ObjectOutputStream(out);

            oo.writeObject(src);



            ByteArrayInputStream in = new ByteArrayInputStream(out.toByteArray());

            ObjectInputStream oi = new ObjectInputStream(in);

            dst = oi.readObject();

        } catch (IOException e) {

            e.printStackTrace();

        } catch (ClassNotFoundException e) {

            e.printStackTrace();

        }       

        return dst;

    }

}