java继承问题 经常遇到保存下来

import java.util.ArrayList;
import java.util.HashMap;

public class Client {

/**
* @param args
*/
public static void main(String[] args) {
test1();
System.out.println("==================================");
test2();
}

public static void test1(){
ParentObj po = new ParentObj();
ArrayList list = new ArrayList();
po.doWork(list);

po = new ChildObj();
po.doWork(list);

ChildObj co = new ChildObj();
co.doWork(list);

}

public static void test2(){
ParentObj po = new ParentObj();
HashMap map = new HashMap();
po.doSport(map);

po = new ChildObj();
po.doSport(map);

ChildObj co = new ChildObj();
co.doSport(map);
}
}

import java.util.ArrayList;
import java.util.Map;

public class ParentObj {
public void doWork(ArrayList list) {
System.out.println("ParentObj doWork...");
}

public void doSport(Map map) {
System.out.println("ParentObj doSport...");
}
}
import java.util.HashMap;
import java.util.List;

public class ChildObj extends ParentObj {

public void doWork(List list) {
System.out.println("ChildObj doWork...");
}

public void doSport(HashMap map) {
System.out.println("ChildObj doSport...");
}

}
运行结果:
ParentObj doWork...
ParentObj doWork...
ParentObj doWork...
==================================
ParentObj doSport...
ParentObj doSport...
ChildObj doSport...