求帮忙分析一个泛型问题。 - 黑马程序员技术交流社区

package test;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;
import java.util.ArrayList;
import java.util.Collection;
import java.util.HashSet;
import java.util.Iterator;
public class Test2 {
public static void main(String[] args)throws Exception {
ArrayList<Worker> al = new ArrayList<Worker>();
al.add(new Worker("张三"));
al.add(new Worker("李四"));
al.add(new Worker("王五"));
show1(al);
show2(al);
show3(al);
show4(al);
HashSet<Student> hs = new HashSet<Student>();
hs.add(new Student("赵六"));
hs.add(new Student("冯七"));
hs.add(new Student("天一"));
show1(hs);
show2(hs);
show3(hs);
show4(hs);
}
private static <Person> void show1(Collection<Person> al) throws NoSuchMethodException, SecurityException, IllegalAccessException, IllegalArgumentException, InvocationTargetException {//此处为什么不能用Person,必须用通用符吗，为什么也不能用Object,不是说父类指向子类吗，为什么写父类类型不行？
Iterator<Person> it = al.iterator();
while(it.hasNext()) {
Person p = it.next();//<font color="#ff0000">这里的p不知道是什么。没法调用Person类里的getName()方法。其他3种泛型的show方法里p都可以调用getName()方法。</font>
Method method = p.getClass().getMethod("getName");
System.out.println(method.invoke(p));
}
}
private static <T>void show2(Collection<T> al) {
Iterator<T> it = al.iterator();
while(it.hasNext()) {
Person p = (Person)it.next();
System.out.println(p);
System.out.println( p.getName());
}
}
private static<Object> void show3(Collection<Object> al) {
Iterator<Object> it = al.iterator();
while(it.hasNext()) {
Person p = (Person)it.next();
System.out.println(p);
System.out.println(p.getName());
}
}//这么改，改成Object类型，为什么程序是对的
private static<Worker> void show4(Collection<Worker> al) {
Iterator<Worker> it = al.iterator();
while(it.hasNext()) {
Person p = (Person)it.next();
System.out.println(p);
System.out.println( p.getName());
}
}//如果类型改成Worker或者Student类型，为什么运行是对的，改成他们的父类就是错的呢，改成Object类型就是对的呢？？？
}

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package test;
public class Person {
private String name;
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public Person(String name) {
super();
this.name = name;
}
}

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package test;
public class Student extends Person {
public Student(String name) {
super(name);
}
}

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package test;
public class Worker extends Person {
public Worker(String name) {
super(name);
}
}

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