package 泛型限定;
public class Person {
private String name;
public Person() {
// TODO Auto-generated constructor stub
}
public Person(String name) {
super();
this.name = name;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public void eat(){
System.out.println("人在吃饭");
}
public String toString()
{
return this.getName();
}
}
@SuppressWarnings("all")
class Student extends Person{
private String name;
public Student() {
// TODO Auto-generated constructor stub
}
public Student(String name) {
super();
this.name = name;
}
public void study(){
System.out.println("学生在学习");
}
}
@SuppressWarnings("all")
class Worker extends Person{
private String name;
public Worker() {
// TODO Auto-generated constructor stub
}
public Worker(String name) {
super();
this.name = name;
}
public void work(){
System.out.println("工人在工作");
}
}
///////////////////////
package 泛型限定;
import java.util.ArrayList;
import java.util.Iterator;
public class TextPerson {
public static void main(String[] args) {
ArrayList list=new ArrayList<>();
ArrayList list02=new ArrayList<>();
ArrayList list03=new ArrayList<>();
list.add(new Person("汪洋"));
list.add(new Person("刘林"));
list.add(new Person("张兆年"));
list02.add(new Student("学生一"));
list02.add(new Student("学生二"));
list02.add(new Student("学生三"));
list03.add(new Worker("工人一"));
list03.add(new Worker("工人二"));
list03.add(new Worker("工人三"));
paint(list);
paint(list02);
paint(list03);
System.out.println("==============");
paint2(list02);
paint2(list);
}
public static void paint(ArrayList<? extends Person> list){
Iterator<? extends Person> it=list.iterator();
while(it.hasNext())
{
System.out.println(it.next().getName());
}
}
public static void paint2(ArrayList<? super Student> list){
Iterator<? super Student> it=list.iterator();
while(it.hasNext())
{
System.out.println(it.next().toString());
}
}
}
///////////////////
运行结果为:
汪洋
刘林
张兆年
null
null
null
null
null
null
null
null
null
汪洋
刘林
张兆年
问题来了,为什么会出现null呢,是我继承的地方有什么没有弄懂吗,求解答!