java中类型通配符上限定(? extends T)指什么呢?

//定义一个Person类
class Person
{
	private String name;
	public Person(String name){
		this.name = name;
	}
	public String getNmae(){
		return this.name;
	}
}
// 定义一个Student 并继承 Person
class Student extends Person
{
	Student(String name){
		super(name);
	}
}

main方法如下
public static void main(String[] args) {
		
		Arraylist<Person> a1 = new ArrayList<Person>();
		a1.add(new Person("java265-1"));
		a1.add(new Person("java265-2"));
		a1.add(new Person("java265-3"));

		printMethod(a1);
		 
		//a2中存储的Person
		//存在继承的话
		//也能放Student
		//等号右边只能存Student
		//无法存储worker.类型安全问题。左右两边要一致
		ArrayList<Student> a2 = new ArrayList<Student>();
		a2.add(new Student("java265--1"));
		a2.add(new Student("java265--2"));
		a2.add(new Student("java265--3"));
		printMethod(a2); // ArrayList<Person> a2 = new ArrayList<Student>();
	}
	
	public static void printMethod(ArrayList< Person> a1){
		Iterator<Person> it = a1.iterator();
		while(it.hasNext()){
			System.out.println(it.next().getNmae());
		}
	}

如果想一个方法既能打印Student,Person
此时我们可以使用"上限符"

? extends T为泛型上限定:
   可以接收T和T的子类

 
	public static void printMethod(ArrayList<? extends Person> a1){
		Iterator<? extends Person> it = a1.iterator();
		while(it.hasNext()){
			System.out.println(it.next().getNmae());
		}
	}