【Java SE】? extends T 和 ? super T

class Pair<T> {
    T value;
    public T getFirst() {
        return value;
    }
    public void setFirst(T value) {
        this.value = value;
    }
}
class Person { }
class Employee extends Person { }
class Manager extends Employee { }

Person person = new Person();
Employee employee = new Employee();
Manager manager = new Manager();

Pair<? super Employee> p1 = new Pair<>();
p1.setFirst(employee);// 存放自己和子类
Employee e1 = p1.getFirst();// 错误，超类限定符get失效

Pair<? extends Employee> p2 = new Pair<>();
p2.setFirst(employee);// 错误，子类限定符set失效
Employee e2 = p2.getFirst();// 接收自己和父类

 

? super T 超类限定符，下界限定符，导致get失效

元素是T的父类，

? extends T 子类限定符，上界限定符，导致set失效