Java下界类型List作为参数

Question

Hello I got two classes: Person class and Employee class which extends Person they look really simple

public class Person {
    private String name;
    private String surname;

    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public String getSurname() {
        return surname;
    }
    public void setSurname(String surname) {
        this.surname = surname;
    }
    public Person(String name, String surname) {
        super();
        this.name = name;
        this.surname = surname;
    }


}

public class Employee extends Person {
    private String salary;
    private String position;
    public String getSalary() {
        return salary;
    }
    public void setSalary(String salary) {
        this.salary = salary;
    }
    public String getPosition() {
        return position;
    }
    public void setPosition(String position) {
        this.position = position;
    }
    public Employee(String name, String surname, String salary, String position) {
        super(name, surname);
        this.salary = salary;
        this.position = position;
    }



}

I want to write a function which will be adding Person to lower bounded parametrized List

public class AuxiliaryClass {
    public static void addToList(List<? super Employee> list){
        Person osoba1 = new Person("Czeslaw","Spiewa");
        list.add(osoba1);
    }


}

But eclipse doesn't allow for it with error message: "The method add(capture#1-of ?super Employee)in the type List is not applicable for the arguments (Person)

Answer 1

List<? super Employee> List<? super Employee> means "a list of some unknown but concrete class X, that is a superclass of Employee .

It should be clear now why you cannot add an object of type Person to this list: Person is not X .

Perhaps, we could be of more help, if you explained what it is you are really trying to do here.

Update : Actually, I take it back. Person is X , because it is a subclass of Employee , which is a subclass of X . It should be possible to add it to the list. Looks like eclipse is just full of it. Javac (1.8) lets me do this, and so does intellij.

Sorry for the confusion.

Answer 2

You don't allowed to put Person instances into the List because when you call your function you can provide for example List collection. If you will be able to put Person into this collection, this would cause ClassCastException on accessing this collection content, because every collection item will be treated as Employee (but Person instance cannot be casted to Employee).

Answer 3

You should take a look at: https://en.wikipedia.org/wiki/Liskov_substitution_principle .

In this case you want to put Persion in list of Employee but not every persion is a Employee and that is why it fails. You would be able to to that if the list was expecting Persons or anything that extends Person.

List<? extends Person>

Then you could add a person but also a employee since every employee is a person.

Answer 4

To be clear I don't want to add to the List an object which is subtype of type Person. I want to add an object which is supertype (ancestor) of Employee type. I want to know if operation described above is possible?