Lỗi kế thừa trong java

class Person
{
    String name;
    String birthday;

    String getName()
    {
        return name;
    }
    void setName(String ten)
    {
        name = ten;
    }
    void setBirthday(String sn)
    {
        birthday = sn;
    }
    public String toString()
    {
        String temp = "name: " + name + " ; " + " ngay sinh: " + birthday;
        return temp;
    }
}

class Employee extends Person
{
    double salary;

    double getSalary()
    {
        return salary;
    }
    void setSalary(double hesoluong)
    {
        salary = hesoluong;
    }
    public String toString()
    {
        String temp = super.toString() + " ; he so luong: " + salary;
        return temp;
    }
}

class Manager extends Employee
{
    Employee assistant;

    void setAssistant(Employee assis)
    {
        assistant = assis;
    }
    public String toString()
    {
        String temp = "asistant: " + super.toString();
        return temp;
    }
}

public class b5_page_150
{
    public static void main(String[] args)
    {
        Person[] ps = new Person[3];
        ps[0] = new Person();
        ps[1] = new Employee();
        ps[2] = new Manager();

        ps[0].setName("ruoi muoi");
        ps[0].setBirthday("26/7/1999");

        System.out.println( ps[0].toString() );

        ps[1].setName("trau bo");
        ps[1].setBirthday("2605/199x");
        ps[1].setSalary(9.9);

        System.out.println( ps[1].toString() );

    }
}

tại sao em gọi hàm setSalary thì nó báo cant not find sysbol nhỉ