add object to ArrayList (object is from extends other class)

[mBull]

hi

i'm trying do develop a school

i created afew classes already:
school.class
person.class
student.class extends person
employee.class extends person

in the school class i want to create a method to add a student to the arraylist of persons and a method that only counts the objects wich are from the student class and another method wich is used to count the object from the employee class.

public class School {
    private ArrayList<Persons> persons = new ArrayList<Persons>();

	public void addemployee (Employee employee){
		if (employee !=null){
			personen.add(employee);
			}
// i think this doesn't work?
	}



public int countEmployee{
//don't know how to count these
}

Can you give me some hints?

[Steve11235]

There are several ways to do what you want to do. Here are a few.

Create separate lists for employees and students. You can combine the lists on demand in a getPersons() method.

Add counters for employees and students. Update the counters in the addEmployee(), addStudent(), etc. methods.

Loop through the list to count students and employees on demand.

[mBull]

yes, i had come to that solution aswell but I realy want only just one list of persons (the object student is sligthly different from the object employee) so isn't it possible for java to recognice each object?

[Steve11235]

If you have one list of Person, you can use the instanceof operator to determine which sub-type you are dealing with.

With two lists, you implement a method like this:

public List<Person> getPersons() {
  final List<Person> personList = new ArrayList<Person>();
  personList.addAll(studentList);
  personList.AddAll(employeeList);
  return personList;
}

This approach allows for much more flexibility as your design grows. You should always create a new List and return it, rather than the instance object, as the caller can modify the list you return.