Thread: Why does my code not work?

Why does my code not work?

package demo;
 
import java.util.List;
 
public class Student {
    private String naam;
    private int num;
    private List<Student> students;
 
    public Student() {
    }
 
    public Student(String naam, int num) {
        this.naam = naam;
        this.num = num;
    }
 
    public void setStudents(Student student){
        students.add((Student) students);
    }
 
    public List<Student> getStudents() {
        return students;
    }
 
    public static void main(String[] args) {
        Student student = new Student();
        student.setStudents(new Student("Jen", 27));
        student.setStudents(new Student("Ruurs", 28));
 
        List<Student> stud = student.getStudents();
 
        for (Student students: stud){
            System.out.println("Name "+ students.naam + " number "+ students.num);
        }
 
    }
}

I don't get a clear error message

Task :Student.main() FAILED
Exception in thread "main" java.lang.NullPointerException
at demo.Student.setStudents(Student.java:19)
at demo.Student.main(Student.java:28)

FAILURE: Build failed with an exception.

* What went wrong:
Execution failed for task ':Student.main()'.
> Process 'command 'C:/Program Files/Java/jdk-11.0.2/bin/java.exe'' finished with non-zero exit value 1

Did you mean student in the add method?

[code]
public void setStudents(Student student){
students.add((Student) student);
}
[code]