Adding a Object in a Objectlist (ArrayList)!

Question

i have three classes. 1.Class:`

public class Handler {
private String name;
private String short_name;
private int semester; 
private int modul_number; 
private String prof; 
private int credits; 
private double note; 
private ArrayList<Handler_date> dates;
public Handler() {
}
public Handler(String _name, String _short_name, int _semester, int _modul_number,
 String _prof, int _credits, double _mark) {
 this.modul_number=_modul_number;
 this.name = _name;
 this.short_name = _short_name;
 this.semester = _semester;
 this.prof = _prof;
 this.credits = _credits;
 this.note= _mark;
 dates = new ArrayList<Handler_date>();
}
public void add_date(String _room, int _time, 
 String _day) {
 Handler_date temp = new Handler_date(_room, _time, 
 _day);
 dates.add(temp);
}`

and the 2.class (Elementclass):

`public class Handler_date {
private String room; 
private int time; 
private String day; 
public Handler_date() {
}
public Handler_date(String _room, int _time, 
 String _day) {
 this.room = _room;
 this.time = _time;
 this.day = _day;
}

}

I want to add a modul, but i get a NullPointerException for dates.add(temp);

i call the method with templist.search_modul_number(modulnumber).add_date("room", 1, "monday");

My Handler-Objects are saved in a extra Objectlist/class to an arrayList ...private ArrayList<Handler> handlerlist; Anyone an idea what I am doing wrong?

Thanks!

• Jeffrey: What's the full stack trace of the NullPointerException?

Answer 1

Most likely you're not initializing private ArrayList<Handler_date> dates;.

You have a no-args constructor for Handler which doesn't create a list.

Change it to:

public Handler() {
 dates = new ArrayList<Handler_date>();
}

Answer 2

Your dates arraylist is not initialized. It is null, so you get this exception.

Are you calling new Handler(); - in that constructor, you don't init dates.

And in the future, please attach the full stacktrace to get help.

Answer 3

Your dates field is not initialized. Probably because you created Handler instance using no-arg constructor.