Add Object to array list. Get error when try add obect to ArrayList

Question

Could somebody answer why there is a problem with my array list. I have a classes: List, People and Main (to run everything).

In List I am creating a new ArrayList to hold objects of type People.

In Main I am making new List object, then make new People object and then calling from List object add method, and at this point I get a nullPointerException exception.

public class Main {
    public static void main(String[] args) {

        List l = new List();       // making new List object 
        People p = new People();   // making new People object

        l.addPeople(p);           // calling from List object "addPeople" method and
    }            

                 // parsing People object "p"
 }




import java.util.ArrayList;

public class List {

        public List(){           //constructor
    }

        ArrayList<People>list;      // new ArrayList to hold objects of type "People"

    public void addPeople(People people){   
        list.add(people);               // I get error here
    }
} 

public class People {

    public People(){         // constructor
    }
}

Thanks everyone, I thought that there is problem in constructor just could not understand where. Now it works, thank a lot to all once more time. — edi
– edi, Commented Apr 15, 2011 at 2:04

Ben · Accepted Answer · 2011-04-15 01:53:51Z

6

In the constructor:

list = new ArrayList<People>();

answered Apr 15, 2011 at 1:53

Ben

2,9472 gold badges26 silver badges33 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Vincent Ramdhanie · Accepted Answer · 2011-04-15 01:53:56Z

2

You did not instantiate the list at any time. In your constructor do this:

   public List(){           //constructor
          list = new ArrayList<People>();
   }

answered Apr 15, 2011 at 1:53

Vincent Ramdhanie

103k23 gold badges143 silver badges196 bronze badges

Comments

Larry Watanabe · Accepted Answer · 2011-04-15 01:53:58Z

2

I'm not sure if this is relevant, but it's a bad idea to name your class "List" since this will hide the List interface.

answered Apr 15, 2011 at 1:53

Larry Watanabe

10.2k9 gold badges45 silver badges48 bronze badges

2 Comments

edi Over a year ago

That's just for testing, in big program I wouldn't.

maaartinus Over a year ago

@Edgar You'd better never do that. The only things you can "gain" is to confuse yourself (and the people you show it to) and to have to rewrite it for use in a "big program".

SLaks · Accepted Answer · 2011-04-15 01:53:41Z

1

You need to put an ArrayList instance into your list field.

answered Apr 15, 2011 at 1:53

SLaks

891k182 gold badges1.9k silver badges2k bronze badges

Collectives™ on Stack Overflow

Add Object to array list. Get error when try add obect to ArrayList

4 Answers 4

Comments

Comments

2 Comments

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

4 Answers 4

Comments

Comments

2 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related