NullPointerException in a simple ArrayList.add function?

Question

I'm getting used to java again and was experimenting with collections. I have the following very basic code but I can't seem to find the reason why I get an Nullpointer exception:

   import java.util.*;

    public class Event

{
    private ArrayList<String> fans;

    public Event()
    {
        ArrayList<String> fans = new ArrayList<String>();
    }

    public void registerUser(String user)
    {    
        fans.add(user);
    }
}

Thanks in advance everyone!

rgettman · Accepted Answer · 2014-12-02 21:35:04Z

4

You've initialized a local fans in your constructor, so your instance variable fans is not explicitly initialized, so it's still null in registerUser.

Change

ArrayList<String> fans = new ArrayList<String>();

to

fans = new ArrayList<String>();

answered Dec 2, 2014 at 21:35

rgettman

179k30 gold badges282 silver badges365 bronze badges

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

Comments

Alexis C. · Accepted Answer · 2014-12-02 21:34:59Z

4

You are shadowing your class field in your constructor. Remove the datatype declaration.

public Event(){
    fans = new ArrayList<String>();
}

answered Dec 2, 2014 at 21:34

Alexis C.

94.3k22 gold badges173 silver badges179 bronze badges

Collectives™ on Stack Overflow

NullPointerException in a simple ArrayList.add function?

2 Answers 2

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related