Beginner question about ArrayList

[kesi]

Hi all, I'm trying to implement an array list of generic data types.

Essentially I'm only using the following methods and getting the error:

Exception in thread "main" java.lang.NullPointerException
at ASet.toString(ASet.java:51)
at java.lang.String.valueOf(String.java:2838)
at java.lang.StringBuilder.append(StringBuilder.java: 132)
at TestASet.main(TestASet.java:8)

Can anyone tell me what I'm doing wrong here? Thanks in Advance!

import java.util.*;

public class ASet<T> implements SetInterface<T>
{

    private static Random rand = new Random();
    private ArrayList<T> store;

    public boolean isEmpty()
    {
	if (store.isEmpty()== true)
	    return true;
	else
	    return false;
    }

    public String toString()
    {
	return "{" + store.toString() + "}";
    }
}

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

ASet set4 = new ASet<Integer>();

      System.out.print("set4 = " + set4 + "\n");                     //toString() in action

      if(set4.isEmpty())                                             //isEmpty() in action
         System.out.println(set4 + " is empty");
      else
         System.out.println(set4 + " is not empty");
}}

[kesi]

OK, I see something see something obvious. It's null because I never instantiate the arraylist. Now I have to figure out how to do that. It's been awhile since I've had to do any of this. :)

[quad64bit]

store was never created in the ASet class. So,

if (store.isEmpty()== true)
	    return true;
	else
	    return false;

is checking the size of a null object. Create the array in the constuctor:

public class ASet<T> implements SetInterface<T>
{

    private static Random rand = new Random();
    private ArrayList<T> store;
    
    public ASet(){
        store = new ArrayList<T>();
    }

Also, why is your Random static? If this class NEEDS to be static for some reason, then you need to put object creation in a static{} block instead, but generally you should keep away from static -- also, create your objects in the constructor as a general rule of thumb, instead of creating then during declaration. For example, instead of:

public class ASet<T> implements SetInterface<T>
{
    private static Random rand = new Random();
    ...
}

Do this instead:

public class ASet<T> implements SetInterface<T>
{
    private Random rand;

    public ASet(){
         rand = new Random();
    }
    ...
}

[kesi]

I figured it out but thanks for the help!