arraylist in self-designed class errors. please help me

Question

This is my Class

  1 //  This class reads in integers and puts the values into a set
  2 
  3 import java.util.ArrayList;
  4 import java.util.*;
  5 
  6 class Set {
  7 
  8     private ArrayList<Integer> members;
  9     private static int quantity = 0;
 10 
 11     // Constructors
 12     public Set()    {
 13         new ArrayList<Integer>();
  7 
  8     private ArrayList<Integer> members;
  9     private static int quantity = 0;
 10 
 11     // Constructors
 12     public Set()    {
 13         new ArrayList<Integer>();
 14     }
 15 
 16     public Set(int member){
 17         new ArrayList<Integer>();
 18         addMember(member);
 19         quantity++;
 20     }
 21     //  Accessors
 22     public static int getQuantity() {
 23         return quantity;
 24     }
 25 
 26     //  Mutators
 27     public void addMember(int member)   {
 28         members.add(member);
 29     }
 30     // toString() method
 31     public String toString() {
 32         int i;
 33         String str = "[";
 34         System.out.print("[");
 35         for(i=0; i<getQuantity(); i++){
 36             str += members.get(i);
 37             if(i+1 == getQuantity())
 38                 str += "]";
 39             else
 40                 System.out.print(", ");
 41                 str += ", ";
 42         }
 43         return str;
 44     }
 45 
 46     // Return true if 'this' is a subset of 'set', 
 47     // otherwise return false.
 48     public boolean isSubset(Set set) {
 49         if(this.members.contains(set))
 50             return true;
 51         else
 52             return false;
53     }
 54     // Return true if 'this' is equals to 'obj',
 55     // Otherwise return false
 56     public boolean equals(Set set)  {
 57         return (members.contains(set) && set.members.contains(this));
 58     }
 59 }

And this is my Test case.

  1 // This program reads two sets of integers A and B, and determines
  2 // if A is a subset of B, and if A is same as B.
  3 
  4 import java.util.Scanner;
  5 import java.util.ArrayList;
  6 
  7 public class TestSet {
  8 
  9     public static void main(String[] args) {
 10         Scanner sc = new Scanner(System.in);
 11         int i, setAnum, setBnum;
 12         
 13         System.out.print("Enter number of elements in set A: ");
 14         setAnum = sc.nextInt();
 15         ArrayList<Integer> list1 = new ArrayList<Integer>();
 16         
 17         System.out.print("Enter elements for set A: ");
 18         for(i=0; i<setAnum; i++)    
 19             list1.add(sc.nextInt());
 20             
 21         Set setA = new Set();
 22         for(i=0; i<setAnum; i++)
 23             setA.addMember(list1.get(i));
 24             
 25         System.out.print("Enter number of elements in set A: ");
 26         setBnum = sc.nextInt();
 27         ArrayList<Integer> list2 = new ArrayList<Integer>();
 28         
 29         System.out.print("Enter elements for set A: ");
 30         for(i=0; i<setBnum; i++)    
 31             list2.add(sc.nextInt());
 32             
 33         Set setB = new Set();
 34         for(i=0; i<setAnum; i++)
 35             setB.addMember(list2.get(i));
 36             
 37         System.out.println("Set A: " + setA);
 38         System.out.println("Set B: " + setB);
 39         
 40         if (setA.isSubset(setB)) {
 41             System.out.println("Set A is a subset of set B.");
 42         }   
 43         else {
 44             System.out.println("Set A is not a subset of set B.");
 45         }   
 46         if (setA.equals(setB))
 47             System.out.println("Set A is equal to set B.");
 48         else
 49             System.out.println("Set A is not equal to set B.");
 50     }
 51 
 52 }

I keep getting this error

Exception in thread "main" java.lang.NullPointerException
        at Set.addMember(Set.java:28)
        at TestSet.main(TestSet.java:23)

I can read and pinpoint where the problem is but I don't know what to correct it to. This is the first time I'm writing a user-defined class with an ArrayList inside. Previously I've only be writing classes with primitive data types. So I'm confused with a lot of things like what the constructors accessors and mutators are supposed to look like, when to use the 'this' reference. Someone answered and I inferred that if I'm using an arraylist here with the attribute 'members', I should use the reference to 'members' rather than 'this' because that's the attribute that I want to play around with. I know it's not a hard and fast rule but I sorta get the picture. Please help!!

You should avoid "star imports". While technically valid, they are deprecated. In your particular case, you should be aware of the fact that there is already a Java interface called `Set` in the package `java.util`. — Mad Physicist, Feb 12 '14 at 16:40
@MadPhysicist Do you any reliable source saying "start imports" are deprecated? I don't think so. At least eclipse doesn't mark them as deprecated. — Rohit Jain, Feb 12 '14 at 16:41
@RohitJain http://stackoverflow.com/questions/147454/why-is-using-a-wild-card-with-a-java-import-statement-bad — Mad Physicist, Feb 12 '14 at 16:44
@MadPhysicist Where is it mentioned in that question that it is deprecated? It is just talking about best practices. — Rohit Jain, Feb 12 '14 at 16:50

score 1 · Answer 1 · answered Feb 12 '14 at 16:39

The constructor:

public Set()    {
    new ArrayList<Integer>();
}

doesn't do anything. It just creates an object, and discards it. So the members instance variable is still null. Change the constructor to:

public Set()    {
    members = new ArrayList<Integer>();
}

And please choose a different name for your class. Set is already an interface in Java API.

arraylist in self-designed class errors. please help me

1 Answers1