Exception in thread 'main' java.lang.NullPointerException

Question

Hi I'm very new to Java and have this HW problem, we are asked to build a Class RandomBag -- a generic bag container which allows 'RandomPick' of an element. I keep getting this error I have little idea to fix, any help is greatly appreciated!

  public RandomBag() {
      ArrayList<E> Bag=new ArrayList<E>();
      Random Rand =new Random();
  }
  public RandomBag(int seed) { 
      ArrayList<E> Bag=new ArrayList<E>();
      Random Rand =new Random(seed);

Paul Boddington · Accepted Answer · 2015-04-22T02:01:57.077

1

You are never initializing the variables rand or bag, so they are always null. In your constructors you need to use rand and bag, not Rand or Bag. For example:

public RandomBag() {
    bag = new ArrayList<E>();
    rand = new Random();
}

edited Apr 22 '15 at 02:01

answered Apr 22 '15 at 01:59

Paul Boddington

37,127
10
65
116

you haven't even read the code, right? – Apr 22 '15 at 02:00
whats wrong with it @Paul? this seems correct – Baby Apr 22 '15 at 02:03
@Baby as far as i can see, there are two constructors, and both initialize bag aswell as rand – Apr 22 '15 at 02:05
@Paul no. Look closely. case-sensitive – Baby Apr 22 '15 at 02:06
1

@Paul also he declares them as new variables by defining the type, so even if they had the same name it would't define the ones he declares above. – Epicblood Apr 22 '15 at 02:10
oh, right didn't see that – Apr 22 '15 at 02:12

Epicblood · Answer 2 · 2015-04-22T02:23:31.507

A few things, in your constructors change

ArrayList<E> Bag=new ArrayList<E>();
Random Rand =new Random();

to

bag=new ArrayList<E>();
rand = new Random(seed);

few things to remember: variables are case sensitive. if you add a type, you are defining, not setting, ie:

ArrayList<E> bag=new ArrayList<E>();

is defining a new variable bag while

bag=new ArrayList<E>();

sets the existing variable bag to new ArrayList<E>();

score 0 · Answer 3 · answered Apr 22 '15 at 02:05

0

You are trying to initialise new variable in constructor. Assign it to variables that you have declared instead.

  public RandomBag(int seed) { 
  bag=new ArrayList<E>();
  rand =new Random(seed);

}

answered Apr 22 '15 at 02:05

Sunil Rajashekar

350
2
18

score 0 · Answer 4 · answered Apr 22 '15 at 02:18

Change your constructors:

 public RandomBag() {
      bag = new ArrayList<E>();
      rand = new Random();
  }
  public RandomBag(int seed) { 
      bag = new ArrayList<E>();
      rand = new Random(seed);
  }

The output is:

10 10 11 12 13 14 15 16 17 Random pick: 15 Random pick: 16 Random pick: 13 10 10 11 12 14 17**

score -1 · Answer 5 · answered Apr 22 '15 at 02:05

-1

In your constructors... Replace

ArrayList<E> Bag=new ArrayList<E>();
  Random Rand =new Random(seed);

with

Bag=new ArrayList<E>();
   Rand =new Random(seed);

answered Apr 22 '15 at 02:05

Daniel Stancu

52
2
8

cannot find symbol Bag. cannot find symbol Rand – Baby Apr 22 '15 at 02:13
bag, not Bag, sorry, and rand, not Rand. – Daniel Stancu Apr 22 '15 at 02:14
The idea is that you declare this instance variables in your class, but you do not initialize them in your constructors, but you declare other 2 local (to the constructors) variables and initialize them there, which has nothing to do with your instance variables (bag and random). – Daniel Stancu Apr 22 '15 at 02:17
you can edit your post, not posting as a comment. – Baby Apr 22 '15 at 02:18

Exception in thread 'main' java.lang.NullPointerException

5 Answers5