Checking if a HashMap is empty without causing NullPointerException / IndexOutOfBoundsException?

Question

So am doing a mail server kind of program for my java course, (I would call myself a beginner in Java) and while I have completed all the required parts, there are some problems with the program that while the assignment doesn't require it, I would like to know why this is behaving the way and how to fix it. So to summarize it I have two methods that check how many mail items a specific user has and another one that returns the mail item that is first in the users mailbox, (by LIFO rule).

So while if a user both A.Has a mailbox and B.Has at least 1 mail in said mailbox there isn't a problem, this doesn't really work when a and/or b isn't true. And that is what I tried to fix. I thought it would be as simple as checking both if the mailbox.containsKey(who)==false and/or mailbox.get(who).get(0) == null would be enough but when I do this i get java.lang.IndexOutOfBoundsException: Index: 0, Size: 0.

I've tried multiple types of either checking if it's ==true/false null etc.. I've tried to just throw the exception all together but nothing seems to work and while maybe throwing the exception could work in some way I would want to solve it in another way as the throw command seems a bit lazy

Here is my method in which I check both A and B:

private boolean errorHandling(String who){  
    if (mailbox.containsKey(who)==false ||  mailbox.get(who).get(0) == null    ){ 
        return true;  
    }
    else{
        return false;
    }
}

and this method is utilized by these two methods:

public MailItem getNextMailItem(String who)
{
    if (!errorHandling(who) ){   
        MailItem item2 = mailbox.get(who).get(0);
        mailbox.get(who).remove(0);
        return item2;
    } else{
        System.out.println("that user dosent have a mailbox");
        return null;
    }
}

public int howManyMailItems(String who)
{
    if (!errorHandling(who) ){  //To check if user has a mailbox
        return mailbox.get(who).size();
    }
    else{
        System.out.println("that user dosent have a mailbox");
        return 0;
    }
}

I am not sure if you need more of my code to help me or not. Tell me if that is the case but to clarify, the "mailbox" is the hashmap and the who parameter is the person you want to check his or her mailbox. Thank you.

How about MailItem mail = mailbox.get(who);, followed by if (mail != null). That's in general, for the specific case of #howManyMailItems: mailbox.getOrDefault(who, 0); will return 0 if there isn't a key mapped to a value. You can also make use of Map#computeIfAbsent so that you can always retrieve a Mailbox and the values are never null. — Rogue
– Rogue, Commented Oct 4, 2019 at 16:40
Your check for "email exists" is wrong. mailbox.get(who).get(0) == null would only be true if you add null to the mail list and I hope you don't do that. I guess you have a LinkedList there, so you can check the JavaDoc if that class provides something for you to check if the list is empty or contains something: docs.oracle.com/javase/7/docs/api/java/util/LinkedList.html (also check the inherited methods). — Tom
– Tom, Commented Oct 4, 2019 at 16:42
In followup to tom's comment, List#isEmpty is useful here. Lists/Sets don't just return null for a non-existant value, you need to check that the index you request (0 in this case) is actually contained in the collection by comparing #size to the index you want. — Rogue
– Rogue, Commented Oct 4, 2019 at 16:44
Thanks for the helpful comments am gonna investigate more on this and see if that resolves my issue, appreciate your time! — DJ Oakman
– DJ Oakman, Commented Oct 4, 2019 at 16:55
In addition to everything else, don't compare booleans; use !mailbox.containsKey. — chrylis -cautiouslyoptimistic-
– chrylis -cautiouslyoptimistic-, Commented Oct 4, 2019 at 17:04

Ryuzaki L · Accepted Answer · 2019-10-04 16:56:24Z

1

There are couple of problems in our code which i listed below

First Problem

The if condition is wrong and which will be true even if map doesn't contain key

if (mailbox.containsKey(who)==false ||  mailbox.get(who).get(0) == null)

  // mailbox.containsKey(who) will return false if map doesn't contain key
  // then false==false ---> always true

So you are returning true even key is not present in map, And there is not necessity of errorHandling method. you can just use containsKey

if (containsKey(who) ){      // if key exists then proceed
   MailItem item2 = mailbox.get(who).get(0);
   mailbox.get(who).remove(0);
   return item2; }
else{                   // else don't proceed 
    System.out.println("that user dosent have a mailbox");
    return null;
}

You can even avoid this `if condition by using getOrDefault

default V getOrDefault(Object key, V defaultValue)

Second Problem

Second part of if condition is wrong mailbox.get(who).get(0) == null you might have empty list for that key, so you can refactor your code as below

List<MailItem> listItem = mailbox.get(who, new ArraysList<>());  //get list if key exists or else get empty list

 if(!listItems.isEmpty()) {
      return listItems.remove(0);    //if list has items delete at index 0
   }
 else{
    System.out.println("that user dosent have a mailbox");
    return null;
  }

edited Oct 4, 2019 at 16:56

answered Oct 4, 2019 at 16:44

Ryuzaki L

40.4k14 gold badges81 silver badges111 bronze badges

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

1 Comment

DJ Oakman Over a year ago

Thanks for the helpful reply, I realize the problem now, I will also investigate this getOrDefault more in-depth, to see how too utilize it in my program!

Collectives™ on Stack Overflow

Checking if a HashMap is empty without causing NullPointerException / IndexOutOfBoundsException?

1 Answer 1

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related