NullPointerException while using put method of HashMap

Question

The following code is giving me a NullPointerException. The problem is on the following line:

... 
dataMap.put(nextLine[0], nextLine[6]);

What is strange is that I have run this code without the above line and the call to nextLine[0] and nextLine[6] work exactly as expected - that is they give me back elements of a csv file. I declare and initialise the HashMap with the code

HashMap<String, String> dataMap = null;

earlier in the method

  String[] nextLine;
  int counter=0;
  while (counter<40) {
    counter++;

    System.out.println(counter);
    nextLine = reader.readNext(); 
    // nextLine[] is an array of values from the line
    System.out.println(nextLine[0] + " - " + nextLine[6] +" - " + "etc...");
    dataMap.put(nextLine[0], nextLine[6]);
  }
  return dataMap;
}

I just wonder about the "24" points for a such trivial newbie coding problem — , Feb 10 '21 at 13:03

score 47 · Accepted Answer · edited Feb 18 '17 at 09:29

47

HashMap<String, String> dataMap = new HashMap<String,String>();

Your dataMap variable isn't initialized at this point. You should be getting a compiler warning about that.

edited Feb 18 '17 at 09:29

Ole V.V.

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answered Apr 09 '09 at 16:05

Codingscape

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Isn't the problem really that it's *explicitly* initialised ? To null ? – Brian Agnew Apr 09 '09 at 16:13
I think Codingscape means to say "is not instantiated" – matt b Apr 09 '09 at 17:34
1

Actually, there is a hint here. He didn't get a warning because he initialized it to null to override the compiler warning he certainly was getting. You should make it a habit NOT to initialize variables to a "Neutral" value by default, only set them to their real value so the compiler can warn. – Bill K Apr 09 '09 at 17:39

score 5 · Answer 2 · answered Apr 09 '09 at 16:04

5

Where is datamap initialised ? It's always null.

To clarify, you declare the variable and set it to null. But you need to instantiate a new Map, whether it's a HashMap or similar.

e.g.

datamap = new HashMap();

(leaving aside generics etc.)

answered Apr 09 '09 at 16:04

Brian Agnew

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Yes, it's very strange that dereferencing a pointer that is explicitly null would throw a NullPointerException. – Apocalisp Apr 09 '09 at 16:06

score 3 · Answer 3 · answered Apr 09 '09 at 16:05

3

dataMap is declared but not initialized. It can be initialized with

datamap = new HashMap();

answered Apr 09 '09 at 16:05

Sheldon

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score 1 · Answer 4 · answered Apr 09 '09 at 16:05

1

Well, there are three objects accessed on that line. If nextLine[0] and nextLine[6] aren't null, because the println call above worked, then that leaves dataMap. Did you do dataMap = new HashMap(); somwehere?

answered Apr 09 '09 at 16:05

jcopenha

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score 0 · Answer 5 · answered Feb 10 '21 at 13:00

My case was different as Usually, hashmaps throw the null pointer exception when trying to read a key that is not there e.g I have a

HashMap<String, Integer> map = new HashMap<String, Integer>();

Which is empty or have keys except "someKey", So when I try to

map.get("someKey") == 0;

Now this will produce a nullPointerExeption due to matching a null with some integer.

What should I do instead?

Ans: I should instead check for null like

map.get("someKey") != null;

Now the runtime error Nullpointer would not raise!

score -1 · Answer 6 · answered Apr 09 '09 at 16:06

-1

Um, what exactly do you expect when you do this?

HashMap<String, String> dataMap = null;
...
dataMap.put(...)

answered Apr 09 '09 at 16:06

Michael Borgwardt

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I was expecting that null is kind of a place holder, but when you use put the value being "put" make the HashMap no longer equal to null – Ankur Apr 09 '09 at 16:09
If put =null everywhere will my program write itself? – mP. Apr 10 '09 at 04:51
...and why a minus point here? I begin to doubt the objectivity of Stackoverflow community. – Feb 10 '21 at 13:05

NullPointerException while using put method of HashMap

6 Answers6