views:

1185

answers:

5

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;
}
+4  A: 

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.)

Brian Agnew
Yes, it's very strange that dereferencing a pointer that is explicitly null would throw a NullPointerException.
Apocalisp
+3  A: 

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

datamap = new HashMap();

Sheldon Young
+1  A: 

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?

jcopenha
+7  A: 
HashMap<String, String> dataMap = new HashMap<String,String>

Your dataMap variable is not initialized. You should be getting a compiler warning about that.

Codingscape
Isn't the problem really that it's *explicitly* initialised ? To null ?
Brian Agnew
I think Codingscape means to say "is not instantiated"
matt b
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
A: 

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

HashMap<String, String> dataMap = null;
...
dataMap.put(...)
Michael Borgwardt
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
so you're expecting magic?
JasonTrue
If put =null everywhere will my program write itself?
mP