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; } 

Answer 1

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.

Answer 2

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.)