CODEWITHSUNDEEP
javaarraysnullpointerexceptionhashmap
Henrik Hillestad Løvold
Henrik Hillestad Løvold

Reputation: 1291

Java - NullPointerException when referencing an object in a HashMap

I am currently writing a command line program that is supposed to take in lines from a text file and add data to an array of HashMaps. I currently get a NullPointerException when running this method.

Public class Vaerdata {
String[] linje;
String line;
String line2;
HashMap<String, Stasjon> stasjonsmap = new HashMap<String, Stasjon>();
HashMap<String, Stasjon>[] regionmap = (HashMap<String, Stasjon>[]) new HashMap<?, ?>[6];


void init() throws IOException{
    BufferedReader br = new BufferedReader(new FileReader("stasjoner_norge.txt"));
    BufferedReader brData = new BufferedReader(new FileReader("klimadata2012.txt"));
    for(int i = 0; i < 10; i++){
        brData.readLine();
    }
    br.readLine();
    while((line = br.readLine()) != null){
        linje = line.split("\\s+");
        stasjonsmap.put(linje[1], new Stasjon(Integer.parseInt(linje[1]), linje[2], Integer.parseInt(linje[3]), linje[4], linje[5], linje[6]));
        }
        if(linje[6].equals("AGDER")){
            System.out.println(stasjonsmap.get(linje[1])); //DEBUG
            regionmap[1].put(stasjonsmap.get(linje[1]).navn, stasjonsmap.get(linje[1]));
            System.out.println(regionmap[1].get(stasjonsmap.get(linje[1]).navn)); //DEBUG
        }
    }  
}

My NullPointerException happens in this line: 

regionmap[1].put(stasjonsmap.get(linje[1]).navn, stasjonsmap.get(linje[1]));

So my question is: When I have declared the array of HashMaps with <String, Stasjon> (Stasjon is an object of my Stasjon class, taking info about certain weather stations), why do I get an error in that line? The object in stasjonsmap.get(linje[1]) has been declared, and I can't understand why it won't allow me to make a reference to this object in my second hashmap.

Every line in the text file, after line one (which I skip in my program) look like this:
36200 TORUNGEN_FYR 12 ARENDAL AUST-AGDER AGDER

In advance; thanks for your help.

Upvotes: 2

Views: 937

Answers (3)

Axel
Axel

Reputation: 14159

You have created an array of Hashmaps of size 6, but all items in that array are still initialized to null.

You could instead do it like this (which also gets rid of the ugly cast):

  private static HashMap<String, Stasjun>[] initRegionMap(HashMap<String, Stasjun>... items) {
    return items;
  }

  HashMap<String, Stasjun>[] regionmap = initRegionMap ( 
      new HashMap<String, Stasjun>(),
      new HashMap<String, Stasjun>(),
      new HashMap<String, Stasjun>(),
      new HashMap<String, Stasjun>(),
      new HashMap<String, Stasjun>(),
      new HashMap<String, Stasjun>()
    );

Upvotes: 0

Kumar Vivek Mitra
Kumar Vivek Mitra

Reputation: 33534

- I think you have not initialized the HashMap, and you called put() method.

- As HashMap is an Object its default value is null, so you need to initialize it.

Upvotes: 0

jlordo
jlordo

Reputation: 37813

When you initialize your array of HashMap here

HashMap<String, Stasjon>[] regionmap = (HashMap<String, Stasjon>[]) new HashMap<?, ?>[6];

all values in the array are null.

You then try to call the put method of HashMap on a null-reference.

First you have to initialize your HashMaps somehow:

for (int i = 0; i < regionmap.length; i++) {
    regionmap[i] = new HashMap<String, Stasjon>();
}

Upvotes: 5

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