CODEWITHSUNDEEP
javahashmap
user2904796
user2904796

Reputation: 267

Java HashMap Error

I have working on a problem where I am given a map (called myMap) where the keys are the names of authors, and the values are the names of the co-authors that the written has published articles with. I have to create a method that returns a String array containing the degrees of separation between a particular author, named ERDOS, and the other ones. I have written up the code below to do so; I am using a breadth first search. 

   HashMap<String,Boolean> visitedMap= new HashMap<String,Boolean>();
   HashMap<String,Integer> distanceMap=new HashMap<String,Integer>();

     for(String name: myMap.keySet()){
         visitedMap.put(name, false);
     }
     for(String name:myMap.keySet()){
         distanceMap.put(name, -1);
     }
     distanceMap.put("ERDOS", 0);
     Queue<String> q= new LinkedList();
     q.add("ERDOS");
     visitedMap.put("ERDOS", true);


     int i=0;
     while(!q.isEmpty()){
         String a = q.remove();
         i=i+1;
         ArrayList<String> current= myMap.get(a);
         for(String z:current){
             System.out.println(current);
             System.out.println(visitedMap);
             if(visitedMap.get(current)==true){
                 q.add(z);
                 distanceMap.put(z, i);
                 visitedMap.put(z,true);
             }
             else{
                 break;
             }
         }
      }

      ArrayList<String> answer= new ArrayList<String>();
      for(String y:distanceMap.keySet()){
          answer.add("y"+" "+Integer.toString(distanceMap.get(y)));
      }
      String[] realAnswer= answer.toArray(new String[answer.size()]);
      Arrays.sort(realAnswer);
      return realAnswer;

}

However, I keep getting a null-pointer error when I call (visitedMap.get(current)==true). The two print statements before this line show that current is definitely present in visitedMap, and so I really don't understand the cause of this null-pointer exception. I would really appreciate any help. Thanks a lot!

Upvotes: 0

Views: 139

Answers (1)

Jesper
Jesper

Reputation: 206816

current is an ArrayList<String>, while the keys in your map visitedMap are String objects. Therefore, visitedMap.get(current) will always return null.

You probably wanted to use: visitedMap.get(z) instead of visitedMap.get(current).

Upvotes: 2

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