NullPointerException at adding int to ArrayList

Question

I have a function which is supposed to work with given ArrayList<String> and return the output as ArrayList<Integer>. The problem is NullPointerException is raised on hourList.add(hourInt); and I don't know why.

public ArrayList<Integer> takeTime(ArrayList<String> logs)  {

    ArrayList<Integer> hourList=null;
    Integer hourInt;

    for(String line: logs) {

        String[] matrix = line.split(" ");     
        String[] hour = matrix[3].split(":");
        //  System.out.print(hour[0]+"\n");

       String s = hour[0].replaceFirst("^0+(?!$)", "");

        hourInt = Integer.parseInt(s);
       System.out.print(hourInt+",");

        hourList.add(hourInt);   // <--- NullPointerException here
    }
  return hourList;  
}

System.out.print(hourInt+",");:

  0,0,0,0,1,1,2,3,4,4,4,4,4,5,5,5,5,5,6,6,6,6,6,6,6,6,6,6,6,6,6,6,7,7,7,7,7,7,7,7,8,8,8,8,8,8,8,8,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,10,10,10,10,10,10,10

A generic answer on NPE might be a good reference point but this question was specific to `ArrayList` and it's initialization. — anubhava, May 14 '21 at 18:58

score 8 · Accepted Answer · edited May 14 '21 at 22:09

8

You need to construct your ArrayList while declaring it:

ArrayList<Integer> hourList = new ArrayList<>();

Otherwise hourList.add will give NPE since reference hourList is null.

edited May 14 '21 at 22:09

Arvind Kumar Avinash

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answered Jul 03 '15 at 10:20

anubhava

761,203
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score 1 · Answer 2 · answered Jul 03 '15 at 10:20

1

You have to initialize the ArrayList:

List<Integer> hourList = new ArrayList<Integer>;

answered Jul 03 '15 at 10:20

Crazyjavahacking

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score 1 · Answer 3 · answered Jul 03 '15 at 10:21

1

You are trying to call a function of a null object. Try:

ArrayList<Integer> hourList=new ArrayList<Integer>();

answered Jul 03 '15 at 10:21

Adam Voga

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NullPointerException at adding int to ArrayList

3 Answers3