将 int 添加到 ArrayList 时出现 NullPointerException

Question

我有一个函数应该使用给定ArrayList<String>并将输出返回为ArrayList<Integer>. 问题是NullPointerException提出来的hourList.add(hourInt);，我不知道为什么。

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

Answer 1

您需要ArrayList在声明时构建您的：

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

否则hourList.add将给出 NPE，因为引用hourList为空。