java - 将对象添加到 arrayList

Question

我在用 Java 向 arrayList 添加对象时遇到问题。当我运行我的代码时，出现以下错误。这是我的两个文件的片段。如果有人指出我的错误，我将非常感激。谢谢，乔

House.addRoom(House.java:18)处的java.lang.NullPointerException House.(House.java:36)

//房间等级

  public Room () {
    Scanner scan = new Scanner(System.in);
    scan.useDelimiter("\n");

    System.out.println("Enter description of room:");
    description = scan.next();

    System.out.println("Enter length of room:");
    length = scan.nextDouble();

    System.out.println("Enter width of room:");
    width = scan.nextDouble();
   }

//房屋等级

public class House {
  private static ArrayList<Room> abode;

   public void addRoom (){
     abode.add(new Room ());
   }
   public House () {
    idNum = ++internalCount;
    Scanner scan = new Scanner(System.in);
    scan.useDelimiter("\n");

    System.out.println("Enter address of house:");
    address = scan.next();

    System.out.println("Enter number of rooms:");
    numRooms = scan.nextInt();

    System.out.println("Enter type of house:");
    houseType = scan.next();

    for (int i=1; i<=numRooms; i++){
      addRoom();
    }
  }
}

Answer 1

您需要初始化您的arraylist在向其中添加元素之前。可能在构造函数中初始化

private static ArrayList<Room> abode;

public House()
{
 abode = new ArrayList<String>();
//rest of your code 
}

顺便说一句，对接口(interface)进行编码比对实现进行编码始终是一个好习惯:

即List<Room> abode = new ArrayList<String>();