我收到 null 指針異常錯誤，我找不到源

Question

我正在創建一個程序，機場工作人員可以輸入飛機和航班信息，之后機場用戶可以打印出輸入的飛機和航班信息。 運行第一個方法startAirportPanel()，並在閱讀器掃描器中輸入1添加飛機信息后，輸入飛機ID和飛機容量后會出現null指針異常錯誤。

結果：

Airport panel
---------------


Choose operation: 
[1] Add airplane
[2] Add flight
[x] Exit
1
Give plane ID: 
GHUA-HG
Give plane capacity: 
22

在此處輸入 22 並按回車后，我會得到 null 指針異常。

這是 object 航班：

import java.util.ArrayList;

public class Flights {

    public HashMap<String,Integer> plane = new HashMap<String,Integer>();
    public HashMap<String,String> flight = new HashMap<String,String>();

    public Flights(){

        this.plane = plane;
        this.flight = flight;

    }
    public void planeMap(String id, Integer capacity){
        plane.put(id, capacity);
    }

    public void flightMap(String id, String departure, String destination){
        String flight1 = departure + "-" + destination;
        flight.put(id, flight1);
    }

    public ArrayList planeList(){
        ArrayList<String> keylist = new ArrayList<String>(plane.keySet());
        ArrayList<Integer> valuelist =  new ArrayList<Integer>(plane.values());
        ArrayList<String> newlist = new ArrayList<String>();

        for(int i = 0 ; i < keylist.size() ; i++){
            newlist.add(keylist.get(i) + " (" + valuelist.get(i) + ")");
        }

        for(int i = 0 ; i < newlist.size() ; i++){
            System.out.println(newlist.get(i));
        }
        return newlist;
    }

    public ArrayList flightList(){
        ArrayList<String> keylist = new ArrayList<String>(flight.keySet());
        ArrayList<String> valuelist =  new ArrayList<String>(flight.values());
        ArrayList<String> newlist = new ArrayList<String>();

        for(int i = 0; i < keylist.size(); i++){
            newlist.add(keylist.get(i) + " (" + plane.containsKey(keylist.get(i)) + ") " + "(" + valuelist.get(i) + ")");

        }
        for(int i = 0; i < newlist.size() ; i++){
            System.out.println(newlist.get(i));
        }
        return newlist;
    }

    public int planeInfo(String id){

        if(plane.containsKey(id)){
        return plane.get(id);
    }
        return 0;
}

} 

這是用戶界面：

import java.util.HashMap;
import java.util.Scanner;
import java.util.ArrayList;

public class UserInterface {
    private Flights fly;
    private Scanner reader = new Scanner(System.in);


    public UserInterface(){
        this.fly = fly;
        this.reader = reader;




    }

    public void startFlightService(){



        System.out.println("Flight service ");
            System.out.println("--------------");
            System.out.println();
            System.out.println();

         while(true){

            System.out.println("Choose operation: ");
            System.out.println("[1] Print planes");
            System.out.println("[2] Print flights");
            System.out.println("[3] Print plane info");
            System.out.println("[x] Print Quit");

            if(reader.equals("x")){
                break;
            }
            if(Integer.parseInt(reader.nextLine()) == 1){
                printPlane();
            }
            if(Integer.parseInt(reader.nextLine()) == 2){
                printFlight();
            }
            if(Integer.parseInt(reader.nextLine()) == 3){
                printPlaneInfo();
            }
                else continue;

        }

    }

    public void startAirplanePanel(){


        System.out.println("Airport panel");
        System.out.println("---------------");
        System.out.println();
        System.out.println();



        while(true){
        System.out.println("Choose operation: ");
        System.out.println("[1] Add airplane");
        System.out.println("[2] Add flight");
        System.out.println("[x] Exit");

        String input = reader.nextLine();


        if(Integer.parseInt(input) == 1){
            addPlane();


        } if(Integer.parseInt(input) == 2){
            addFlight();

        }
        if(input.equals("x")){
            break;
        }
    }

}

    public void addPlane(){
        System.out.println("Give plane ID: ");
        String id = reader.nextLine();
        System.out.println("Give plane capacity: ");
        int capacity = Integer.parseInt(reader.nextLine());

        fly.planeMap(id,capacity);


    }

    public void addFlight(){
        System.out.println("Give plane ID: ");
        String id = reader.nextLine();
        System.out.println("Give departure airport code: ");
        String departure = reader.nextLine();
        System.out.println("Give destination airport code: ");
        String destination = reader.nextLine();



        fly.flightMap(id,departure,destination);
    }

    public void printPlane(){

        for(int i = 0; i < fly.planeList().size(); i++){
            System.out.println(fly.planeList());
        }






    }

    public void printFlight(){
        for(int i = 0; i < fly.flightList().size(); i++){
            System.out.println(fly.flightList());
        }

    }

    public void printPlaneInfo(){
        System.out.print("Give plane ID: ");
        String id = reader.nextLine();

        System.out.println(id + " (" + fly.planeInfo(id) + ")");
    }

}

最后，主class啟動程序：

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        // Write your main program here. Implementing your own classes will be useful.

        Scanner reader = new Scanner(System.in);

        Flights flightss = new Flights();
        UserInterface hello = new UserInterface();

        hello.startAirplanePanel();
        }
}
        ```

Answer 1

構造函數的這一部分沒有意義：

this.fly = fly;
this.reader = reader;

閱讀器已經被實例化，因此您只需將當前值替換為：當前值。 然而，蒼蠅沒有被實例化。 因此，您只需將 null 設置為 null。 無論您在飛行中撥打什么電話，都會導致 NPE。

刪除this.reader = reader; 並通過實際實例化更改有關 fly 的行。

編輯：

所以，你的構造函數應該是這樣的：

public UserInterface(){
        this.fly = new Flights();
  }

我還會重新檢查您的航班構造函數。

Answer 2

有問題的代碼部分是（在 UserInterface class 中）：-

private Flights fly;
private Scanner reader = new Scanner(System.in);


public UserInterface(){
    this.fly = fly;
    this.reader = reader;

}

只有 fly 變量類型 (Flights) 被聲明，沒有航班 class object 的任何分配。

使用“新”關鍵字修復此問題以實例化航班 class：-

private Flights fly = new Flights();
private Scanner reader = new Scanner(System.in);


public UserInterface(){
    this.fly = fly;
    this.reader = reader;
}

但是，可以按照@Stultuske 的建議以更好的方式重構代碼。