[英]I'm getting a null pointer exception error and I can't find the source
我正在创建一个程序,机场工作人员可以输入飞机和航班信息,之后机场用户可以打印出输入的飞机和航班信息。 运行第一个方法startAirportPanel(),并在阅读器扫描器中输入1添加飞机信息后,输入飞机ID和飞机容量后会出现null指针异常错误。
结果:
Airport panel
---------------
Choose operation:
[1] Add airplane
[2] Add flight
[x] Exit
1
Give plane ID:
GHUA-HG
Give plane capacity:
22
在此处输入 22 并按回车后,我会得到 null 指针异常。
这是 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;
}
}
这是用户界面:
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启动程序:
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();
}
}
```
构造函数的这一部分没有意义:
this.fly = fly;
this.reader = reader;
阅读器已经被实例化,因此您只需将当前值替换为:当前值。 然而,苍蝇没有被实例化。 因此,您只需将 null 设置为 null。 无论您在飞行中拨打什么电话,都会导致 NPE。
删除this.reader = reader;
并通过实际实例化更改有关 fly 的行。
编辑:
所以,你的构造函数应该是这样的:
public UserInterface(){
this.fly = new Flights();
}
我还会重新检查您的航班构造函数。
有问题的代码部分是(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 变量类型 (Flights) 被声明,没有航班 class object 的任何分配。
使用“新”关键字修复此问题以实例化航班 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 的建议以更好的方式重构代码。
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