如何修复我的空指针异常

Question

Doing a code for a class assignment, and im getting null pointer exception on these lines(bolded and italicized in my code): I don't understand how these lines are pointing at a blank spot, like the first one is my code trying to set a name for a certain seat, but apparantly i can't because it is null, even though i have i set to 0, first name set to an empty string, etc. helP!

public class Passenger {
    String FirstName;
    String LastName;

    void getName() {
        FirstName = " ";
        LastName = " ";
    }

    void getName(String first, String last) {
        FirstName = first;
        LastName = last;
    }

    String getFirst() {
        return FirstName;
    }

    String getLast() {
        return LastName;
    }

    void setFirstName(String tempfirst) {
        FirstName = tempfirst;
    }

    void setLastName(String templast) {
        LastName = templast;
    }

    void printNames() {
        System.out.printf("%s %s\n", this.getFirst(), this.getLast());
    }

    public static void main(String[] args) {
    }
}

public class WaitingList extends Passenger {
    Passenger[] waitlist = new WaitingList[10];

    WaitingList(){
        for(int i = 0; i<10;i++){
            ***waitlist[i].setFirstName("");***
            waitlist[i].setLastName("");
        }}

    void printWaitingList() {
        for (int j = 0; j < 10; j++)
            waitlist[j].printNames();
    }

    void setPassenger(int i, Passenger tempPass) {
        waitlist[i] = tempPass;
    }

    Passenger getPassenger(int i) {
        return waitlist[i];
    }

    public static void main(String[] args) {
    }

}

import java.io.Console;
import java.util.Scanner;

public class SeatingChart extends WaitingList {
    Passenger[][] pass = new Passenger[10][4];

    SeatingChart() {
        for (int row = 0; row < 10; row++) {
            for (int col = 0; col < 4; col++) {
                pass[row][col] = new Passenger();
            }
        }
    }

    void printSeatingChart() {
        for (int row = 0; row < 10; row++) {
            for (int col = 0; col < 4; col++) {
                System.out.print(pass[row][col].getFirst() + "\t");
                System.out.print(pass[row][col].getLast() + "\n");
            }
        }
    }

    String getSeatChoicefromUser() {
        Scanner scanner = new Scanner(System.in);
        System.out.print("Enter preferred seat (row, column): ");
        String seat = scanner.next();
        return seat;
    }

    void setPassenger(int row, int col, Passenger temppass) {
        pass[row][col] = temppass;
    }

    Passenger getPassenger(int row, int col) {
        return pass[row][col];
    }

    Passenger search(String first, String last) {
        Passenger search = new Passenger();
        for (int row = 0; row < 10; row++) {
            for (int col = 0; col < 4; col++) {
                String tempfirst = pass[row][col].getFirst();
                String templast = pass[row][col].getLast();
                if (tempfirst.equals(first)) {
                    if (templast.equals(last)) {
                        search = pass[row][col];
                        removePassenger(row, col);
                    }
                } else {
                    search = null;
                    System.out.print("\nPassenger not found\n");
                }
            }
        }
        return search;
    }

    void removePassenger(int row, int col) {
        pass[row][col].setFirstName(null);
        pass[row][col].setLastName(null);
    }

    Passenger reconcileWithWaitingList() {
        int i = 0;
        Passenger pass = new Passenger();
        String tempfirst = waitlist[i].getFirst();
        String templast = waitlist[i].getLast();
        if (tempfirst.equals(null)) {
            if (templast.equals(null))
                System.out.print("\nWaiting list is empty\n");
        } else {
            pass = waitlist[i];
            for (i = 0; i < 10; i++)
                waitlist[i] = waitlist[i + 1];
        }
        return pass;
    }

    public void addPassengerToWaitingList(int wrow, int wcol) {
        Passenger addPass = new Passenger();
        Scanner sc = new Scanner(System.in);
        System.out.print("Enter first name: ");
        String tempfirst = sc.next();
        System.out.print("Enter last name: ");
        String templast = sc.next();
        addPass = search(tempfirst, templast);
        if (addPass == null) {
            String tempfirst1 = pass[wrow][wcol].getFirst();
            String templast1 = pass[wrow][wcol].getLast();
            if (tempfirst1.equals(null)) {
                if (templast1.equals(null)) {
                    pass[wrow][wcol].setFirstName(tempfirst1);
                    pass[wrow][wcol].setLastName(templast1);
                }
            }
        } else
            System.out.print("Passenger already present!");
    }

    public static void main(String[] args) {
    }
}

Answer 1

Your waitlist array holds null elements. You must first initialize the elements before trying to use them.

Passenger[] waitlist = new WaitingList[10];

WaitingList(){
    for(int i = 0; i<10;i++){
       waitlist[i] = new WaitingList();  // *** first create an element 
       waitlist[i].setFirstName("");   // *** before using it.
       waitlist[i].setLastName("");
    }
}

Think of an object array similar to a parking lot. When you create the array that is equivalent to creating the lot, and at first the lot, though it holds a lot of spaces, is empty. You can't drive any cars in the lot until you first put cars in the lot. Likewise, you can't use any of the array items until they refer to honest to goodness objects.

---

Edit
Regarding your comment,

that gave me a stack overflow error on the same line

That's a whole new error -- you've got some recursive code in there. That problem is due to this code:

public class WaitingList extends Passenger {
   Passenger[] waitlist = new WaitingList[10];

• First off, a WaitingList should not be a subclass of a Passenger. This makes no sense since the concept of a waiting list is not a specialized case of the concept of a passenger. In other words, it does not fulfill the "is-a" criteria for inheritance. Rather it fulfills the "has-a" criteria -- a waiting list has passengers, not is-a passenger. So use composition instead.
• Next the WaitingList class should not create and fill an array of WaitingLists as this will cause infinite recursion with each new WaitingList object creating 10 new WaitingList objects, each of which will create 10 more WaitingList objects, each of which will... do you see the problem? Instead the WaitingList class should likely inherit from no other classes, and should hold an array (or ArrayList) of Passenger objects, not WaitingList objects.

Your code has lots of problems and you may wish to restart this project.

Answer 2

---

Hey you declared array of references What you need to do is initialize those null references before using them

for(int i=0; i< waitlist.length;++i){
    waitlist[i] = new waitlist();
}

then call your function

Answer 3

Replace your Passenger class code with it. Have Fun :)

 public class Passenger {
        String FirstName = "";
        String LastName = "";
        void getName(){
            FirstName="";
            LastName="";}
        void getName(String first, String last){
            if(first != null)
            {    
                 FirstName=first;
            }
         else
         {
           FirstName = "";
         }
         if(last != null)
         {
            LastName=last;
         }
         else
         {
           LastName = "";
         }
        }
        String getFirst(){
            return FirstName;}
        String getLast(){
            return LastName;}
        void setFirstName(String tempfirst){
           if(tempfirst == null)
           {
             FirstName = "";
           }
           else
           {        
             FirstName=tempfirst;
           }
        }
        void setLastName(String templast){
           if(templast == null)
           {
             LastName="";
           }
           else
           {
            LastName=templast;
           }
        }
        void printNames(){
            System.out.printf("%s %s\n", this.getFirst(), this.getLast());
        }