每当数组成员访问抛出NullPointerException [英] NullPointerException being thrown whenever array member is accessed

问题描述

我有那个不断被抛出每次我试图执行以下code时异常的问题。

I am having an issue with an exception that keeps being thrown every time I attempt to execute the following code.

下面是驱动程序，下面我会给你客房和 playerEnters 方法构造。

Below is the driver, and below that I will give you the constructor for Room and the playerEnters method.

import java.util.Random;
import java.util.Scanner;

public class ZorkPlayer
{

    public static void main (String [ ] args)
    {
        // create a start screen followed by introduction
        int choice = 0;
        while(choice != 3)
        {
            choice = menu();
            switch (choice)
            {
                case 1:
                    //begin new game
                    newGame();
                    break;
                case 2:
                    //change difficulty level
                    break;
                case 3:
                    //exit the program
                    break;
                default: 
                    //invalid choice
                    break;
            }
        }
    }

    /**
     * Creates a menu and gets a choice from the user <br>        
     */
    public static int menu ()
    {
        Scanner kb = new Scanner(System.in);
        System.out.println("Welcome to Zork!");
        System.out.println("What would you like to do?");
        System.out.println("\t1- Start a new game");
        System.out.println("\t2- Choose difficulty level (not yet available)");
        System.out.println("\t3- Exit the program");
        int choice = kb.nextInt();
        return choice;
    }

    public static void newGame()
    {
        final int DEFAULT_ROOMS = 5;
        Random rng = new Random ();
        int numRooms = rng.nextInt(5) + DEFAULT_ROOMS;
        int playerPos = 0;
        Room dungeon[] = new Room[numRooms];

        dungeon[playerPos].playerEnters();
        for (int i = 0; i < dungeon.length; i++)
        {
            System.out.print (dungeon[i].getMap ( ));
        }
    }
}

构造室

private int monster = 0; //initializes with no monster spawned
private int player = 0; //initializes with no player present
private int weapon = 0; //initializes with no weapon present

public Room()
{
    //creates a new room and determines whether the new room has a monster or not
    Random rng = new Random();
    int monsterSpawn = rng.nextInt (2); //determines whether or not a monster will spawn in the room
    if (monsterSpawn == 0)
    {
        new Monster(); //if a monster is spawned, creates a new monster and sets the value of monster to present
        setMonster(1);  
    }
    else
        setMonster(0); //otherwise leaves the value of monster to default
    setPlayer(0); //sets the presence of player to false
    setWeapon(0); //sets the presence of weapon to false
}

playerEnters方法

playerEnters method

public void playerEnters()
{
    setPlayer(1);
}

的 NullPointerException异常每当 playerEnters 方法被调用驱动程序被抛出，然后再当的GetMap 方法被调用。

The NullPointerException is thrown whenever the playerEnters method is called in the driver, and then again when the getMap method is called.

推荐答案

忽略一切，这就是问题所在：

Ignoring everything else, this is the problem:

Room dungeon[] = new Room[numRooms];
dungeon[playerPos].playerEnters();

您创建一个客房数组，但不要创建阵列中的任何客房的对象。因此，地牢[playerPos] 是空。

You create a Room array, but don't create any Room objects in that array. Therefore dungeon[playerPos] is null.

您需要填充客房对象数组。

for (int i = 0; i < numRooms; i++)
{
    dungeon[i] = new Room();
}

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