[英]NullPointerException being thrown whenever array member is accessed
我遇到一个异常问题,每次尝试执行以下代码时都会抛出异常。
下面是驱动程序,下面我将为您提供Room
和playerEnters
方法的构造函数。
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 ( ));
}
}
}
房间的构造者
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
public void playerEnters()
{
setPlayer(1);
}
每当在驱动程序中调用playerEnters
方法时抛出NullPointerException
,然后在调用getMap
方法时再次抛出NullPointerException
。
忽略其他一切,这就是问题所在:
Room dungeon[] = new Room[numRooms];
dungeon[playerPos].playerEnters();
您创建一个Room
数组,但不要在该数组中创建任何Room
对象。 因此dungeon[playerPos]
为null
。
您需要使用Room
对象填充该数组。
for (int i = 0; i < numRooms; i++)
{
dungeon[i] = new Room();
}
您需要实例化房间。
Room dungeon[] = new Room[numRooms];
dungeon[0]=new Room();
...
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