对象数组的空指针异常 [英] Null pointer exception for Array of Objects

问题描述

我不熟悉使用对象数组，但无法弄清楚自己在做什么，为什么我总是收到Null指针异常.我正在尝试使用一组设置为打开或关闭的聚光灯对象来创建Theater类.但是-每当我在该数组上调用时，都会得到一个空指针异常.

I am new to using arrays of objects but can't figure out what I am doing wrong and why I keep getting a Null pointer exception. I am trying to create an Theatre class with an array of spotlight objects that are either set to on or off. But - whenever I call on this array I get a null pointer exception.

package theatreLights;

public class TheatreSpotlightApp {


public static void main(String[] args) {

    Theatre theTheatre = new Theatre(8);

    System.out.println("element 5 " + theTheatre.arrayOfSpotlights[5].toString());

}

}

package theatreLights;

public class Theatre {

spotlight[] arrayOfSpotlights;

public Theatre(int N){

  arrayOfSpotlights =  new spotlight[N];

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

}
}


package theatreLights;

public class spotlight {
int state;

public  spotlight(){    
    state = 0;  
}

public void turnOn(){
    state = 1;  
}

void turnOff(){ 
    state = 0;
}

public String toString(){
    String stringState = "";
    if(state == 0){
        stringState = "is off";

    }
    else if(state==1){
        stringState = "is on";
    }

    return stringState;

}
}

在创建数组时，我必须做一些基本的错误，但无法弄清楚.

I must be doing something basic wrong in creating the array but can't figure it out.

推荐答案

替换

arrayOfSpotlights[i].turnOn();

使用

arrayOfSpotLights[i] = new Spotlight();
arrayOfSpotlights[i].turnOn();    

行

arrayOfSpotlights =  new spotlight[N];

将创建一个聚光灯阵列.但是，它将不会使用聚光灯填充此阵列.

will create an array of spotlights. It will however not populate this array with spotlights.

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