Null pointer exception for Array of Objects

Question

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

Answer 1

replace

arrayOfSpotlights[i].turnOn();

with

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

The line

arrayOfSpotlights =  new spotlight[N];

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

Answer 2

You are not creating an spotlight objects.

arrayOfSpotlights =  new spotlight[N];

This just creates an array of references to spotlights, not the objects which are referenced.

The simple solution is

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

BTW You should use TitleCase for class names.

You could write your class like this, without using cryptic code like 0 and 1

public class Spotlight {
    private String state;

    public Spotlight() {
        turnOff();
    }

    public void turnOn() {
        state = "on";  
    }

    void turnOff() { 
        state = "off";
    }

    public String toString() {
        return "is " + state;
    }
}

Answer 3

When you do "arrayOfSpotlights = new spotlight[N];" you init an array of length N, what you need to do is also init each object in it:

for i=0; i<N; i++
    arrayOfSpotlights[i] = new spotlight();
    arrayOfSpotlights[i].turnOn();

Hope I'm correct :)

Answer 4

You declared the array arrayOfSpotlights, but didn't initialize the members of the array (so they are null - and you get the exception).

Change it to:

public class Theatre {
    spotlight[] arrayOfSpotlights;

    public Theatre(int N){

      arrayOfSpotlights =  new spotlight[N];

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

    } 
}

and it should work.