current community

your communities

more stack exchange communities

company blog
Stack Exchange Inbox Reputation and Badges
tour help
Stack Overflow is a community of 4.7 million programmers, just like you, helping each other.

Join them; it only takes a minute:

Sign up
Join the Stack Overflow community to:
  1. Ask programming questions
  2. Answer and help your peers
  3. Get recognized for your expertise
up vote 0 down vote favorite

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;

}
}

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

share|improve this question
    
Arrays are initialized with null values. You need to assign a new spotlight to each entry of the array, like so: array[2] = new spotlight(). Side note: class names by convention (not syntax) should start with a capital letter. – G. Bach Feb 3 '13 at 20:48
1  
Making your array package private is kind of risky, even if the array is final, the references can still be changed. package private is better than public, but you still need to take extra care that things aren't changing when you don't expect them to. – Kurtymckurt Feb 3 '13 at 20:58
    
(I have to mention, ignoring coding conventions is a really bad idea.) – Tom Hawtin - tackline Feb 3 '13 at 21:07
up vote 3 down vote

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.

share|improve this answer
    
Thank you this worked perfectly – user2037935 Feb 3 '13 at 20:56
up vote 2 down vote

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 :)

share|improve this answer
    
arrayOfSpotlights[i] = new spotlight(); Wow thanks so much to everyone - it was this line in the for loop that was missing and now the whole program ( which was a lot longer! ) works - thanks again – user2037935 Feb 3 '13 at 20:54
1  
Apart from the required brackets, it is correct. – G. Bach Feb 3 '13 at 20:55
    
Long live pseudo code :) – Odinn Feb 4 '13 at 6:34
up vote 1 down vote

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;
    }
}
share|improve this answer
    
Thank you - I will try these methods without the cryptic code also – user2037935 Feb 3 '13 at 20:57
    
@user2037935 Once upon a time saving a few bytes mattered and using 0 and 1 instead of on and off was important. e.g. in the Apollo 11 mission they ran out of memory while trying to land the shuttle. ;) – Peter Lawrey Feb 3 '13 at 21:00
up vote 0 down vote

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.

share|improve this answer

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.