Null pointer exception

[DBaskov]

Hello, I'm trying to create a class called "Bottle" and it's a class were give values to each Bottle instance and then you can do things like multiplying, dividing, adding, subtracting them. I'm having trouble with my add/subtract/divide/multiply methods, since every time I try to run them I get a null pointer exception. I had also created a method called checkNumber, to make sure the number value for Bottle classes is not null. Please help me resolve this issue. The program is compiled like this:

package bottleclass;

import java.util.Scanner;

/**
*
* @author
*/
public class Bottle {

public int number;
public int bottle;
public Bottle other;
public Bottle answer;

private int read() {

int check;
Scanner input = new Scanner(System.in);
System.out.println("Enter the number of bottles: ");
check = input.nextInt();
if(check > 0) {
number = check;
}
else {
System.out.println("Number you had entered is invalid");
}

return number;

}

private void checkNumber() {

System.out.println(number);
}

private void setInt(int number) {
this.number = number;
}

private void setBottle(Bottle other) {
this.other = other;

}

private Bottle add(Bottle other) {
this.other = other;

answer.number = this.number + other.number;
return answer;
}

private Bottle subtract(Bottle other) {

this.other = other;


answer.number = this.number - other.number;
return other;

}



public static void main(String[] args) {
Bottle b1 = new Bottle();
b1.read();
b1.checkNumber();
Bottle b2 = new Bottle();
b2.read();
b2.checkNumber();
b2 = b2.add(b1);
Bottle BottleAve = new Bottle();

//BottleAve = BottleAve.subtract(b2)


}

}

[Norm]

every time I try to run them I get a null pointer exception.

Please copy full text of error message and paste it here.

Also please wrap your code in code tags to preserver formatting. See: http://www.java-forums.org/misc.php?do=bbcode#code

[DBaskov]

Ok, let me try to post it:

The error is:

Exception in thread "main" java.lang.NullPointerException
        at bottleclass.Bottle.add(Bottle.java:55)
        at bottleclass.Bottle.main(Bottle.java:78)
Java Result: 1

package bottleclass;

import java.util.Scanner;

/**
 *
 * @author    
 */
public class Bottle {
    
    public int number;
    public int bottle;
    public Bottle other;
    public Bottle answer;
    
    private int read() {
        
        int check;
        Scanner input = new Scanner(System.in);
          System.out.println("Enter the number of bottles: ");
          check = input.nextInt(); 
          if(check > 0) {
              number = check;
          }
          else {
              System.out.println("Number you had entered is invalid");
          }
          
          return number;  
        
    }
    
    private void checkNumber() {
       
        System.out.println(number);
    }
    
    private void setInt(int number) {
        this.number = number;
    }
    
    private void setBottle(Bottle other) {
        this.other = other;
        
    }
    
    private Bottle add(Bottle other) {
        this.other = other;
        
        answer.number = this.number + other.number;
        return answer;
    }
    
    private Bottle subtract(Bottle other) {
        
        this.other = other;
        
        
        answer.number = this.number - other.number;
        return other;
        
    }
    
   
    
    public static void main(String[] args) {
        Bottle b1 = new Bottle();
        b1.read();
        b1.checkNumber();
        Bottle b2 = new Bottle();
        b2.read();
        b2.checkNumber();
        b2 = b2.add(b1);
        Bottle BottleAve = new Bottle();
        
        //BottleAve = BottleAve.subtract(b2)
        
       
    }

}

[Norm]

java.lang.NullPointerException
at bottleclass.Bottle.add(Bottle.java:55)

What variable is null at line 55? Find that variable. Has it ever been given a value? Back track in your code to see why is doesn't have a value.

[DBaskov]

What variable is null at line 55? Find that variable. Has it ever been given a value? Back track in your code to see why is doesn't have a value.

At line 55 I believe the variable is called answer.number

It's a class reference variable that has int number variable attached to it, and I though by using operations of two variable and assigning it to that variable would give it the value...

[Fubarable]

At line 55 I believe the variable is called answer.number

It's a class reference variable that has int number variable attached to it, and I though by using operations of two variable and assigning it to that variable would give it the value...

Look back and see where or if you have you initialized "answer"?

[DBaskov]

Look back and see where or if you have you initialized "answer"?

I made it into an instance variable, doesn't that initializes it or do I need to assign a value regardless?

[Norm]

doesn't that initializes it

The compiler will initialize primitives like int and boolean.
Otherwise you are responsible.
What data type is answer?

[DBaskov]

The compiler will initialize primitives like int and boolean.
Otherwise you are responsible.
What data type is answer?

The answer is a class reference variable: Bottle answer;
If I should assign value to it, what should it be or how? I never really dealt with class reference variable that much to be honest...

[Norm]

If I should assign value to it, what should it be or how?

If you have no use for the variable, take it out of your code.
If you do have a use for it, then what is that usage?

The "how" is by using an assignment statement with new:
AClass yourVar = new AClass(); // create an instance of the AClass class

[DBaskov]

If you have no use for the variable, take it out of your code.
If you do have a use for it, then what is that usage?

The "how" is by using an assignment statement with new:
AClass yourVar = new AClass(); // create an instance of the AClass class

Well what I'm trying to do is create a method called "add" for say that returns a "Bottle(class reference variable)" and I want to take one instance of a class Bottle and add it to another by using one of the instances as a parameters, I thought by adding those two instances and assigning them to variable called
"answer" and returning it would give me the value of those two object instances being added...

[Norm]

I thought by adding those two instances and assigning them to variable called
"answer" and returning it would give me the value of those two object instances being added

The code you posted does not do that.
The assignment was not to the variable answer. To assign a value to answer you would code:
answer = <value to put in answer>
Your were adding the contents of variables with two classes and storing the value inside of another instance of the class. However, you have never created that instance (it had a null value) so you got the NullPointerException.
You need to create an instance of the Bottle class and put its reference in the answer variable.

[DBaskov]

The code you posted does not do that.
The assignment was not to the variable answer. To assign a value to answer you would code:
answer = <value to put in answer>
Your were adding the contents of variables with two classes and storing the value inside of another instance of the class. However, you have never created that instance (it had a null value) so you got the NullPointerException.
You need to create an instance of the Bottle class and put its reference in the answer variable.

Well I couldn't assign the solution of adding those two variables because they have int(number) variable in them, and java does not allow assigning int variables to object referenced variable unless I use something like this:

answer.number = this.number + other.number;
return answer;

It won't let me return answer.number , nor would it let me do: answer = this.number + other.number because number is an int. I declared variable Bottle answer; as an instance variable btw.

[Norm]

What does the add() method return? Where will you get the thing to be returned?
Answer: YOU WILL HAVE TO CREATE ONE.
See post #8 for some syntax for creating an instance of a class.

[DBaskov]

Hmm, It seems to be working now. I didn't it think it was necessary for me to instantiate the instance of a class in the method because I already had it as an instance variable. Basically what I did is put: Bottle answer = new Bottle() in that method and it seems to be working fine. Thanks for the help. Hopefully I won't need any more for a while lol.