Exception in thread "main" java.lang.Nullpointerexception: Calculator

Question

I started studying Object Orientation and decided to make a simple calculator program. At the moment I just made the sum to see if the way I was doing is right.

Follows the code:

package javaapplication100;
import java.util.Scanner;

/**
 *
 * @author Marcielli
 */
public class JavaApplication100 {

    public static void main(String[] args) {

        Scanner input = new Scanner(System.in);
        Calculadora calc = new Calculadora();

        System.out.println("Digite:\n+ - Somar\n* - Multiplicar\n/ - Dividir\n- - Subtrair");
        String escolha = input.nextLine();

        switch (escolha) {
            case "+":
                System.out.println("Somar quantos numeros? ");
                int a = input.nextInt();            

                calc.setNumerosSomar(a);//Esta acusando erro aqui.
                //calc.setNumerosSomar(calc.qntNumeros = input.nextInt());                
                System.out.println("A soma: "+calc.getNumerosSomar());
                break;
            case "*":
                break;
            case "/":
                break;
            case "-":
                break;
            default:
                break;
        }
    }
}

class Calculadora {

    Scanner input = new Scanner(System.in);

    protected double[] numerosSomar;
    private double[] numerosMultiplicar;
    private double[] numerosDividir;
    private double[] numerosSubtrair;
    public int qntNumeros;
    public double resSoma;
    public double resMultiplicar;
    public double resDividir;
    public double resSubtrair;

    public void setNumerosSomar(int a) {

        this.qntNumeros = a;

        for(int i=0; i<qntNumeros; i++) {

            System.out.println("Digite um numero: ");
            numerosSomar[i] = input.nextDouble(); //Esta acusando erro aqui.

        }

       for(int i=0; i<qntNumeros; i++) {

            resSoma+=numerosSomar[i];

        }     

    }

    public double getNumerosSomar() {

        return resSoma;

    }
}

I’m really not sure if I’m doing anything right, because I’m still trying to understand the concept of Object Orientation as a whole.

The problem I’m having is the mistake:

Exception in thread "main" java.lang.Nullpointerexception.

I left a comment on the lines that the IDE is pointing out that it is in trouble. But just in case they follow below:

calc.setNumerosSomar(a);

numerosSomar[i] = input.nextDouble();

My question here is this, is it a mistake of logic? Because I believe that the problem is not in the method, because if I typed 2 in qntNumeros (variable used to determine when the for will stop), this variable will be valid 2.

The loop for is of 0 < qntNumeros(que vale 2). Here it seems to me there is no mistake.

I also see no mistake in the line numerosSomar[i] = input.nextDouble();, because I really believe that’s how you store a value in a double-type vector.

Looking at that question Exception in thread "main" java.lang.Nullpointerexception I believe that I’m trying to write data into a null state, and that I need to initialize the vector first. But I didn’t understand how I should initialize the vector before.

Answer 1

The error is simple. You are not declaring the initial value of the number array.

protected double[] numerosSomar = new double[100];
private double[] numerosMultiplicar = new double[100];
private double[] numerosDividir = new double[100];
private double[] numerosSubtrair  = new double[100];

I tested with the change and gave everything OK

Obs.: this value "100" within the array declaration is the maximum value it can allocate or "How many numbers you want..."