Thread: NullPointerException:null problem

Hello,

I'm trying to make a simple calculator with Java and everything compiles well but when I try to run it, I get an error on the line
String expression = keyboard.nextLine();
saying "NullPointerException:null"

I read a few webpages saying that it is trying to access something but that something is null? :S

Thanks

import java.util.Scanner;
 
public class Calculator
{
    private static Scanner keyboard;
 
    public static void main(String[] args)
    {
 
        System.out.println("Welcome to calculator 1.0");
        System.out.println();
 
        System.out.print("Enter an expression: ");
        String expression = keyboard.nextLine();
 
        // The user must enter an expression such as 2+3.
        // You can assume that only 1-digit numbers are typed so
        // that you can use expression.charAt(0) to get the first
        // number and so on.
 
        int number1; // the first number
        int number2; // the second number
        char operation; // the operation (+,-,*,/)
 
        number1 = expression.charAt(0);
        operation = expression.charAt(1);
        number2 = expression.charAt(2);
 
        // Now calculate the answer and print it out.
        //
        // Use an if/else statement to handle each operation
        int answer;
        if (operation == '+')
            answer = number1+number2;
        else if (operation == '-')
            answer = number1-number2;
        else if (operation == '*')
            answer = number1*number2;
        else if (operation == '/')
            answer = number1/number2;
 
            else
        {
            System.out.println("Unsupported operation: " + operation);
            return;
        }
 
        System.out.println("The answer is" +answer);
    }
}

you forgot the correct instantiation of the object for the Scanner class



you only have this
,

private static Scanner keyboard;

it should be like this,

private static Scanner keyboard = new Scanner(System.in);

Indeed, you have a null reference, when you attempt to call a method on a null reference you will be thrown a NullPointerException.

    String myString = null;
    myString.length();

// Json

sir json i notice his error and i corrected it.. but i need some brief explanation about the
correct instantiation that i've posted

can you please explain it how? and why?

tnx

Here is a scary bit of code for you though.

Try running this.

public class MyClass {
 
    public static void myMethod() {
        System.out.println("No null pointer exception?");
    }
 
 
    public static void main(final String[] arguments) {
        MyClass myclass = null;
        myClass.myMethod();
    }
}

// Json

My eclipse compiles fine with that, you should be getting a compiler warning though.

You can call statics using "this" as well if you like, during runtime the JVM will just reference the class rather than the instance anyways.

// Json