java.lang.NullPointerException when adding two numbers

Question

I'm new to java and am trying to write a simple program that basicly asks for user input and then returns the average. I keep getting java.lang.NullPointerException when trying to add. Why is this? This is the code I have so far.

import java.io.*;
class Numbers {
    public static void main(String[] args) {
            System.out.println("Hello USER! This is TRON, state the number of NUMBERS you wish to enter and I will return the Average..."); // Display the string.
            BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
            Integer loopnum = null;
            Integer i = null;
            Long num = null;
            Long sum = null;
            Long avg = null;

            try {
                loopnum = Integer.parseInt(br.readLine());

            }
            catch(IOException e){
                System.out.println("Error!");
                System.exit(1);
            }
            System.out.println("OK now enter your numbers.");
            for (i=1;i<=loopnum; i++) {
                try {
                System.out.println("Enter number "+i+":");
                num = Long.parseLong(br.readLine());
                sum += num;
                }
                catch(IOException e){
                    System.out.println("Error!");
                    System.exit(1);
                }
            }

                avg = sum / loopnum;
                System.out.println("TRON here, Your average is: " + avg);




    }
}

Answer 1

Initialize! Change declaration to Long sum = 0L at the beginning of your code. This line is problematic in your loop:

sum += num; 

it's same as sum = sum + num; Evaluation is done from the right to left and you see it tries to add variable num with a null variable sum.

Answer 2

You should initialize your Long variables in 0L , not null . Even better, use the primitive type long instead of the wrapper type Long and int instead of Integer , you don't need to use wrapper objects here! Do this:

int loopnum = 0;
int i = 0;
long num = 0L;
long sum = 0L;
long avg = 0L;

The above will get rid of the NullPointerException and will avoid the unnecessary boxing/unboxing of values between primitive and object data types.

Answer 3

All the other answers fail to address why you're getting the null pointer exception. You are using the Long object instead of the long primitive. Thus anytime you use a direct arithmetic operation (such as + ), the compiler will unbox Long into long by invoking Long.longValue() . So sum += sum gets translated to:

sum = Long.valueOf(sum.longValue() + sum.longValue());

Since sum is null you'll end up with a null pointer exception. Most people avoid this by just using the primitve long .

Answer 4

initialize your sum variable with a non null value. 0L perhaps.

replace this line sum += num with sum = sum + num .. this is more readable.