beginner java, help me fix my program?

I am trying to make a calculator for college gpa's. I cut out all like 20 if statements that just say what each letter grade is. I fixed my first program for anybody looking at this again. The program now works开发者_如何学C, but regardless of the letters i type in the gpa it returns is a 2.0 . If anybody sees anything wrong it would be very much appreciated...again. Thanks

import java.util.Scanner;

public class universityGPA {
    public static void main(String args[]){

        int classes = 4;
        int units[] = {3, 2, 4, 4};
        double[] grade = new double[4];
        double[] value= new double[4];
        int counter = 0;
        double total = 0;
        double gpa;
        String letter;


        while(classes > counter){
            Scanner gradeObject = new Scanner(System.in);
             letter = gradeObject.next();

            if(letter.equalsIgnoreCase("A+") || letter.equalsIgnoreCase("A")){
                grade[counter] = 4;
            }
            if(letter.equalsIgnoreCase("F")){
                grade[counter] = 0;
            }

            value[counter] = grade[counter] * units[counter];
            counter++;
        }

        for(int i = 0; i < classes; i++  ){
            total += value[i];
        }

        gpa = total/classes;
        System.out.println("You gpa is " +gpa);
    }
}

You forgot to initialize grade. The NullPointerException is telling you that grade is null. The exception is thrown the first time you try to use grade, in the statment grade[counter] = 4;. Allocate as much space as you need with new.

Initialization of grade can be done statically as well dynamically:

double []grade = new double[4];

or

double []grade = new double[classes];

Do the same for value as well.

Here are a few pointers for cleaning up your code:

• Try to be more consistent with your formatting. Make sure everything is properly indented and that you don't have lingering spaces at the beginnings or endings of lines (line 18).
• You should declare variables as close to the first spot you use them as possible. This, along with making your code much more readable, minimizes the scope. For instance, on line 18, you initialize letter, but it is never used outside the scope of the while statement. You should declare the variable right there, along with the initializer (String letter = gradeObject.next()).
• Declaring arrays in the type name[] form is discouraged. It is recommended to use the type[] name form instead.
• Try to separate your program into distinguished sections. For instance, for this program, you can clearly see a few steps are involved. Namely, you first must grab some input, then parse it, then calculate the return value. These sections can be factored out into separate methods to clean up the code and promote reuse. While it may not seem to yield many benefits for such a simple program, once you start working on larger problems this organization will be absolutely mandatory.

NullPointerException means you are trying to access something that does not exist.

Since your grade[] is null, accessing it on line 21 by grade[counter] actually means you are accessing something that has yet to be created.

You need to initialize the array, so it actually has an instance.