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Thread: Why does my two-dimensional array not accepting any values?

  1. #1
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    Default Why does my two-dimensional array not accepting any values?

    I'm creating a 2D array program that accepts values from users and to perform two operations, such as Adding and Multiplying the array, then print the results. However, the program is not accepting any values, it prints the following error:

    Exception in thread "main" java.lang.NullPointerException
    at twodimensionalarray.Matrix.getMatrix(Matrix.java:2 7)
    at twodimensionalarray.TwoDimensionalArray.main(TwoDi mensionalArray.java:18)

    package twodimensionalarray;
     
    import java.util.Scanner;
     
    public class Matrix {
        double [][]Array;
        int rows;
        int cols;
     
        Matrix(){
            rows = 0;
            cols = 0;
            int [][] Array = new int[rows][cols];
        }
     
        Matrix(int row1, int col1)
    	{
    		rows = row1;
    		cols = col1;
    	}
     
        public void getMatrix(){
            Scanner entry = new Scanner(System.in);
            System.out.println("Enter elements for array: ");
            for(int i = 0; i < rows; i++){
                for(int j = 0; j < cols; j++){
                    Array[i][j] = entry.nextDouble();
                }
            }
        }
     
        public void displayMatrix(){
            for(int i = 0; i < rows; i++){
                for(int j = 0; j < cols; j++){
                    System.out.printf("%2f  " + Array[i][j]);
                }
                System.out.println();
            }   
        }
     
        public Matrix addMartix(Matrix first){
            Matrix results = new Matrix();
     
            if (rows != first.rows || cols != first.cols){
                System.out.println("Unable to add matrix!");
            }else{
                for(int i = 0; i < rows; i++){
                    for(int j = 0; j < cols; j++){
                        results.Array[i][j] = (int) (Array[i][j] + Array[i][j]);
                    }
                }
            }
            return results;
        }
     
        public Matrix multiplyMartix(Matrix first){
            Matrix results = new Matrix();
     
            if (rows != first.rows || cols != first.cols){
                System.out.println("Unable to multiply matrix!");
            }else{
                for(int i = 0; i < rows; i++){
                    for(int j = 0; j < cols; j++){
                        results.Array[i][j] = (int) (Array[i][j] * Array[i][j]);
                    }
                }
            }
            return results;
        }
    }
     
     
    package twodimensionalarray;
     
    import java.util.Scanner;
     
    public class TwoDimensionalArray {
        public static Matrix results;
        public static void main(String[] args) {
            int rows = 0, cols = 0;
     
            Scanner entry = new Scanner(System.in);
     
            System.out.println("Enter number of rows: ");
            rows = entry.nextInt();
            System.out.println("Enter number of columns: ");
            cols = entry.nextInt();
     
            Matrix a = new Matrix(rows, cols);
            a.getMatrix();
     
     
            results = a.addMartix(a);
            results.displayMatrix();
     
            results = a.multiplyMartix(a);
            results.displayMatrix();
        }
     
    }

  2. #2
    Super Moderator Norm's Avatar
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    Default Re: Why does my two-dimensional array not accepting any values?

    Exception in thread "main" java.lang.NullPointerException
    at twodimensionalarray.Matrix.getMatrix(Matrix.java:2 7)
    What variable has the null value when line 27 is executed?

    Note: Array is a poor name for a variable. Variable names should begin with a lowercase letter.
    Also Array is the name of a Java SE class.

    These statements create a local array with 0 rows and 0 columns:
       Matrix(){
            rows = 0;
            cols = 0;
            int [][] Array = new int[rows][cols];  // Array is local to the constructor.
        }

    The variable named Array declared at the class level is never given a value so its value would be null.
    If you don't understand my answer, don't ignore it, ask a question.

  3. #3
    Super Moderator Norm's Avatar
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    Default Re: Why does my two-dimensional array not accepting any values?

    Also posted here: https://coderanch.com/t/734404/java/...values#3415346

    Please read: http://www.javaprogrammingforums.com...s-posting.html
    If you don't understand my answer, don't ignore it, ask a question.

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