java - Exception in main thread. Do not know the reason - 


package examples; import java.util.scanner;  public class matrixmultiplication {     public static void main(string[] args) {

the below 4 sections identifies user input rows , columns of 2 matrices.

        scanner userrows1 = new scanner(system.in);         system.out.println("enter number of rows matrix 1: ");         int rows1 = userrows1.nextint();          scanner usercolumns1 = new scanner(system.in);         system.out.println("enter number of columns matrix 2");          int columns1 = usercolumns1.nextint();          scanner userrows2 = new scanner(system.in);         system.out.println("enter number of rows matrix 2: ");         int rows2 = userrows2.nextint();          scanner usercolumns2 = new scanner(system.in);         system.out.println("enter number of columns matrix 2");          int columns2 = usercolumns2.nextint();

this sets objects matrix1 , matrix2 belonging class matrix

        matrix matrix1 = new matrix(rows1, columns1);         matrix matrix2 = new matrix(rows2, columns2);          matrix1.showmatrix();         system.out.println("\n \n");          matrix2.showmatrix();      }  }   class matrix {      int rows;     int columns;     int[][] values;      public matrix(int r, int c) {         rows = r;         columns = c;          int[][] values = new int[r][c];

this served allow user input values of matrix 1 one. set values of matrix value simplicity.

        int i;         int j;         for(i = 0; < r; i++) {             for(j = 0; j < c; j++) {                 //scanner userelement = new scanner(system.in);                 //system.out.println("enter number:");                 //int element = userelement.nextint();                 values[i][j] = 1;              }           }        }       public void showmatrix() {         int k;         int l;         for(k = 0; k < rows; k++) {             for(l = 0; l < columns; l++) {                 system.out.println(values[k][l] + " ");              }             system.out.println("\n");          }       }   }   code above. in final method in class matrix (the method showmatrix), trying print out matrix. however, using general values matrix here , says:  exception in thread "main" java.lang.nullpointerexception     @ examples.matrix.showmatrix(matrixmultiplication.java:75)     @ examples.matrixmultiplication.main(matrixmultiplication.java:29)  can diagnose issue? i'm still new java.

you've not instantiate field [][]values (there local declaration of int[][] values).

public matrix(int r, int c) {   rows = r;   columns = c;    int[][] values = new int[r][c]; <--  remove   values = new int[r][c];   .... }

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