I have an exercise in java whose text is:
The Matrix class represents an array of integers. Its skeleton is as follows: public class Matrix { private int [] [] mat; / * creates a Matrix object whose content is represented by the matrix matrix. * / public Matrix (int [] [] mat) {...} / * returns the sum of the elements of the matrix represented by the object on which the method is invoked. * / public int sumElements () {...} / * Returns the submatrix obtained from the one represented by the receiving object (this) by removing row i and column j * / public Submatrix matrix (int i, int j) {...} / * Returns a two-dimensional array of integers whose element (i, j) is the sum of the elements of the submatrix obtained from the receiving object (this) by removing row i and column j. * / public int [] [] subscriptValue () {...} / * Returns a textual representation of the array * / public String toString () {...}
Write the Matrix class and a Matrix Test class that contains only the main method and that performs the following actions: Makes the user enter a two-dimensional array of integers a and creates a Matrix object m. View the created object to the user. Asks the user to enter an index of row i and an index of column j and displays the user the submatrix obtained from m by deleting row i and row j. Show the user the values of all the submatrixes obtained by removing for each possible pair (i, j) the row i and column j.
I don't understand why the compiler gives me this error: "Exception in thread" main "java.lang.NullPointerException: Cannot read the array length because" this.mat "is null"
package Matrice1;
public class Matrice1 {
private int[][] mat;
public Matrice1(int[][] m) {
this.mat = new int[mat.length][mat[0].length];
for(int i = 0; i < mat.length; i++) {
for(int j = 0; j < mat[0].length; j++) {
this.mat[i][j] = m[i][j];
}
}
}
public int sommaElementi() {
int somma = 0;
for(int i = 0; i < this.mat.length; i++) {
for(int j = 0; j < this.mat[0].length; j++) {
somma += this.mat[i][j];
}
}
return somma;
}
public Matrice1 sottomatrice(int i, int j) {
int [][] n = new int[this.mat.length -1][this.mat[0].length -1];
for(int h = 0; h < n.length; h++) {
for(int k = 0; k < n[h].length; k++) {
int a = h;
int b = k;
if(a >= i) {
a++;
}
if(b >= j) {
b++;
}
n[h][k] = this.mat[a][b];
}
}
Matrice1 m1 = new Matrice1(n);
return m1;
}
public int[][] valoreSottomatrici(){
int[][] valore = new int[this.mat.length][this.mat[0].length];
for(int i = 0; i < this.mat.length; i++) {
for(int j = 0; j < this.mat[0].length; j++) {
Matrice1 m = sottomatrice(i, j);
valore[i][j] = m.sommaElementi();
}
}
return valore;
}
public String toString() {
String s = "";
for(int i = 0; i < this.mat.length; i++) {
for(int j = 0; j < this.mat[0].length; j++) {
s += this.mat[i][j] + " ";
}
s += "\n";
}
return s;
}
}
package Matrice1;
public class ProvaMatrice {
public static void main(String[] args) {
InputWindow in = new InputWindow();
OutputWindow out = new OutputWindow();
int k, h;
k = in.readInt("Numero righe: ");
h = in.readInt("Numero colonne: ");
int[][] m1 = new int[k][h];
for(int i = 0; i < m1.length; i++) {
for(int j = 0; j < m1[i].length; j++) {
m1[i][j] = in.readInt("Inserisci valore in posizione " + i + " " + j +" : ");
}
}
Matrice1 mat = new Matrice1(m1);
out.write("Matrice inserita: " + mat.toString());
}
}
