public class Main {

    public static void main(String[] args) throws MatrixIndexException {
        Matrix m1 = new Matrix1D(3, 3);
        Matrix m2 = new Matrix1D(3, 3);
        
        MatrixUtils.fillByRandom(m1);
        MatrixUtils.fillByRandom(m2);
        
        Matrix m = MatrixUtils.matrixMultiple(m1, m2);
        
        System.out.println(m.toString());
        MatrixUtils.writeMatrixToFile(m, "file.txt");
        Matrix m3 = MatrixUtils.loadMatrixFromFile("file.txt");
        System.out.println(m3.toString());
    }
}

package matrix;

import java.util.Random;
import java.util.Scanner;
import java.io.InputStreamReader;
import java.io.IOException;
import java.io.FileReader;
import java.io.FileWriter;

public class MatrixUtils {

    public static void fillByRandom(Matrix m) {
        int rowCount = m.getRowCount();
        int colCount = m.getColCount();
        Random random = new Random();

        for (int i = 1; i <= rowCount; i++) {

            for (int j = 1; j <= colCount; j++) {
                try {
                    m.put(i, j, random.nextInt(50));
                } catch (MatrixIndexException e) {
                    System.err.println(e.getMessage());

                }

            }
        }
    }

    public static void fillByNumber(Matrix m, int number) {
        int rowCount = m.getRowCount();
        int colCount = m.getColCount();

        for (int i = 1; i <= colCount; i++) {

            for (int j = 1; j <= rowCount; j++) {
                try {
                    m.put(i, j, number);

                } catch (MatrixIndexException e) {
                    System.err.println(e.getMessage());

                }

            }
        }

    }

    public static Matrix fillByKeyboard() {
        Scanner scan = new Scanner(System.in);
        System.out.println("Enter The Number Of Matrix Rows:");
        int rowCount = scan.nextInt();
        System.out.println("Enter The Number Of Matrix Columns:");
        int colCount = scan.nextInt();
        try {
            Matrix m = new Matrix1D(rowCount, colCount);
            System.out.println("Enter Matrix Data:");
            for (int i = 1; i <= rowCount; i++) {
                for (int j = 1; j <= colCount; j++) {
                    int value = scan.nextInt();
                     m.put(i, j, value);
                }
            }
            return m;

        } catch (MatrixIndexException ex) {
            System.err.println(ex.getMessage());

        }
        return null;

    }
    
    public static Matrix matrixMultiple(Matrix a, Matrix b) throws MatrixIndexException {
       
        int aRows = a.getRowCount();
        int aColumns = a.getColCount();
        int bRows = b.getRowCount();
        int bColumns = b.getColCount();

        if (aColumns != bRows) {
            throw new IllegalArgumentException("A:Rows: " + aColumns + " did not match B:Columns " + bRows + ".");
        }

        Matrix c = new Matrix1D(aRows, bColumns);
        for (int i = 0; i < aRows; i++) {
            for (int j = 0; j < bColumns; j++) {
                c.put(i+1,j+1,0);
            }
        }

        for (int i = 0; i < aRows; i++) { // aRow
            for (int j = 0; j < bColumns; j++) { // bColumn
                for (int k = 0; k < aColumns; k++) { // aColumn
                    c.put(i+1,j+1,c.get(i+1,j+1) + a.get(i+1, k+1) * b.get(k+1, j+1));
                }
            }
        }

        return c;   
    }

    public static void writeMatrixToFile(Matrix m, String filename) {

        try(FileWriter writer = new FileWriter(filename, false)){
            String strForWrite = "";
            
            writer.write(String.valueOf(m.getRowCount()));
            writer.write("\n");
            writer.write(String.valueOf(m.getColCount()));
            writer.write("\n");

            for (int i = 1; i <= m.getRowCount(); i++){
                for (int j = 1; j <= m.getColCount(); j++){
                    
                    strForWrite += m.get(i,j) + " ";
                }
                strForWrite += "\n";
            }

            writer.write(strForWrite);
            writer.flush();
        } catch (IOException e) {
            e.printStackTrace();
        } catch(MatrixIndexException e) {
             e.printStackTrace();
        }
    }

    public static Matrix loadMatrixFromFile(String filename) {

        try(FileReader reader = new FileReader(filename)) {
            String strFromFile = "";

            while (reader.ready()) {
                strFromFile += (char) reader.read();
            }

            String[] arr = strFromFile.split(" |\n");
            Matrix result = new Matrix1D(Integer.valueOf(arr[0]), Integer.valueOf(arr[1]));

            int row = 1;
            int col = 1;

            for (int i = 2; i < arr.length; i++){
                if (!arr[i].equals("")){
                    result.put(row, col, Integer.valueOf(arr[i]));
                    col++;
                    if (col == result.getColCount() + 1){
                        col = 1;
                        row++;
                    }
                }
            }

            return result;
        } catch (Exception e) {
            e.printStackTrace();
        }
        return null;
    }
}

public class Main extends Exception {

    public static void main(String[] args) throws MatrixIndexException {

        Matrix1D m = new Matrix1D(100, 100);
        
        MatrixUtils.fillByRandom(m);
        
        MatrixUtils.writeMatrixToFile(m, "C://Data//1.txt");
        
        System.out.println(m.toString());

3
3
2558 1272 672 
1499 2393 485 
1235 2164 708