import java.util.Arrays; import java.util.concurrent.ThreadLocalRandom;

public class Matrix { private int[][] data;

public Matrix(int rows, int cols) {
    if (rows <= 0 || cols <= 0) throw new IllegalArgumentException("Invalid size");
    this.data = new int[rows][cols];
}

public Matrix(int[][] data) {
    if (data == null || data.length == 0 || data[0].length == 0)
        throw new IllegalArgumentException("Invalid array");
    int r = data.length, c = data[0].length;
    this.data = new int[r][c];
    for (int i = 0; i < r; i++) {
        if (data[i].length != c) throw new IllegalArgumentException("Jagged array");
        System.arraycopy(data[i], 0, this.data[i], 0, c);
    }
}

public void populateRandom() {
    ThreadLocalRandom rnd = ThreadLocalRandom.current();
    for (int i = 0; i < data.length; i++)
        for (int j = 0; j < data[0].length; j++)
            data[i][j] = rnd.nextInt(1, 11);
}

public Matrix add(Matrix other) {
    if (other == null) throw new IllegalArgumentException("null");
    if (data.length != other.data.length || data[0].length != other.data[0].length)
        throw new IllegalArgumentException("Dimension mismatch");
    int r = data.length, c = data[0].length;
    Matrix res = new Matrix(r, c);
    for (int i = 0; i < r; i++)
        for (int j = 0; j < c; j++)
            res.data[i][j] = this.data[i][j] + other.data[i][j];
    return res;
}

public Matrix multiply(Matrix other) {
    if (other == null) throw new IllegalArgumentException("null");
    int r1 = data.length, c1 = data[0].length, r2 = other.data.length, c2 = other.data[0].length;
    if (c1 != r2) throw new IllegalArgumentException("Dimension mismatch");
    Matrix res = new Matrix(r1, c2);
    for (int i = 0; i < r1; i++) {
        for (int k = 0; k < c1; k++) {
            int a = this.data[i][k];
            if (a == 0) continue;
            for (int j = 0; j < c2; j++) {
                res.data[i][j] += a * other.data[k][j];
            }
        }
    }
    return res;
}

@Override
public String toString() {
    StringBuilder sb = new StringBuilder();
    for (int i = 0; i < data.length; i++) {
        sb.append(Arrays.toString(data[i]));
        if (i != data.length - 1) sb.append(System.lineSeparator());
    }
    return sb.toString();
}

public int rows() { return data.length; }
public int cols() { return data[0].length; }

public static void main(String[] args) {
    Matrix A = new Matrix(2, 3);
    Matrix B = new Matrix(3, 2);
    A.populateRandom();
    B.populateRandom();
    System.out.println("A:\n" + A);
    System.out.println("B:\n" + B);
    System.out.println("A * B:\n" + A.multiply(B));
    Matrix C = new Matrix(new int[][]{{1,2,3},{4,5,6}});
    Matrix D = new Matrix(new int[][]{{7,8,9},{10,11,12}});
    System.out.println("C + D:\n" + C.add(D));
}

}