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In Java, we have already defined, exception classes such as ArithmeticException, NullPointerException etc. These exceptions are already set to trigger on pre-defined conditions such as when you divide a number by zero it triggers ArithmeticException.

In Java, we can create our own exception class and throw that exception using throw keyword. These exceptions are known as user-defined or custom exceptions.

Problem statement: Realize a Java class Matrix to represent bi-dimensional matrices of real numbers. The class should export the following methods:

Matrix(int n, int m): Constructor that creates a matrix of size nxm, with all values initially set to 0;
Matrix product(Matrix m): It returns the matrix that is the product of the object and of m, if the two matrices have compatible dimensions, and null otherwise; ExceptionWrongMatrixDimension that is thrown in the method check() if the dimension of the matrix is wrong for the multiplication of the matrix.

Example:

Java

// Java program to create user defined
// exceptions

import java.util.Scanner;

// User defined exception class to store the exception
// message
class ExceptionWrongMatrixDimension extends Exception {

    public ExceptionWrongMatrixDimension(String str)
    {
        // stores the exception message to be displayed
        super(str);
    }
}

class twoDimensionalMatrix {

    void Matrix(int n, int m)
        throws ExceptionWrongMatrixDimension
    {
        // initialize matrix to be processed
        int[][] matrix = { { 1, 2 },
                           {
                               4,
                               5,
                           } };

        System.out.println("\nMatrix is :");

        for (int i = 0; i < m; i++) {
            for (int j = 0; j < n; j++) {
                System.out.print(matrix[i][j] + " ");
            }
            System.out.println();
        }

        int rows = 2;
        int cols = 2;

        if (n != rows) {

            // throw keyword for an exception in a method
            throw new ExceptionWrongMatrixDimension(
                "Invalid matrix dimensions to multiply");
        }
        else {
            int[][] m_matrix = { { 6, 3 },
                                 {
                                     9,
                                     2,
                                 } };

            System.out.println("\nMatrix to multiply is :");

            for (int i = 0; i < rows; i++) {
                for (int j = 0; j < cols; j++) {

                    System.out.print(m_matrix[i][j] + " ");
                }
                System.out.println();
            }

            System.out.println("\nMatrix to multiply is :");

            for (int i = 0; i < rows; i++) {
                for (int j = 0; j < cols; j++) {
                    System.out.print(m_matrix[i][j] + " ");
                }
                System.out.println();
            }

            int c[][] = new int[m][n];

            for (int i = 0; i < rows; i++) {
                for (int j = 0; j < rows; j++) {
                    c[i][j] = 0;
                    for (int k = 0; k < rows; k++) {
                        c[i][j] += matrix[i][j]
                                   * m_matrix[k][j];
                    }
                }
            }

            System.out.println(
                "\n\nMatrix after multiplication is");

            for (int i = 0; i < rows; i++) {
                for (int j = 0; j < cols; j++) {

                    // prints the third matrix containing
                    // the multiplied values
                    System.out.print(c[i][j] + " ");
                }
                System.out.println();
            }
        }
    }
}

public class Main {
    public static void main(String args[])
    {
        twoDimensionalMatrix matrix
            = new twoDimensionalMatrix();

        try {

            // block of code to be tested for errors while
            // it is being executed.
            System.out.println("Enter the number of rows");

            int n = 2;

            System.out.println(
                "Enter the number of columns");
            int m = 2;
            matrix.Matrix(n, m);
        }

        catch (ExceptionWrongMatrixDimension e) {

            // block of code to be executed, if an error
            // occurs in the try block.
            System.out.println(
                "ExceptionWrongMatrixDimension:");

            // returns a method object. The name parameter
            // is passed as a string.
            System.out.println(e.getMessage());
        }
    }
}

Output

Enter the number of rows
Enter the number of columns

Matrix is :
1 2 
4 5 

Matrix to multiply is :
6 3 
9 2 

Matrix to multiply is :
6 3 
9 2 


Matrix after multiplication is
15 10 
60 25

User Defined Exceptions using Constructors in Java

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