Matrix rotation

MatrixUtils.java

public class MatrixUtils {

    public static <T> boolean isSquareMatrix(T[][] matrix) {
        if (matrix.length == 0) return true;
        for (int i = 0; i < matrix.length; i++) {
            if (matrix[i].length != matrix.length) return false;
        }
        return true;
    }

    public static <T> void rotateClockwise90(T[][] matrix) throws NotASquareMatrixException {
        if (matrix.length == 0) return;
        else if (!isSquareMatrix(matrix)) throw new NotASquareMatrixException();

        int n = matrix.length;
        for (int i = 0; i < n / 2; i++) {
            for (int j = i; j < n - i - 1; j++) {
                T temp = matrix[i][j];
                matrix[i][j] = matrix[n - 1 - j][i];
                matrix[n - 1 - j][i] = matrix[n - 1 - i][n - 1 - j];
                matrix[n - 1 - i][n - 1 - j] = matrix[j][n - 1 - i];
                matrix[j][n - 1 - i] = temp;
            }
        }
    }

    public static void test3by3() throws NotASquareMatrixException {
        Integer[][] matrix = new Integer[][]{
                new Integer[]{1, 2, 3},
                new Integer[]{4, 5, 6},
                new Integer[]{7, 8, 9}
        };
        rotateClockwise90(matrix);
    }

    public static void test4by4() throws NotASquareMatrixException {
        Integer[][] matrix = new Integer[][]{
                new Integer[]{1, 2, 3, 4},
                new Integer[]{5, 6, 7, 8},
                new Integer[]{9, 10, 11, 12},
                new Integer[]{13, 14, 15, 16}
        };
        rotateClockwise90(matrix);
    }

    public static void main(String[] args) throws NotASquareMatrixException {
        test3by3();
        test4by4();
    }
}

MatrixUtilsTest.java

import org.junit.Test;

import static org.junit.Assert.assertTrue;

public class MatrixUtilsTest {

    private <T> boolean equals(T[][] m1, T[][] m2) {
        if (m1.length == 0 && m2.length == 0) return true;
        if (m1.length != m2.length) return false;
        for (int i = 0; i < m1.length; i++) {
            if (m1[i].length != m2[i].length) return false;
            for (int j = 0; j < m1[0].length; j++) {
                if (!m1[i][j].equals(m2[i][j])) return false;
            }
        }
        return true;
    }

    @Test
    public void test3By3Matrix() throws NotASquareMatrixException {
        Integer[][] matrix = new Integer[][]{
                new Integer[]{1, 2, 3},
                new Integer[]{4, 5, 6},
                new Integer[]{7, 8, 9}
        };

        Integer[][] expected = new Integer[][]{
                new Integer[]{7, 4, 1},
                new Integer[]{8, 5, 2},
                new Integer[]{9, 6, 3}
        };

        MatrixUtils.rotateClockwise90(matrix);
        assertTrue(equals(expected, matrix));
    }

    @Test
    public void test4By4Matrix() throws NotASquareMatrixException {
        Integer[][] matrix = new Integer[][]{
                new Integer[]{5, 1, 9, 11},
                new Integer[]{2, 4, 8, 10},
                new Integer[]{13, 3, 6, 7},
                new Integer[]{15, 14, 12, 16}
        };

        Integer[][] expected = new Integer[][]{
                new Integer[]{15, 13, 2, 5},
                new Integer[]{14, 3, 4, 1},
                new Integer[]{12, 6, 8, 9},
                new Integer[]{16, 7, 10, 11}
        };

        MatrixUtils.rotateClockwise90(matrix);
        assertTrue(equals(expected, matrix));
    }

    @Test(expected = NotASquareMatrixException.class)
    public void testNotASquareMatrixException() throws NotASquareMatrixException {
        Integer[][] matrix = new Integer[][]{
                new Integer[]{1, 2, 3},
                new Integer[]{4, 5, 6, 10},
                new Integer[]{7, 8, 9}
        };
        MatrixUtils.rotateClockwise90(matrix);
    }
}

NotASquareMatrixException.java

public class NotASquareMatrixException extends Exception {
    public NotASquareMatrixException() {
        this("Given matrix is not a square matrix");
    }

    public NotASquareMatrixException(String message) {
        super(message);
    }
}