Matrix rotation

Matrix.java

// -*- compile-command: "javac -Xlint Matrix.java && java Matrix" -*-
import java.lang.Math;

/**
 * Square matrix class.
 *
 * @author Jon-Michael Deldin <[email protected]>
 */
public class Matrix {
    private int N;
    private char[][] mat;

    public Matrix(char[][] mat) {
        this.mat = mat;
        this.N   = mat.length;
    }

    /**
     * Rotate the matrix 90º clockwise.
     *
     */
    public void rotate90cw() {
        char[][] ret = new char[N][N];
        int newX;

        for (int i = 0; i < N; i++) {
            for (int j = 0; j < N; j++) {
                newX = N - 1 - i;
                ret[j][newX] = mat[i][j];
            }
        }
        mat = ret;
    }

    public String toString() {
        StringBuilder sb = new StringBuilder();

        for (int i = 0; i < N; i++) {
            for (int j = 0; j< N; j++) {
                sb.append(mat[i][j]);
                sb.append(" ");
            }
            sb.append("\n");
        }

        return sb.toString();
    }

    public void print() {
        System.out.print(this);
    }

    public static void main(String[] args) {
        Matrix mat = new Matrix(new char[][] {
                {'a', 'b', 'c'},
                {'d', 'e', 'f'},
                {'g', 'h', 'i'},
            });
        mat.print();

        System.out.println();
        mat.rotate90cw();
        mat.print();
    }
}