matrice di stringhe

public class MatriceDiStringhe {

    private String[][] mat;

    public MatriceDiStringhe (String[][] m){
        this.mat = new String[m.length][m.length];

        for (int i=0; i<m.length; i++) {
            for (int j=0; j<m.length; j++) {
                this.mat[i][j] = m[i][j];
            }
        }
    }

    public String primaInRiga (int i){
        String[] riga = mat[i];
        String minima = riga[0];
        for (int j = 1; j < riga.length; j++) {
            if (riga[j].compareTo(minima) < 0) {
                minima = riga[j];
            }
        }
        return minima;
    }

    public void estraiSottomatrice(MatriceDiStringhe mds){
        for (int i=0; i<mat.length; i++) {
            for (int j=0; j<mat[i].length; j++) {
                if (Character.isLowerCase(mat[i][j].charAt(0))) {
                    this.mat[i][j] = mds.mat[i][j];
                }
            }
        }
    }

    public String toString() {
        String s = "";

        for (int i=0; i<mat.length; i++) {
            for (int j=0; j<mat[i].length; j++) {
                s += this.mat[i][j] + " ";
            }
            s += "\n";
        }
        return s;
    }

}