Suduko solver 3

package suduko_solver;

import javax.swing.*;
import java.awt.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.event.KeyAdapter;
import java.awt.event.KeyEvent;

public class Suduko_solver extends JFrame implements ActionListener {

    private final int MAX_PERMITTED_TEXT_LENGTH = 1;
    private final Container C;
    private final int[][] grid = new int[10][10];
    private final JTextField tf[][] = new JTextField[10][10];
    private JButton btn, btn2;
    private Font f;

    boolean isSafe(int num, int r, int c) {
        int i, j, rs, cs;
        for (i = 0; i < 9; i++) {
            if (grid[r][i] == num) {
                return false;
            }
        }
        for (i = 0; i < 9; i++) {
            if (grid[i][c] == num) {
                return false;
            }
        }
        rs = r - r % 3;
        cs = c - c % 3;
        for (i = 0; i < 3; i++) {
            for (j = 0; j < 3; j++) {
                if (grid[rs + i][cs + j] == num) {
                    return false;
                }
            }
        }
        return true;
    }

    boolean solve() {
        //     System.out.println("hello");
        boolean flag;
        flag = false;
        int i, j = 0;
        for (i = 0; i < 9; i++) {
            for (j = 0; j < 9; j++) {
                if (grid[i][j] == 0) {
                    flag = true;
                    break;
                }
            }
            if (flag) {
                break;
            }
        }
        int rval = i;
        int cval = j;
        if (!flag) {
            return true;
        } else {
            for (i = 1; i <= 9; i++) {
                if (isSafe(i, rval, cval)) {
                    grid[rval][cval] = i;
                    if (solve()) {
                        return true;
                    }
                    grid[rval][cval] = 0;
                }
            }
        }
        return false;
    }

    Suduko_solver() {
        C = this.getContentPane();
        C.setLayout(null);
        f = new Font("Arial", Font.BOLD, 25);
        JButton btn = new JButton("Solve");
        btn.setBounds(180, 520, 80, 50);
        C.add(btn);

//        JButton btn2 = new JButton("Clear");
//        btn2.setBounds(250, 520, 80, 50);
//        C.add(btn2);

        int a, b;
        a = 10;
        b = 60;
        for (int i = 0; i < 9; i++) {
            a = 10;
            for (int j = 0; j < 9; j++) {
                grid[i][j] = 0;

                //-> Permitted input text length check for each textfield in grid :: START
                JTextField currField = tf[i][j] = new JTextField();//-> referencing currentField
                currField.addKeyListener(new KeyAdapter() { //-> adding listener
                    @Override
                    public void keyTyped(KeyEvent e) { //-> key pressed event
                        if (currField.getText().length() >= MAX_PERMITTED_TEXT_LENGTH) //-> max permitted text length check
                            e.consume();// Consumes this event so that it will not be processed in the default manner by the source which originated it.
                    }
                });
                //-> Permitted input text length check for each textfield in grid :: END

                tf[i][j].setFont(f);
                tf[i][j].setHorizontalAlignment(JTextField.CENTER);
                tf[i][j].setBounds(a, b, 50, 50);
                C.add(tf[i][j]);
                a += 50;
            }
            b += 50;
        }
        btn.addActionListener(this);
        //   btn2.addActionListener(this);

    }

    @Override
    public void actionPerformed(ActionEvent e) {

        // System.out.println("hellow...2");

        //  if (e.getSource() == btn) {

        String str, str2;
        boolean key = true;

        for (int i = 0; i < 9; i++) {

            for (int j = 0; j < 9; j++) {
                str = tf[i][j].getText();
                if (str.length() > 0) {
                    grid[i][j] = Integer.parseInt(str);
                    if (grid[i][j] > 9 && grid[i][j] < 1) {
                        key = false;
                        str2 = "Wrong input in row " + (i + 1) + " colom " + (j + 1);
                        JOptionPane.showMessageDialog(null, str2);
                    }
                }
            }

        }

        if (solve() && key) {
            for (int i = 0; i < 9; i++) {

                for (int j = 0; j < 9; j++) {
                    str = "" + grid[i][j];
                    tf[i][j].setText(str);
                }

            }
        } else {
            JOptionPane.showMessageDialog(null, "No solution exists for this input");

        }
//        }
//else {
//
//            Suduko_solver frm = new Suduko_solver();
//            frm.setVisible(true);
//            frm.setBounds(100, 100, 480, 600);
//            frm.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
//            frm.setResizable(false);
//        }

    }

    public static void main(String[] args) {
        Suduko_solver frm = new Suduko_solver();
        frm.setVisible(true);
        frm.setBounds(100, 100, 480, 600);
        frm.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frm.setResizable(false);
    }

}