public void actionPerformed(ActionEvent e) {
                String x = t1.getText();
                String y = t3.getText();
                String o = t4.getText();
                if (x.isEmpty()) {
                    JOptionPane.showMessageDialog(TestFibMethodRange.this, "This field cannot be empty", "Required Field", JOptionPane.ERROR_MESSAGE);
                } else if (y.isEmpty()) {
                    JOptionPane.showMessageDialog(TestFibMethodRange.this, "This field cannot be empty", "Required Field", JOptionPane.ERROR_MESSAGE);
                } else if (o.isEmpty()) {
                    JOptionPane.showMessageDialog(TestFibMethodRange.this, "This field cannot be empty", "Required Field", JOptionPane.ERROR_MESSAGE);
                } else {
                    if (isInteger(x) && isInteger(y) && isInteger(o)) {
                        int min = Integer.valueOf(x);
                        int max = Integer.valueOf(y);
                        int nSequence = Integer.valueOf(o);
                        for (int i = min; i <= nSequence; i++) {
                            if (x3 <= max) {
                                if (x3 >= min) {
                                    t2.setText(t2.getText() + " " + x3);
                                }
                                x1 = x2;
                                x2 = x3;
                                x3 = x1 + x2;
                            }
                        }
                    }
                }

            }