Handler.java

import javafx.beans.value.ChangeListener;
import javafx.beans.value.ObservableValue;
import javafx.event.ActionEvent;
import javafx.event.EventHandler;
import javafx.geometry.Insets;
import javafx.scene.Node;
import javafx.scene.control.Button;
import javafx.scene.control.ButtonType;
import javafx.scene.input.MouseButton;
import javafx.scene.input.MouseEvent;
import javafx.scene.layout.Background;
import javafx.scene.layout.BackgroundFill;
import javafx.scene.layout.CornerRadii;
import javafx.scene.layout.GridPane;
import javafx.scene.paint.Color;

public class Handler {

    private Window window = new Window();
    private int n = 4;
    private int[][] cellValues = new int[n][n];

    public void helpButton(Button help, GridPane grid) {

        help.setOnAction(new EventHandler<ActionEvent>() {
            @Override
            public void handle(ActionEvent actionEvent) {

                if(help.getText().equals("Enable Help")) {
                    help.setText("Disable Help");

                    highlight(grid);

                    for(Node cell : grid.getChildren()) {
                        enableHighlight((Cell) cell, grid);
                    }

                } else {
                    help.setText("Enable Help");
                    setTransparent(grid);
                }
            }
        });
    }

    public void submitButton(Button submit) {
        submit.setOnAction(new EventHandler<ActionEvent>() {
            @Override
            public void handle(ActionEvent actionEvent) {

                for(int i = 0; i < getN(); i ++) {
                    for(int j = 0; j < getN(); j ++) {
                        System.out.print(getCellValues()[i][j]);
                    }
                    System.out.println();
                }

                if(verify()) {
                    window.win("Congratulations!", "You won the game!");
                } else {
                    window.win("Too bad...", "You lost the game");
                }
            }
        });
    }

    public void clearBoard(Button clear, GridPane grid) {

        clear.setOnAction(new EventHandler<ActionEvent>() {
            @Override
            public void handle(ActionEvent actionEvent) {

                window.getAlertDialog("Are you sure you want to clear the board?").showAndWait().ifPresent(response -> {
                    if(response == ButtonType.YES) {
                        clear(grid);
                    }
                });
            }
        });
    }

    public void doubleClick(Cell cell) {
        cell.getTextField().setOnMouseClicked(new EventHandler<MouseEvent>() {
            @Override
            public void handle(MouseEvent mouseEvent) {
                if(mouseEvent.getButton().equals(MouseButton.PRIMARY)) {
                    if(mouseEvent.getClickCount() == 2) {
                        window.showButtons(cell, getN());
                    }
                }
            }
        });
    }

    public void memoriseValues(Cell cell) {
        cell.getTextField().textProperty().addListener(new ChangeListener<String>() {
            @Override
            public void changed(ObservableValue<? extends String> observableValue, String oldValue,
                                String newValue) {
                if (cell.isNumeric(newValue)) {
                    if (Integer.parseInt(newValue) > getN() || Integer.parseInt(newValue) == 0) {
                        cell.getTextField().clear();
                    } else {
                        getCellValues()[cell.getRow()][cell.getColumn()] = Integer.parseInt(newValue);
                    }
                } else if (!newValue.matches("\\d*")) {
                    cell.getTextField().setText(newValue.replaceAll("[^\\d]", ""));
                }

            }
        });
    }

    public void enableHighlight(Cell cell, GridPane grid) {

        cell.getTextField().textProperty().addListener(new ChangeListener<String>() {
            @Override
            public void changed(ObservableValue<? extends String> observableValue, String s, String t1) {

                setTransparent(grid);
                highlight(grid);
            }
        });
    }

    public void draw(GridPane grid) {

        for(int i = 0; i < getN(); i ++) {
            for (int j = 0; j < getN(); j++) {
                Cell cell = new Cell(getN(), i, j);

                drawLines(i, j, cell);
                doubleClick(cell);
                memoriseValues(cell);

                grid.add(cell, j, i, 1, 1);
            }
        }
    }

    public void drawLines(int i, int j, Cell cell) {

        cell.setStyle("-fx-border-color: black; -fx-border-width: 1 1 0 0;");

        if(j == 0) {
            cell.setStyle("-fx-border-color: black; -fx-border-width: 1 1 0 1;");
        }
        if(i == getN() - 1 && j == 0) {
            cell.setStyle("-fx-border-color: black; -fx-border-width: 1 1 1 1;");
        }
        if( i == getN() - 1 && j != 0) {
            cell.setStyle("-fx-border-color: black; -fx-border-width: 1 1 1 0;");
        }
    }

    public void clear(GridPane grid) {
        for(int i =0; i < n; i ++) {
            for(int j = 0; j < n; j ++) {
                cellValues[i][j] = 0;
                getCell(grid, i, j).getTextField().clear();
                getCell(grid, i, j).setBackground(new Background(new BackgroundFill(Color.TRANSPARENT,
                        CornerRadii.EMPTY, Insets.EMPTY)));
            }
        }
    }

    public void setTransparent(GridPane grid) {
        for(Node cell : grid.getChildren()) {
            ((Cell) cell).setBackground(new Background(new BackgroundFill(Color.TRANSPARENT,
                    CornerRadii.EMPTY, Insets.EMPTY)));
        }
    }

    public void highlight(GridPane grid) {

        for(int i = 0; i < n; i ++) {
            for(int j = 0; j < n; j ++) {
                if(cellValues[i][j] != 0) {

                    if(!verifyRow(cellValues[i][j], i)) {
                        highlightRow(i, grid, true);
                    }

                    if(!verifyColumn(cellValues[i][j], j)) {
                        highlightColumn(j, grid, true);
                    }
                }
            }

        }
    }

    public boolean verify() {

        for(int i = 0; i < n; i ++) {
            for(int j = 0; j < n; j ++) {
                if(cellValues[i][j] !=0 ) {
                    if(verifyRow(cellValues[i][j], i) == false || verifyColumn(cellValues[i][j], j) == false) {
                        System.out.println(cellValues[i][j] + " " + i + " " + j);
                        return false;
                    }
                }
            }
        }

        return true;
    }

    public boolean verifyRow(int x, int row) {

        int count = 0;

        for(int i = 0; i < n; i ++) {
            if(cellValues[row][i] == x) {
                count++;
            }
        }

        if(count == 1) {
            return true;
        }

        return false;
    }

    public boolean verifyColumn(int x, int column) {

        int count = 0;

        for(int i = 0; i < n; i ++) {
            if(cellValues[i][column] == x) {
                count++;
            }
        }

        if(count == 1) {
            return true;
        }
        return false;
    }

    public void highlightRow(int row, GridPane grid, boolean highlight) {

        if(highlight) {
            for(int i = 0; i < n; i ++) {
                getCell(grid, row, i).setBackground(new Background(new BackgroundFill(Color.YELLOW,
                        CornerRadii.EMPTY, Insets.EMPTY)));
            }
        } else {
            for(int i = 0; i < n; i ++) {
                getCell(grid, row, i).setBackground(new Background(new BackgroundFill(Color.TRANSPARENT,
                        CornerRadii.EMPTY, Insets.EMPTY)));
            }
        }

    }

    public void highlightColumn(int column, GridPane grid, boolean highlight) {

        if(highlight) {
            for(int i = 0; i < n; i ++) {
                getCell(grid, i, column).setBackground(new Background(new BackgroundFill(Color.YELLOW,
                        CornerRadii.EMPTY, Insets.EMPTY)));
            }
        } else {
            for(int i = 0; i < n; i ++) {
                getCell(grid, i, column).setBackground(new Background(new BackgroundFill(Color.TRANSPARENT,
                        CornerRadii.EMPTY, Insets.EMPTY)));
            }
        }
    }

    public Cell getCell(GridPane grid, int i, int j) {
        for(Node cell : grid.getChildren()) {
            if(((Cell) cell).getRow() == i && ((Cell) cell).getColumn() == j) {
                return (Cell) cell;
            }
        }
        return null;
    }

    public int[][] getCellValues() {
        return cellValues;
    }

    public int getN() {
        return n;
    }

    public void setN(int n) {
        this.n = n;
    }

}