Class that is in charge of all the functionalities of the game, inclusive drawing.

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;
+    }
+
+}