diff -r e70a0e3555fb -r 576e7a2861ae src/main/java/de/uapcore/sudoku/Field.java
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/src/main/java/de/uapcore/sudoku/Field.java	Sat Jul 25 14:01:28 2020 +0200
@@ -0,0 +1,177 @@
+/*
+ * Copyright 2013 Mike Becker. All rights reserved.
+ * 
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ * 
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ * 
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE
+ * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+ * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+ * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+ * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+ * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+ * POSSIBILITY OF SUCH DAMAGE.
+ */
+
+package de.uapcore.sudoku;
+
+import javax.swing.*;
+import java.awt.*;
+import java.awt.image.BufferedImage;
+
+/**
+ *
+ * @author mike
+ */
+public final class Field extends JPanel {
+    private SudokuTextField[][] cells;
+    
+    public Field() {
+        setBackground(Color.WHITE);
+        
+        setLayout(new GridBagLayout());
+        GridBagConstraints c = new GridBagConstraints();
+        c.insets = new Insets(5, 5, 5, 5);
+        
+        cells = new SudokuTextField[9][9];
+        for (int x = 0 ; x < 9 ; x++) {
+            for (int y = 0 ; y < 9 ; y++) {
+                cells[x][y] = new SudokuTextField();
+                c.gridx = x;
+                c.gridy = y;
+                add(cells[x][y], c);
+            }
+        }
+    }
+
+    @Override
+    public void paint(Graphics graphics) {
+        final int w = getWidth();
+        final int h = getHeight();
+        final int cw = w / 9;
+        final int ch = h / 9;
+        
+        BufferedImage img = new BufferedImage(w, h, BufferedImage.TYPE_INT_RGB);
+        Graphics2D g = img.createGraphics();
+        g.setBackground(Color.WHITE);
+        g.clearRect(0, 0, w, h);
+        
+        g.setColor(Color.BLACK);
+        g.drawRect(1, 1, w-2, h-2);
+        g.drawRect(2, 2, w-4, h-4);
+        for (int x = cw ; x < w ; x += cw) {
+            for (int y = ch ; y < h ; y += ch) {
+                g.drawLine(x, 2, x, h-2);
+                g.drawLine(2, y, w-2, y);
+                if ((x / cw) % 3 == 0) {
+                    g.drawLine(x+1, 2, x+1, h-2);
+                }
+                if ((y / ch) % 3 == 0) {
+                    g.drawLine(2, y+1, w-2, y+1);
+                }
+            }
+        }
+        
+        graphics.drawImage(img, 0, 0, this);
+        super.paintChildren(graphics);
+    }
+    
+    public boolean isCellEmpty(int x, int y) {
+        return getCellValue(x, y) == 0;
+    }
+    
+    public int getCellValue(int x, int y) {
+        return cells[x][y].getValue();
+    }
+    
+    public void setCellValue(int x, int y, int v) {
+        cells[x][y].setValue(v);
+    }
+    
+    public void clearCellValue(int x, int y) {
+        setCellValue(x, y, 0);
+    }
+    
+    public void setCellModified(int x, int y, boolean modified) {
+        cells[x][y].setModified(modified);
+    }
+    
+    public void setAllCellsModified(boolean modified) {
+        for (int x = 0 ; x < 9 ; x++) {
+            for (int y = 0 ; y < 9 ; y++) {
+                cells[x][y].setModified(modified);
+            }
+        }
+    }
+    
+    public boolean isAnyCellModified() {
+        for (int x = 0 ; x < 9 ; x++) {
+            for (int y = 0 ; y < 9 ; y++) {
+                if (cells[x][y].isModified()) {
+                    return true;
+                }
+            }
+        }
+        return false;
+    }
+    
+    public void clear() {
+        for (int x = 0 ; x < 9 ; x++) {
+            for (int y = 0 ; y < 9 ; y++) {
+                cells[x][y].setValue(0);
+            }
+        }
+    }
+    
+    public int[][] getSquare(int x, int y) {
+        if (x < 0 || x > 2 || y < 0 || y > 2) {
+            throw new IllegalArgumentException("Invalid square coordinates");
+        }
+        int[][] square = new int[3][3];
+        
+        for (int u = 0 ; u < 3 ; u++) {
+            for (int v = 0 ; v < 3 ; v++) {
+                square[u][v] = getCellValue(3*x+u, 3*y+v);
+            }
+        }
+        
+        return square;
+    }
+    
+    public int[] getRow(int y) {
+        if (y < 0 || y > 8) {
+            throw new IllegalArgumentException("Invalid row number");
+        }
+        int row[] = new int[9];
+        
+        for (int x = 0 ; x < 9 ; x++) {
+            row[x] = getCellValue(x, y);
+        }
+        
+        return row;
+    }
+    
+    public int[] getColumn(int x) {
+        if (x < 0 || x > 8) {
+            throw new IllegalArgumentException("Invalid column number");
+        }
+        int column[] = new int[9];
+        
+        for (int y = 0 ; y < 9 ; y++) {
+            column[y] = getCellValue(x, y);
+        }
+        
+        return column;
+    }
+}