diff -r e70a0e3555fb -r 576e7a2861ae src/main/java/de/uapcore/sudoku/SudokuTextField.java
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/src/main/java/de/uapcore/sudoku/SudokuTextField.java	Sat Jul 25 14:01:28 2020 +0200
@@ -0,0 +1,136 @@
+/*
+ * 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.event.FocusAdapter;
+import java.awt.event.FocusEvent;
+import java.awt.event.KeyAdapter;
+import java.awt.event.KeyEvent;
+
+/**
+ *
+ * @author mike
+ */
+public final class SudokuTextField extends JTextField {
+    
+    private boolean modified;
+    
+    public SudokuTextField() {
+        setBorder(null);
+        setBackground(Color.WHITE);
+        
+        setFont(new Font("Dialog", Font.PLAIN, 18));
+        setHorizontalAlignment(JTextField.CENTER);
+        
+        Dimension dim = new Dimension(40,40);
+        setPreferredSize(dim);
+        setMinimumSize(dim);
+        setMaximumSize(dim);
+        
+        addKeyListener(new KeyAdapter() {
+            private void handle(KeyEvent e) {
+                char c = e.getKeyChar();
+                if (!e.isAltDown() && !e.isControlDown() &&
+                        !Character.isISOControl(c)) {
+                    // Perform clean input check
+                    if (getText().length() > 0 && getSelectedText() == null) {
+                        if (c != KeyEvent.CHAR_UNDEFINED) {
+                            e.consume();
+                        }
+                    } else {
+                        if (c < '1' || c > '9') {
+                            e.consume();
+                        } else {
+                            setModified(true);
+                        }
+                    }
+                } else {
+                    // We can still be tricked by hotkeys, so do it the hard way
+                    if (!getText().matches("^[1-9]$")) {
+                        setText("");
+                    }
+                }
+            }
+            
+            @Override
+            public void keyPressed(KeyEvent e) {
+                handle(e);
+            }
+
+            @Override
+            public void keyTyped(KeyEvent e) {
+                handle(e);
+            }
+
+            @Override
+            public void keyReleased(KeyEvent e) {
+                handle(e);
+            }
+            
+        });
+        addFocusListener(new FocusAdapter() {
+            @Override
+            public void focusGained(FocusEvent e) {
+                selectAll();
+            }
+            @Override
+            public void focusLost(FocusEvent e) {
+                select(0, 0);
+            }
+        });
+    }
+    
+    public int getValue() {
+        if (getText().length() > 0) {
+            return Integer.valueOf(getText());
+        } else {
+            return 0;
+        }
+    }
+    
+    public void setValue(int v) {
+        if (v == 0) {
+            setText("");
+        } else if (v < 10) {
+            setText(String.valueOf(v));
+        } else {
+            throw new IllegalArgumentException(
+                    "Sudoku numbers must be in range 0-9 (0 means 'not set')");
+        }
+    }
+    
+    public void setModified(boolean modified) {
+        this.modified = modified;
+        setForeground(modified?Color.BLUE:Color.BLACK);
+    }
+    
+    public boolean isModified() {
+        return modified;
+    }
+}