diff -r f1d7de36b01e -r 5179eff8a9b6 src/de/uapcore/sudoku/SudokuTextField.java
--- a/src/de/uapcore/sudoku/SudokuTextField.java	Sat Jan 26 15:48:59 2013 +0100
+++ b/src/de/uapcore/sudoku/SudokuTextField.java	Sat Jan 26 17:42:07 2013 +0100
@@ -13,7 +13,9 @@
  *
  * @author mike
  */
-public class SudokuTextField extends JTextField {
+public final class SudokuTextField extends JTextField {
+    
+    private boolean modified;
     
     public SudokuTextField() {
         setBorder(null);
@@ -29,7 +31,7 @@
         
         addKeyListener(new KeyAdapter() {
             private void handle(KeyEvent e) {
-                if (getText().length() > 0) {
+                if (getText().length() > 0 && getSelectedText() == null) {
                     int c = e.getKeyCode();
                     if (c != KeyEvent.VK_DELETE &&
                             c != KeyEvent.VK_BACK_SPACE &&
@@ -40,6 +42,8 @@
                     char c = e.getKeyChar();
                     if (c < '0' || c > '9') {
                         e.consume();
+                    } else {
+                        setModified(true);
                     }
                 }
             }
@@ -68,4 +72,31 @@
         });
     }
     
+    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;
+    }
 }