diff -r 576e7a2861ae -r 369903afbb29 src/main/java/de/uapcore/sudoku/SudokuTextField.java
--- a/src/main/java/de/uapcore/sudoku/SudokuTextField.java	Sat Jul 25 14:01:28 2020 +0200
+++ b/src/main/java/de/uapcore/sudoku/SudokuTextField.java	Sat Jul 25 15:29:51 2020 +0200
@@ -34,8 +34,7 @@
 import java.awt.event.KeyEvent;
 
 /**
- *
- * @author mike
+ * A custom text field specifically for Sudoku number fields.
  */
 public final class SudokuTextField extends JTextField {
     
@@ -105,15 +104,25 @@
             }
         });
     }
-    
+
+    /**
+     * Returns the current value in the field or zero if the field is empty.
+     *
+     * @return the number from 1 to 9 or zero if empty
+     */
     public int getValue() {
-        if (getText().length() > 0) {
+        if (getText().isEmpty()) {
+            return 0;
+        } else {
             return Integer.valueOf(getText());
-        } else {
-            return 0;
         }
     }
-    
+
+    /**
+     * Sets the field's value.
+     *
+     * @param v the number from 1 to 9 or zero to clear the field
+     */
     public void setValue(int v) {
         if (v == 0) {
             setText("");
@@ -124,12 +133,24 @@
                     "Sudoku numbers must be in range 0-9 (0 means 'not set')");
         }
     }
-    
+
+    /**
+     * Sets the modified state of this field.
+     *
+     * Modified fields are displayed in blue color.
+     *
+     * @param modified a flag indicating whether this field is modified
+     */
     public void setModified(boolean modified) {
         this.modified = modified;
         setForeground(modified?Color.BLUE:Color.BLACK);
     }
-    
+
+    /**
+     * Checks whether this field is in modified state.
+     *
+     * @return true if this field is modified
+     */
     public boolean isModified() {
         return modified;
     }