diff -r 576e7a2861ae -r 369903afbb29 src/main/java/de/uapcore/sudoku/ActionHandler.java
--- a/src/main/java/de/uapcore/sudoku/ActionHandler.java	Sat Jul 25 14:01:28 2020 +0200
+++ b/src/main/java/de/uapcore/sudoku/ActionHandler.java	Sat Jul 25 15:29:51 2020 +0200
@@ -1,16 +1,16 @@
 /*
  * 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
@@ -33,31 +33,42 @@
 import java.io.IOException;
 
 /**
- *
- * @author mike
+ * Handles all user issued actions in the application.
  */
 public final class ActionHandler implements ActionListener {
-    
+
     public static final String SAVE = "save";
     public static final String CHECK = "check";
     public static final String SOLVE = "solve";
-    
+
     public static final String NEW = "new";
     public static final String OPEN = "open";
     public static final String SAVE_AS = "save as";
     public static final String QUIT = "quit";
     public static final String ABOUT = "about";
-    
+
     private Field field;
     private Solver solver;
     private DocumentHandler doc;
-    
+
+    /**
+     * Constructs a new action handler instance.
+     *
+     * @param f a reference to the playing field
+     */
     public ActionHandler(Field f) {
         field = f;
         solver = new Solver();
         doc = new DocumentHandler();
     }
-    
+
+    /**
+     * Prompts the user for a file name.
+     * <p>
+     * If the user chooses an existing file, they are asked whether the file should be overwritten.
+     *
+     * @return true if the user approves a chosen file name
+     */
     private boolean chooseSaveFilename() {
         JFileChooser fc = new JFileChooser(".");
         fc.setMultiSelectionEnabled(false);
@@ -81,7 +92,10 @@
             return false;
         }
     }
-    
+
+    /**
+     * Prompts the user for a file to open and, if approved, loads that file.
+     */
     private void open() {
         JFileChooser fc = new JFileChooser(".");
         fc.setMultiSelectionEnabled(false);
@@ -92,12 +106,22 @@
                 doc.load(field);
             } catch (IOException e) {
                 JOptionPane.showMessageDialog(field,
-                    "Datei konnte nicht geladen werden: "+e.getMessage(),
-                    "Sudoku", JOptionPane.ERROR_MESSAGE);
+                        "Datei konnte nicht geladen werden: " + e.getMessage(),
+                        "Sudoku", JOptionPane.ERROR_MESSAGE);
             }
         }
     }
-    
+
+    /**
+     * Attempts to save the Sudoku field to a file.
+     * <p>
+     * If necessary, the user is prompted for a file name.
+     * <p>
+     * The field must be solvable, otherwise it cannot be saved.
+     *
+     * @param rename true if the user shall always be prompted, even if a file name is already known
+     * @return true if the user approves the chosen file name
+     */
     private boolean save(boolean rename) {
         if (!doc.isFilenameSet() || rename) {
             if (!chooseSaveFilename()) {
@@ -110,8 +134,8 @@
                 doc.save(field);
             } catch (IOException e) {
                 JOptionPane.showMessageDialog(field,
-                    "Datei konnte nicht gespeichert werden: "+e.getMessage(),
-                    "Sudoku", JOptionPane.ERROR_MESSAGE);
+                        "Datei konnte nicht gespeichert werden: " + e.getMessage(),
+                        "Sudoku", JOptionPane.ERROR_MESSAGE);
             }
             return true;
         } else {
@@ -121,7 +145,10 @@
             return false;
         }
     }
-    
+
+    /**
+     * Checks the Sudoku field and displays the result as a dialog box.
+     */
     private void check() {
         if (solver.check(field)) {
             JOptionPane.showMessageDialog(field, "Überprüfung erfolgreich!",
@@ -131,14 +158,22 @@
                     "Sudoku", JOptionPane.WARNING_MESSAGE);
         }
     }
-    
+
+    /**
+     * Solves the field or displays an error dialog if the field is not solvable.
+     */
     private void solve() {
         if (!solver.check(field) || !solver.solve(field)) {
             JOptionPane.showMessageDialog(field, "Das Feld ist nicht lösbar!",
                     "Sudoku", JOptionPane.WARNING_MESSAGE);
         }
     }
-    
+
+    /**
+     * Checks whether there are unsaved changes and asks the user to save the field.
+     *
+     * @return true if there are no unsaved changes or the user actively decides to continue - false, otherwise
+     */
     private boolean saveUnsaved() {
         boolean proceed = false;
         if (field.isAnyCellModified()) {
@@ -155,49 +190,49 @@
         } else {
             proceed = true;
         }
-        
+
         return proceed;
     }
 
     @Override
     public void actionPerformed(ActionEvent e) {
         switch (e.getActionCommand()) {
-        case NEW:
-            if (saveUnsaved()) {
-                doc.clearFilename();
-                field.clear();
-            }
-            break;
-        case OPEN:
-            open();
-            break;
-        case SAVE:
-            save(false);
-            break;
-        case SAVE_AS:
-            save(true);
-            break;
-        case CHECK:
-            check();
-            break;
-        case SOLVE:
-            solve();
-            break;
-        case QUIT:
-            if (saveUnsaved()) {
-                System.exit(0);
-            }
-            break;
-        case ABOUT:
-            JOptionPane.showMessageDialog(field,
-                    "Sudoku - Copyright (c) 2013 Mike Becker\nwww.uap-core.de"+
-                    "\nPublished under the BSD License",
-                    "Sudoku", JOptionPane.INFORMATION_MESSAGE);
-            break;
-        default:
-            throw new UnsupportedOperationException(
-                    "unknown action: "+e.getActionCommand());
+            case NEW:
+                if (saveUnsaved()) {
+                    doc.clearFilename();
+                    field.clear();
+                }
+                break;
+            case OPEN:
+                open();
+                break;
+            case SAVE:
+                save(false);
+                break;
+            case SAVE_AS:
+                save(true);
+                break;
+            case CHECK:
+                check();
+                break;
+            case SOLVE:
+                solve();
+                break;
+            case QUIT:
+                if (saveUnsaved()) {
+                    System.exit(0);
+                }
+                break;
+            case ABOUT:
+                JOptionPane.showMessageDialog(field,
+                        "Sudoku - Copyright (c) 2013 Mike Becker\nwww.uap-core.de" +
+                                "\nPublished under the BSD License",
+                        "Sudoku", JOptionPane.INFORMATION_MESSAGE);
+                break;
+            default:
+                throw new UnsupportedOperationException(
+                        "unknown action: " + e.getActionCommand());
         }
     }
-    
+
 }