diff -r f7433671fec5 -r 1c62c6009161 src/main/java/de/uapcore/sudoku/ActionHandler.java
--- a/src/main/java/de/uapcore/sudoku/ActionHandler.java	Sat Jul 25 15:56:48 2020 +0200
+++ b/src/main/java/de/uapcore/sudoku/ActionHandler.java	Mon Jul 27 10:56:17 2020 +0200
@@ -47,9 +47,9 @@
     public static final String QUIT = "quit";
     public static final String ABOUT = "about";
 
-    private Field field;
-    private Solver solver;
-    private DocumentHandler doc;
+    private final Field field;
+    private final Solver solver;
+    private final DocumentHandler doc;
 
     /**
      * Constructs a new action handler instance.