some miner cleanups according synchronisation

src/main/java/de/neemann/digital/analyse/TruthTableTableModel.java

@@ -17,7 +17,7 @@ public class TruthTableTableModel implements TableModel {
     public static final String[] STATENAMES = new String[]{"0", "1", "x"};
 
     private final TruthTable truthTable;
-    private ArrayList<TableModelListener> listeners = new ArrayList<>();
+    private final ArrayList<TableModelListener> listeners = new ArrayList<>();
 
     /**
      * Creates a new instance

src/main/java/de/neemann/digital/core/memory/DataField.java

@@ -20,7 +20,7 @@ public class DataField {
     private long[] data;
     private final int bits;
 
-    private transient ArrayList<DataListener> listeners;
+    private final transient ArrayList<DataListener> listeners = new ArrayList<>();
 
     /**
      * Creates a new DataField
@@ -158,10 +158,10 @@ public class DataField {
      * @param l the listener
      */
     public void addListener(DataListener l) {
-        if (listeners == null)
+        synchronized (listeners) {
-            listeners = new ArrayList<>();
             listeners.add(l);
         }
+    }
 
     /**
      * Removes a listener
@@ -169,12 +169,9 @@ public class DataField {
      * @param l the listener to remove
      */
     public void removeListener(DataListener l) {
-        if (listeners == null)
+        synchronized (listeners) {
-            return;
-
             listeners.remove(l);
-        if (listeners.isEmpty())
+        }
-            listeners = null;
     }
 
     /**
@@ -183,12 +180,11 @@ public class DataField {
      * @param addr the address which value has changed
      */
     public void fireChanged(int addr) {
-        if (listeners == null)
+        synchronized (listeners) {
-            return;
-
             for (DataListener l : listeners)
                 l.valueChanged(addr);
         }
+    }
 
     /**
      * The listener interface

src/main/java/de/neemann/digital/draw/model/RealTimeClock.java

@@ -4,6 +4,7 @@ import de.neemann.digital.core.*;
 import de.neemann.digital.core.wiring.Clock;
 import de.neemann.digital.gui.ErrorStopper;
 import de.neemann.digital.gui.GuiModelObserver;
+import de.neemann.digital.gui.StatusInterface;
 import de.neemann.digital.gui.sync.Sync;
 import de.neemann.digital.lang.Lang;
 
@@ -21,6 +22,7 @@ public class RealTimeClock implements ModelStateObserver {
     private final ScheduledThreadPoolExecutor executor;
     private final ErrorStopper stopper;
     private final Sync modelSync;
+    private final StatusInterface status;
     private final int frequency;
     private final ObservableValue output;
     private Runner runner;
@@ -31,12 +33,14 @@ public class RealTimeClock implements ModelStateObserver {
      * @param model    the model
      * @param clock    the clock element which is modify
      * @param executor the executor used to schedule the update
+     * @param status   allows sending messages to the status line
      */
-    public RealTimeClock(Model model, Clock clock, ScheduledThreadPoolExecutor executor, ErrorStopper stopper, Sync modelSync) {
+    public RealTimeClock(Model model, Clock clock, ScheduledThreadPoolExecutor executor, ErrorStopper stopper, Sync modelSync, StatusInterface status) {
         this.model = model;
         this.executor = executor;
         this.stopper = stopper;
         this.modelSync = modelSync;
+        this.status = status;
         int f = clock.getFrequency();
         if (f < 1) f = 1;
         this.frequency = f;
@@ -48,7 +52,7 @@ public class RealTimeClock implements ModelStateObserver {
         switch (event) {
             case STARTED:
                 if (frequency > 50)  // if frequency is high it is not necessary to update the GUI at every clock change
-                    output.removeObserver(GuiModelObserver.class);
+                    modelSync.access(() -> output.removeObserver(GuiModelObserver.class));
 
                 int delay = 500000 / frequency;
                 if (delay < 10)
@@ -125,7 +129,7 @@ public class RealTimeClock implements ModelStateObserver {
                     }
                     time = System.currentTimeMillis() - time;
 
-                    System.out.println(counter/time/2+"kHz");
+                    status.setStatus(counter / time / 2 + "kHz");
 
                     System.out.println("thread end");
                 }

src/main/java/de/neemann/digital/gui/Main.java

@@ -59,7 +59,7 @@ import java.util.concurrent.ScheduledThreadPoolExecutor;
  *
  * @author hneemann
  */
-public class Main extends JFrame implements ClosingWindowListener.ConfirmSave, ErrorStopper, FileHistory.OpenInterface, DigitalRemoteInterface {
+public class Main extends JFrame implements ClosingWindowListener.ConfirmSave, ErrorStopper, FileHistory.OpenInterface, DigitalRemoteInterface, StatusInterface {
     private static final ArrayList<Key> ATTR_LIST = new ArrayList<>();
     private static boolean experimental;
 
@@ -684,7 +684,7 @@ public class Main extends JFrame implements ClosingWindowListener.ConfirmSave, E
                     if (c.getFrequency() > 0) {
                         if (modelSync == null)
                             modelSync = new LockSync();
-                        model.addObserver(new RealTimeClock(model, c, timerExecuter, this, modelSync));
+                        model.addObserver(new RealTimeClock(model, c, timerExecuter, this, modelSync, this));
                         realtimeClockRunning = true;
                     }
             if (modelSync == null)
@@ -841,6 +841,11 @@ public class Main extends JFrame implements ClosingWindowListener.ConfirmSave, E
         return windowPosManager;
     }
 
+    @Override
+    public void setStatus(String message) {
+        SwingUtilities.invokeLater(() -> statusLabel.setText(message));
+    }
+
     private class FullStepObserver implements Observer {
         private final Model model;
 

src/main/java/de/neemann/digital/gui/StatusInterface.java

@@ -0,0 +1,15 @@
+package de.neemann.digital.gui;
+
+/**
+ * Interface to acess the status line
+ *
+ * @author hneemann
+ */
+public interface StatusInterface {
+    /**
+     * Set message to the status line
+     *
+     * @param message the message
+     */
+    void setStatus(String message);
+}

src/main/java/de/neemann/digital/gui/components/graphics/GraphicCard.java

@@ -17,6 +17,10 @@ import javax.swing.*;
 import static de.neemann.digital.core.element.PinInfo.input;
 
 /**
+ * Graphic card.
+ * Mostly a RAM module with an additional input bit which selects the visible bank.
+ * So you can use double buffering.
+ *
  * @author hneemann
  */
 public class GraphicCard extends Node implements Element, RAMInterface {

src/main/java/de/neemann/digital/gui/components/listing/ROMListingDialog.java

@@ -83,7 +83,6 @@ public class ROMListingDialog extends JDialog implements Observer {
                     list.setSelectedIndex(line);
                 });
             }
-
             lastAddr = addr;
         }
     }