added the possibility to save history in sudoku, now we can implement back button to go back and forward in game history

src/main/java/src/MainFrame.java

@@ -12,9 +12,13 @@ import java.awt.event.ActionEvent;
 import java.awt.event.ActionListener;
 import java.awt.event.MouseEvent;
 import java.awt.event.MouseListener;
+import java.beans.PropertyChangeListener;
+import java.util.LinkedList;
 
 public class MainFrame extends JFrame {
 
+    LinkedList<Model> history;
+
     private GameField gameField;
 
     private JSONArray currentGameDifficulty;
@@ -24,6 +28,8 @@ public class MainFrame extends JFrame {
 
     public void run() {
 
+        history = new LinkedList<>();
+
         this.setLayout(null);
         this.setVisible(true);
         this.setBounds(300, 200, 500, 600);
@@ -115,9 +121,15 @@ public class MainFrame extends JFrame {
         JButton newButton  = new JButton();
         newButton.setVisible(true);
         newButton.setBounds(395, 5, 100, 25);
-        newButton.setText("New");
+        newButton.setText("Vorwärts");
         rootPanel.add(newButton);
 
+        JButton backButton  = new JButton();
+        backButton.setVisible(true);
+        backButton.setBounds(10, 5, 100, 25);
+        backButton.setText("Rückwärts");
+        rootPanel.add(backButton);
+
         JButton retryButton = new JButton();
         retryButton.setVisible(true);
         retryButton.setText("Retry");
@@ -146,8 +158,8 @@ public class MainFrame extends JFrame {
 
             @Override
             public void mousePressed(MouseEvent e) {
-            gameField.getActivePanel().setValue(Integer.valueOf(number));
-
+                gameField.getActivePanel().setValue(Integer.valueOf(number));
+                saveCurrentGameFieldStateToHistory();
             }
 
             @Override
@@ -171,9 +183,24 @@ public class MainFrame extends JFrame {
         numLabel1.setVisible(true);
         jPanel.add(numLabel1);
 
+
+
         return jPanel;
     }
 
+    public void saveCurrentGameFieldStateToHistory() {
+        Model model = new Model(gameField);
+        history.add(model);
+    }
+
+    public LinkedList<Model> getHistory() {
+        return history;
+    }
+
+    public GameField getGameField() {
+        return gameField;
+    }
+
     public JPanel setupJPanel() {
         JPanel panel = new JPanel();
         panel.setBounds(100, 450, 300, 30);

src/test/java/MainFrameTest.java

@@ -1,4 +1,7 @@
 import org.junit.jupiter.api.*;
+import src.GameField;
+import src.Main;
+import src.MainFrame;
 import src.Model;
 
 import static org.junit.jupiter.api.Assertions.*;
@@ -10,4 +13,15 @@ public class MainFrameTest {
         Model.initJsonFile();
     }
 
+    @Test
+    public void test_history() {
+        MainFrame mainFrame = new MainFrame();
+        mainFrame.run();
+        GameField gameField = mainFrame.getGameField();
+        gameField.setValue(5, 5, 5);
+        mainFrame.saveCurrentGameFieldStateToHistory();
+        int field = mainFrame.getHistory().getFirst().getField(5, 5);
+
+        assertEquals(5, field);
+    }
 }