diff -r 03bec9dcc860 -r 8ae7a8464c52 minesweeper/src/main/java/org/apidesign/demo/minesweeper/MinesModel.java
--- a/minesweeper/src/main/java/org/apidesign/demo/minesweeper/MinesModel.java	Mon Feb 10 16:50:08 2014 +0100
+++ b/minesweeper/src/main/java/org/apidesign/demo/minesweeper/MinesModel.java	Sun Mar 09 15:03:01 2014 +0100
@@ -111,13 +111,23 @@
     }
     
     @ModelOperation static void init(Mines model, int width, int height, int mines) {
-        List<Row> rows = new ArrayList<Row>(height);
-        for (int y = 0; y < height; y++) {
-            Square[] columns = new Square[width];
-            for (int x = 0; x < width; x++) {
-                columns[x] = new Square(SquareType.UNKNOWN, false);
+        List<Row> rows = model.getRows();
+        if (rows.size() != height || rows.get(0).getColumns().size() != width) {
+            rows = new ArrayList<Row>(height);
+            for (int y = 0; y < height; y++) {
+                Square[] columns = new Square[width];
+                for (int x = 0; x < width; x++) {
+                    columns[x] = new Square(SquareType.UNKNOWN, false);
+                }
+                rows.add(new Row(columns));
             }
-            rows.add(new Row(columns));
+        } else {
+            for (Row row : rows) {
+                for (Square sq : row.getColumns()) {
+                    sq.setState(SquareType.UNKNOWN);
+                    sq.setMine(false);
+                }
+            }
         }
         
         Random r = new Random();
@@ -133,8 +143,10 @@
         }
 
         model.setState(GameState.IN_PROGRESS);
-        model.getRows().clear();
-        model.getRows().addAll(rows);
+        if (rows != model.getRows()) {
+            model.getRows().clear();
+            model.getRows().addAll(rows);
+        }
     }
     
     @ModelOperation static void computeMines(Mines model) {