app/src/main/java/gui/SudokuController.java

@@ -127,15 +127,6 @@ public class SudokuController {
 
             // Update the model and view
             model.set(row, col, value);
-
-            // Warn if the board is not solvable (e.g. if the user has made a mistake)
-            // This is very messy, error prone and computationally expensive
-            if(!model.isSolvable()) {
-                model.set(row, col, 0);
-                view.showErrorMessage("Illegal move. The board is not solvable.");
-            }
-
-            // Sync the view with the model
             view.updateView(model.getBoard());
         }
     }

app/src/test/java/sudoku/SolverTest.java

@@ -34,8 +34,7 @@ class SolverTest {
         assertTrue(solver.isLegal(0, 0, 1));
         solver.set(0, 0, 1);
 
-        // Start from one, since setting the same value is legal
+        IntStream.range(0, 9).forEach(i -> {
-        IntStream.range(1, 9).forEach(i -> {
             assertFalse(solver.isLegal(0, i, 1));
             assertFalse(solver.isLegal(i, 0, 1));
         });