Java SE (Java Platform, Standard Edition)

How can I change the attributes of objects that are stored in a grid? I tried this, but it gives me the above error:

Group group = new Group();
double dimension_x=100;
double dimension_y=100;

GridPane grid = new GridPane();
grid.setHgap(1);
grid.setVgap(1);
grid.setPadding(new Insets(36));
grid.setGridLinesVisible(true);

Rectangle temp = new Rectangle(dimension_x,dimension_y);
Rectangle temp2 = new Rectangle(dimension_x,dimension_y);
Rectangle temp3 = new Rectangle(dimension_x,dimension_y);
Rectangle temp4 = new Rectangle(dimension_x,dimension_y);

grid.add(temp, 0,0);
grid.add(temp2, 1,1);
grid.add(temp3, 2,2);
grid.add(temp4, 3,3);

for (final Node node : grid.getChildren())
{

Rectangle tempvar = (Rectangle) node; //Errors out at javafx.scene.Group cannot be cast to javafx.scene.shape.Rectangle
node.getStyleClass().add("box");
//tempvar.setFill(Color.TRANSPARENT);
//tempvar.setStroke(Color.WHITE);

}

is there any way to loop through my grid and change the objects?