Java: Making Circle with a radius and alternating colors with recursion

Question

I'm trying to get my JaxaFX program to make circle with alternating color when the user enters a radius and come out like this (with my alternating colors being forest green and red):

But its only coming out as forest green

public class App extends Application {

    
    private Pane centerpane;            // center pane
    private TextField enterTextField;   // text field for user to enter radius of circle
    private double radius;                  // order for recursion
    public Color color = Color.FORESTGREEN;
    @Override
    public void start(Stage stage)
    {
        BorderPane outerpane = new BorderPane();
        
        //CREATE the CENTER PANE
        centerpane = new Pane();
        centerpane.setBackground(new Background(new BackgroundFill(Color.LIGHTBLUE,null,null)));
        outerpane.setCenter(centerpane);
        
        
        
        //CREATE BOTTOM PANE AS HBOX OF LABEL , TEXTFIELD, AND START BUTTOM
        HBox bottompane = new HBox (20);
        bottompane.setAlignment(Pos.CENTER);
        bottompane.setPadding(new Insets(25,25,25,25));
        Label label = new Label ("Enter radius of largest cirlce: ");
        enterTextField = new TextField();
        Button startButton = new Button("Start");
        startButton.setOnAction(event -> makeCircles());
        bottompane.getChildren().add(label);
        bottompane.getChildren().add(enterTextField);
        bottompane.getChildren().add(startButton);
        outerpane.setBottom(bottompane);
        
        var scene = new Scene(outerpane, 600, 400);
        stage.setScene(scene);
        stage.show();
        
    }
    //Event handler for Start button gets radius, 
    // and calls recursive display
    public void makeCircles()
    {
        //clear the center pane after every use
        centerpane.getChildren().clear();
        try
        {
            radius = Double.parseDouble(enterTextField.getText());
            
            if((radius*2 <= centerpane.getWidth()) && (radius*2 <= centerpane.getHeight()))
            {
                
                displayCircles(radius,color);
                
            }else
            {
                showAlert("Circle does not fit in thec center pane ");
            }
        }catch(NumberFormatException e){
            showAlert("Invalid input");
        }

    }
    //Display the Circle with recursion
    public void displayCircles(double radius, Color color)
    {
        //When the base case radius is 0 it stops
        if(radius > 0)
        {
           //Displays circle
           Circle circle = new Circle(centerpane.getWidth()/2,centerpane.getHeight()/2,radius);
           circle.setStroke(Color.BLACK);
           circle.setFill(color);
           centerpane.getChildren().add(circle);
        }
        else // recursive case 
        {
            Color nextColor;
            if(color.equals(Color.FORESTGREEN)){
                nextColor = Color.RED;
            }
            else{
                nextColor = Color.FORESTGREEN;
            }
            displayCircles(radius - 10, nextColor);
        }
    }
     public void showAlert(String message)
     {
         Alert alert = new Alert(Alert.AlertType.ERROR);
         alert.setTitle("Error");
         alert.setHeaderText(null);
         alert.setContentText(message);
         alert.showAndWait();
     }
    
    
    
    public static void main(String[] args) {
        launch();
    }

}

i can't seem to get it to alternate colors

Answer 1

There are a few issues here to fix.

First, with the recursion. The base case you wrote for the displayCircles method, radius > 0 is not what you want. You want to keep adding circles for as long as radius > 0, so you want to end when the radius isn't bigger than 0. In other words, your base case should be radius <= 0.

Okay, if we fix that and run the code, we see nothing. That's because currently only in the base case is a circle created. In reality, we want to create a circle every time to get the result you're looking for.

So here's the modified displayCircles code:

public void displayCircles(double radius, Color color)
    {
        //When the base case radius is 0 it stops, otherwise run again.
        if(radius > 0)
        {
            // Create and add the circle to UI
            Circle circle = new Circle(centerpane.getWidth()/2,centerpane.getHeight()/2,Math.max(0, radius));
            circle.setStroke(Color.BLACK);
            circle.setFill(color);
            centerpane.getChildren().add(circle);
            
            // Recursively create the next circle.
            Color nextColor;
            if(color.equals(Color.FORESTGREEN)){
                nextColor = Color.RED;
            }
            else{
                nextColor = Color.FORESTGREEN;
            }
            displayCircles(radius - 10, nextColor);
        }
    }

And the output:

I assume this is for school, but in the off chance it isn't it would be better to use a for-loop to create the circles as recursion is overkill here.