Javafx How to locate a specific button in a GridPane

I'm making a checkers game with buttons. To kill another piece, you have to move your piece diagonally over the other piece, but I'm not sure how to ensure that your piece moved over the other piece.

The idea I have for solving this problem is getting the Row and Column of the second button, which is the button that your piece moves to, then subtracting 1 from each Row and Column, then getting the text from that button to test if it is "black" or "red".

first & second = buttons

System.out.println((GridPane.getColumnIndex(second) + " vs " + (GridPane.getColumnIndex(second) - 1)));


    if (GridPane.getColumnIndex(second) > 0) {
        System.out.println("checking if a button has been jumped");
        GridPane.setRowIndex(second, (GridPane.getRowIndex(second) - 1));
        GridPane.setColumnIndex(second, (GridPane.getColumnIndex(second) - 1));
        System.out.println("this is a printing of the second button name for location " + (GridPane.getColumnIndex(second)) + " " + (GridPane.getRowIndex(second)) + " " + second.getText());

        if (second.getText().contains("black")) {
            System.out.println("it's a kill");
        } 
        else {
            System.out.println("no kill");
            GridPane.setRowIndex(second, (GridPane.getRowIndex(second) + 1));
            GridPane.setColumnIndex(second, (GridPane.getColumnIndex(second) + 1));
        }
    }

I'm able to change the Row and Column to what would match the location of the other piece, but when I get the text from that button(second), it doesn't come back as the name "black" or "red", rather just the name of the blank buttons.
My guess is that GridPane's may not work like this, and I just need to come up with another solution, hopefully I won't have to redo my whole code as a 2d array or something.

Solution

You can get a reference to a node in GridPane by iterating over the GridPane children.
Add some simple calculation to find the diagonal between two buttons clicked, if any:

import java.awt.Toolkit;
import javafx.animation.PauseTransition;
import javafx.application.Application;
import javafx.geometry.Insets;
import javafx.scene.Node;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.scene.layout.GridPane;
import javafx.scene.layout.Pane;
import javafx.scene.layout.VBox;
import javafx.scene.text.Text;
import javafx.stage.Stage;
import javafx.util.Duration;

public class FxMain extends Application {

    private static final int COLS = 5, ROWS = 5;
    private int clickCounter = 0;
    private GridPane grid;
    private Button first, second;

    @Override
    public void start(Stage primaryStage){

        VBox root = new VBox(10);
        root.setPadding(new Insets(10));
        root.getChildren().addAll(makeGrid(), 
                new Text("Click 2 buttons to find the \n diagonally between them"));
        primaryStage.setScene(new Scene(root));
        primaryStage.sizeToScene();
        primaryStage.show();
    }

    private Pane makeGrid() {

        grid = new GridPane();
        for(int rowIndex = 0; rowIndex < ROWS ; rowIndex++) {
            //an array to hold buttons of one row
            Node[] nodes = new Node[COLS];
            for(int colIndex = 0; colIndex < COLS ; colIndex++) {
                Button node= new Button(rowIndex+""+colIndex);
                node.setOnAction(e->buttonCliked(node)); //add action listener
                nodes[colIndex]= node;
            }
            grid.addRow(rowIndex, nodes);
        }
        return grid;
    }

    private void buttonCliked(Button button) {

        if(clickCounter == 0){
            first = button;
        }else{
            second = button;
            markNode(findMidDiagonalButton());
        }

        System.out.println(clickCounter + " " + button.getText()    );
        clickCounter=  ++clickCounter %2 ;  // changes values between 0 1
    }

    //change node background for a short while, and then reset it
    private void markNode(Node node) {

        if(node == null) return;
        String style = node.getStyle();
        node.setStyle("-fx-background-color: cornflowerblue;");
        PauseTransition pause = new PauseTransition(Duration.seconds(1));
        pause.play();
        pause.setOnFinished(e-> node.setStyle(style));
    }

    private Node findMidDiagonalButton() {

        int rowDelta = GridPane.getRowIndex(first) - GridPane.getRowIndex(second);
        int colDelta = GridPane.getColumnIndex(first) - GridPane.getColumnIndex(second);

        if( Math.abs(rowDelta) != 2 ||  Math.abs(colDelta) != 2 ){
            Toolkit.getDefaultToolkit().beep();
            return null;
        }

        int rowsSum = GridPane.getRowIndex(first) + GridPane.getRowIndex(second);
        int colsSum = GridPane.getColumnIndex(first) + GridPane.getColumnIndex(second);

        return  getNodeByRowCol(Math.abs(rowsSum / 2), Math.abs(colsSum / 2) );
    }

    public Node getNodeByRowCol (int row, int col) {

        for (Node node : grid.getChildren()) {
            if(GridPane.getRowIndex(node) == row && GridPane.getColumnIndex(node) == col)
                return node;
        }

        return null;
    }

    public static void main(final String[] args) {
        launch(args);
    }
}