swing - Asking for some clarification in java about jlabel and parent

Question

Found this code in the internet, it was posted years ago, so I just decided to ask here for some clarifications for some lines I don't quite understand.

In the mousePressed method, what does he mean by: chessPiece = null is he saying that if the JLabel chessPiece has a image in it then it should be changed to null?

Is chessBoard.findComponentAt(e.getX(), e.getY()) returns the JPanel square?

and lastly, when Component c gets its parent, who is the parent?

The whole code is below:

public class ChessGameDemo extends JFrame implements MouseListener, MouseMotionListener {

    JLayeredPane layeredPane;
    JPanel chessBoard;
    JLabel chessPiece;
    int xAdjustment;
    int yAdjustment;
    private static final String imageFolderPath = "src/resources/images/";

    public ChessGameDemo() {
        Dimension boardSize = new Dimension(600, 600);

        //  Use a Layered Pane for this this application
        layeredPane = new JLayeredPane();
        getContentPane().add(layeredPane);
        layeredPane.setPreferredSize(boardSize);
        layeredPane.addMouseListener(this);
        layeredPane.addMouseMotionListener(this);

         //Add a chess board to the Layered Pane 
        chessBoard = new JPanel();
        layeredPane.add(chessBoard, JLayeredPane.DEFAULT_LAYER);
        chessBoard.setLayout(new GridLayout(8, 8));
        chessBoard.setPreferredSize(boardSize);
        chessBoard.setBounds(0, 0, boardSize.width, boardSize.height);

        for (int i = 0; i < 64; i++) {
            JPanel square = new JPanel(new BorderLayout());
            chessBoard.add(square);

            int row = (i / 8) % 2;
            if (row == 0) {
                square.setBackground(i % 2 == 0 ? Color.blue : Color.white);
            } else {
                square.setBackground(i % 2 == 0 ? Color.white : Color.blue);
            }
        }

          //Add a few pieces to the board
        JLabel piece = new JLabel(new ImageIcon(imageFolderPath + "/pieces/bdg.png"));
        JPanel panel = (JPanel) chessBoard.getComponent(0);
        panel.add(piece);
        piece = new JLabel(new ImageIcon(imageFolderPath + "/pieces/belder.png"));
        panel = (JPanel) chessBoard.getComponent(15);
        panel.add(piece);
        piece = new JLabel(new ImageIcon(imageFolderPath + "/pieces/bhero.png"));
        panel = (JPanel) chessBoard.getComponent(16);
        panel.add(piece);
        piece = new JLabel(new ImageIcon(imageFolderPath + "/pieces/borb.png"));
        panel = (JPanel) chessBoard.getComponent(20);
        panel.add(piece);

    }

    public void mousePressed(MouseEvent e) {
        chessPiece = null;
        Component c = chessBoard.findComponentAt(e.getX(), e.getY());

        if (c instanceof JPanel) {
            return;
        }

        Point parentLocation = c.getParent().getLocation();
        xAdjustment = parentLocation.x - e.getX();
        yAdjustment = parentLocation.y - e.getY();
        chessPiece = (JLabel) c;
        chessPiece.setLocation(e.getX() + xAdjustment, e.getY() + yAdjustment);
        chessPiece.setSize(chessPiece.getWidth(), chessPiece.getHeight());
        layeredPane.add(chessPiece, JLayeredPane.DRAG_LAYER);
    }

        //Move the chess piece around
    public void mouseDragged(MouseEvent me) {
        if (chessPiece == null) {
            return;
        }
        chessPiece.setLocation(me.getX() + xAdjustment, me.getY() + yAdjustment);
    }

         //Drop the chess piece back onto the chess board
    public void mouseReleased(MouseEvent e) {
        if (chessPiece == null) {
            return;
        }

        chessPiece.setVisible(false);
        Component c = chessBoard.findComponentAt(e.getX(), e.getY());

        if (c instanceof JLabel) {
            Container parent = c.getParent();
            parent.remove(0);
            parent.add(chessPiece);
        } else {
            Container parent = (Container) c;
            parent.add(chessPiece);
        }
        ....
    }

Answer 1

In the mousePiece method, what does he mean by: chessPiece = null is he saying that if the JLabel chessPiece has a image in it then it should be changed to null?

I assume you mean mousePressed. By using chessPiece = null, the author is de-referencing the variable, so what ever was assigned to it is no longer reachable through the is variable

Is chessBoard.findComponentAt(e.getX(), e.getY()) returns the JPanel square?

This depends. findComponentAt can search the current container and it's any of it's child containers until it finds a component at the specified position. Technquial, the author is ignoring the component that triggered the event (which should layeredPane) and is walking the chessBoard instead. I suspect they are doing this because if they used layeredPane it would return chessBoard instead.

The method is capable of returning JPanel, JLabel and possibly even null, but given the way that the components are laid out, it's a lower probability.

and lastly, when Component c gets its parent, who is the parent?

This depends. Based on my understanding of the code, I would say it's return a JPanel underneth the JLabel piece.

Have to say, there are easier ways to achieve the same result though...