Deleting drawn line in java graphics

Question

I have a list containing lines that I draw on gui. I want to be able to select a line and delete it by clicking on the line. I can select and delete but the issue is the repaint method erase all the drawn lines when I do that. When I start drawing a new line, the lines minus the previously deleted line reappear.

I feel like it might be because of the iterator but I'm not entirely sure. Can anyone help with a workaround for this? Or am I doing it wrong?

Here's my codes:

import java.awt.Color;
import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.Point;
import java.awt.RenderingHints;
import java.awt.Shape;
import java.awt.event.MouseAdapter;
import java.awt.event.MouseEvent;
import java.awt.event.MouseMotionAdapter;
import java.awt.geom.Line2D;
import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;

import javax.swing.JFrame;
import javax.swing.JPanel;

public class LinesDemo extends JPanel {

private List linesList = new ArrayList<>();
private List lineCollection = new ArrayList<>();
private Line2D line = null;
Point pointStart = null;
Point pointEnd   = null;
// Width and height of rectangular region around mouse
// pointer to use for hit detection on lines
private static final int HIT_BOX_SIZE = 4;

/**
 * Create the panel.
 */
public LinesDemo() {

    addMouseListener(new MouseAdapter() {
        @Override
        public void mousePressed(MouseEvent e) {
            pointStart = e.getPoint();
            line = new Line2D.Double(e.getPoint(), e.getPoint());
            linesList.add(line);
        }

        @Override
        public void mouseReleased(MouseEvent e) {
            pointStart = null;
        }

        @Override
        public void mouseClicked(MouseEvent e) {
            int x = e.getX();
            int y = e.getY();
            getClickedLine(x, y);
        }
    });

    addMouseMotionListener(new MouseMotionAdapter() {
        @Override
        public void mouseMoved(MouseEvent e) {
            pointEnd = e.getPoint();
        }

        @Override
        public void mouseDragged(MouseEvent e) {
            pointEnd = e.getPoint();
            line.setLine(pointStart, pointEnd);
            lineCollection.add(line);
            repaint();
        }
    });
}

private Shape getClickedLine(int x, int y) {

    int boxX = x - HIT_BOX_SIZE / 2;
    int boxY = y - HIT_BOX_SIZE / 2;

    int width = HIT_BOX_SIZE;
    int height = HIT_BOX_SIZE;

    for(Line2D selectedLine:lineCollection) {
        if (selectedLine.intersects(boxX, boxY, width, height)) {
            System.out.println("intersects!");
            removeLine(selectedLine);
            return selectedLine;
        }
    }
    return null;
}

private void removeLine(Line2D line) {
    Iterator it = lineCollection.iterator();
    while(it.hasNext()) {
        Line2D selectedLine = it.next();
        if(selectedLine.equals(line)) {
            it.remove();
            repaint();
        }
    }
}

@Override
public void paintComponent(Graphics g){
    super.paintComponent(g);
    Graphics2D g2d = (Graphics2D) g;
    if(pointStart != null) {
        g2d.setPaint(Color.RED);
        g2d.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);
        for(Shape content : lineCollection){
            g2d.draw(content);
        }
        g2d.dispose();

    }
}

@Override
public Dimension getPreferredSize() {
    return new Dimension(300, 300);
}

public static void main(String[] args) {
    JFrame frame = new JFrame("Moving and Scaling");
    LinesDemo m = new LinesDemo();
    frame.add(m);
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.setSize(300, 300);
    frame.setLocationRelativeTo(null);
    frame.setVisible(true);
}
}

kiheru · Accepted Answer

The reason the lines disappear is that you explicitly suppress their drawing in paintComponent():

if (pointStart != null) {

pointStart has been set to null in mouseReleased, so none of the lines get drawn. Remove that check, and your code should work as you intended.

As a side note, you should create the GUI in the event dispatch thread.

Deleting drawn line in java graphics

Answers (2)

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