How to make half rounded border?

I have a RoundPanel class that extends JPanel and provides a rounded appearance.

JPanel user_icon = new RoundPanel(8);
user_icon.setBackground(Color.decode("#363b41"));
user_icon.setBorder(null);
user_icon.setBounds(160, 135, 40, 46);
container.add(user_icon);
user_icon.setLayout(null);

public class RoundPanel extends JPanel {

    private static final long serialVersionUID = 1L;
    private Shape shape;
    
    public RoundPanel(int size) {
        setOpaque(false);
    }
    
    protected void paintComponent(Graphics g) {
         g.setColor(getBackground());
         g.fillRoundRect(0, 0, getWidth()-1, getHeight()-1, 8, 8);
         super.paintComponent(g);
    }
    
    protected void paintBorder(Graphics g) {
         g.drawRoundRect(0, 0, getWidth()-1, getHeight()-1, 8, 8);
    }
    
    public boolean contains(int x, int y) {
         if (shape == null || !shape.getBounds().equals(getBounds())) {
             shape = new RoundRectangle2D.Float(0, 0, getWidth()-1, getHeight()-1, 8, 8);
         }
         return shape.contains(x, y);
    }
}

want to modify it to have only the left corners rounded. (Half rounded, Half none)

I would, strongly, recommend taking the time to read through Working with Geometry, actually, I also recommend taking the time to read through the entire 2D Graphics trail while you're at it.

No one example/solution will ever meet your requirements 100%, you need to be prepared to put in the time to try different solutions and adapt the ones you like to your needs, this is the mark of a good developer.

So, with a little bit of experimentation (and a lot of reading through my previous answers) I was able to come up with...

import java.awt.Color;
import java.awt.Dimension;
import java.awt.EventQueue;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.RenderingHints;
import java.awt.geom.Path2D;
import java.util.Arrays;
import java.util.HashSet;
import java.util.Set;
import javax.swing.JFrame;
import javax.swing.JPanel;

public class Main {

    public static void main(String[] args) {
        new Main();
    }

    public Main() {
        EventQueue.invokeLater(new Runnable() {
            @Override
            public void run() {
                JFrame frame = new JFrame("Test");
                frame.add(new TestPane());
                frame.pack();
                frame.setLocationRelativeTo(null);
                frame.setVisible(true);
            }
        });
    }

    protected class TestPane extends JPanel {

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

        @Override
        protected void paintComponent(Graphics g) {
            super.paintComponent(g);
            Graphics2D g2d = (Graphics2D) g.create();
            g2d.setRenderingHint(RenderingHints.KEY_ALPHA_INTERPOLATION, RenderingHints.VALUE_ALPHA_INTERPOLATION_QUALITY);
            g2d.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);
            g2d.setRenderingHint(RenderingHints.KEY_COLOR_RENDERING, RenderingHints.VALUE_COLOR_RENDER_QUALITY);
            g2d.setRenderingHint(RenderingHints.KEY_DITHERING, RenderingHints.VALUE_DITHER_ENABLE);
            g2d.setRenderingHint(RenderingHints.KEY_FRACTIONALMETRICS, RenderingHints.VALUE_FRACTIONALMETRICS_ON);
            g2d.setRenderingHint(RenderingHints.KEY_INTERPOLATION, RenderingHints.VALUE_INTERPOLATION_BILINEAR);
            g2d.setRenderingHint(RenderingHints.KEY_RENDERING, RenderingHints.VALUE_RENDER_QUALITY);
            g2d.setRenderingHint(RenderingHints.KEY_STROKE_CONTROL, RenderingHints.VALUE_STROKE_PURE);

            g2d.setColor(Color.RED);
            RoundedShape shape = new RoundedShape(
                    20,
                    100, 200,
                    RoundedShape.Corner.TOP_LEFT,
                    RoundedShape.Corner.TOP_RIGHT,
                    RoundedShape.Corner.BOTTOM_RIGHT,
                    RoundedShape.Corner.BOTTOM_LEFT
            );
            g2d.translate(10, 10);
            g2d.draw(shape);

            g2d.dispose();
        }
    }

    public class RoundedShape extends Path2D.Double {

        enum Corner {
            TOP_LEFT, TOP_RIGHT, BOTTOM_LEFT, BOTTOM_RIGHT;
        }

        private double radius = 20;
        private Set<Corner> corners;
        private Dimension size;

        public RoundedShape(double radius, int width, int height, Corner... corners) {
            this.corners = new HashSet(Arrays.asList(corners));
            this.radius = radius;
            this.size = new Dimension(width, height);

            build();
        }

        protected void build() {
            moveTo(0, 0);
            if (corners.contains(Corner.TOP_LEFT)) {
                moveTo(0, radius);
                curveTo(0, radius, 0, 0, radius, 0);
            }
            if (corners.contains(Corner.TOP_RIGHT)) {
                lineTo(size.width - radius, 0);
                // Draw the curve here
                curveTo(size.width - radius, 0,
                        size.width, 0,
                        size.width, radius);
            } else {
                lineTo(size.width, 0);
            }
            if (corners.contains(Corner.BOTTOM_RIGHT)) {
                lineTo(size.width, size.height - radius);
                // Draw the curve here
                curveTo(size.width, size.height - radius,
                        size.width, size.height,
                        size.width - radius, size.height);
            } else {
                lineTo(size.width, size.height);
            }
            if (corners.contains(Corner.BOTTOM_LEFT)) {
                lineTo(radius, size.height);
                // Draw the curve here
                curveTo(radius, size.height,
                        0, size.height,
                        0, size.height - radius);
            } else {
                lineTo(0, size.height);
            }
            closePath();
        }
    }
}

I suggest having a look at Drawing a curve in Java using mouse drag, as working with Bézier curve always confuses me.

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• How to draw custom rounded rectangles in java?