RoundedBorder.java

import java.awt.Component;
import java.awt.Graphics;
import java.awt.Insets;
import java.awt.Shape;
import java.awt.geom.RoundRectangle2D;
import javax.swing.border.AbstractBorder;

public class RoundedBorder extends AbstractBorder {

    private final int radius;

    public RoundedBorder(int radius) {
        this.radius = radius;
    }

    @Override
    public void paintBorder(Component c, Graphics g, int x, int y, int width, int height) {
        g.setColor(c.getBackground());
        g.fillRoundRect(x, y, width, height, radius, radius);
    }

    @Override
    public Insets getBorderInsets(Component c) {
        return new Insets(this.radius, this.radius, this.radius, this.radius);
    }

    @Override
    public Insets getBorderInsets(Component c, Insets insets) {
        insets.left = insets.top = insets.right = insets.bottom = this.radius;
        return insets;
    }

    @Override
    public boolean isBorderOpaque() {
        return true;
    }

    public Shape getBorderShape(int x, int y, int width, int height) {
        return new RoundRectangle2D.Double(x, y, width - 1, height - 1, radius, radius);
    }
}