import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.MultipleGradientPaint.CycleMethod;
import java.awt.RadialGradientPaint;
import java.awt.geom.Point2D;

import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.SwingUtilities;

public class RadialGradient {

    public RadialGradient() {
        init();
    }
    
    private void init() {
        RoundGradientPanel gp = new RoundGradientPanel();
        gp.add(new JButton("click"));
        gp.setPreferredSize(new Dimension(200, 200));
        JFrame frame = new JFrame("Radialer Gradient");
        frame.add(gp, BorderLayout.CENTER);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.pack();
        frame.setLocationRelativeTo(null);
        frame.setVisible(true);
    }

    public static void main(String[] args) {
        SwingUtilities.invokeLater(() -> new RadialGradient());
    }
}

class RoundGradientPanel extends JPanel {

    public RoundGradientPanel() {
        super();
        setOpaque(true);
    }

    public void paintComponent(Graphics g) {
        Graphics2D g2d = (Graphics2D) g.create();
        int w = getWidth(), h = getHeight();
        Point2D center = new Point2D.Float(w / 2, h / 2);
        float radius = w / 4;
        float[] dist = { 0.0f, 0.5f, 1.0f };
        Point2D focus = new Point2D.Float(w / 2, h / 2);
        Color[] colors = { Color.BLUE, Color.GREEN, Color.YELLOW };
        RadialGradientPaint rgp = new RadialGradientPaint(center, radius,
                focus, dist, colors, CycleMethod.NO_CYCLE);
        g2d.setPaint(rgp);
        g2d.fillRect(0, 0, w, h);
    }
}