import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.GradientPaint;
import java.awt.Graphics;
import java.awt.Graphics2D;

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

public class LinearGradient {

    public LinearGradient() {
        GradientPanel gp = new GradientPanel();
        gp.add(new JButton("click"));
        gp.setPreferredSize(new Dimension(200, 200));
        JFrame frame = new JFrame("Farbverlauf");
        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 LinearGradient());
    }
}

class GradientPanel extends JPanel {

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

    public void paintComponent(Graphics g) {
        Graphics2D g2d = (Graphics2D) g.create();
        // vertikaler Verlauf von rechts oben nach links unten
        GradientPaint gp = new GradientPaint(getWidth(), 0, Color.BLUE, 0,
                getHeight(), Color.YELLOW);
        // horizontaler, wiederholter Verlauf
        // GradientPaint gp = new GradientPaint(getWidth()/2, getHeight()/2,
        // Color.BLUE,
        // getWidth(), getHeight()/2, Color.YELLOW, true);
        // diagonaler Verlauf von links oben nach rechts unten
        // GradientPaint gp = new GradientPaint(0, 0, Color.BLUE, getWidth(),
        // getHeight(), Color.YELLOW);
        // vertikaler Verlauf
        // GradientPaint gp = new GradientPaint(getWidth()/2, 0, Color.BLUE,
        // getWidth()/2, getHeight(), Color.YELLOW);
        g2d.setPaint(gp);
        g2d.fillRect(0, 0, getWidth(), getHeight());
    }
}