如何为 JPanel 创建渐变？ 另外，如何在 Java 中的 class 中调用 class？

Question

I am making a Java GUI and I have searched the internet for 2 hours on how make a gradient for a JPanel. The code below is that I have, but when run the gradient does not show. What is wrong?

I've tried many other posts from similar questions on this throughout the Internet but they don't work. I've tried numerous versions, but I also don't exactly know how to run a class within a class. Can someone help me please?

class TestPanel extends JPanel{
    @Override
    protected void paintComponent(Graphics g) {
        super.paintComponent(g);
        Graphics2D g2d = (Graphics2D) g;
        g2d.setRenderingHint(RenderingHints.KEY_RENDERING, RenderingHints.VALUE_RENDER_QUALITY);
        int w = getWidth();
        int h = getHeight();
        Color color1 = Color.BLUE;
        Color color2 = Color.GREEN;
        GradientPaint gp = new GradientPaint(0, 0, color1, 0, h, color2);
        g2d.setPaint(gp);
        g2d.fillRect(0, 0, w, h);
    }
} //this is nested within the main class

//some code

public static void main(String[] args) {

    SwingUtilities.invokeLater(new Runnable() {
        @Override
        public void run() {
        CreateGUI cg = new CreateGUI();
        cg.create();    //previous method (not mentioned here)           
            CreateGUI.TestPanel tp = cg.new TestPanel(); //problem
            JPanel panel = new JPanel();
            f.add(panel);
            f.setSize(800, 600);
            f.setLocationRelativeTo(null);
        f.getContentPane().setLayout(null);
            f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
            f.setVisible(true);
        }
    });
}

I expect there to be a gradient but there is none; the background of the JPanel is still white

Answer 1

1. The instance of TestPanel is never added to anything
2. null layouts will prevent the component from been sized and positioned, so you won't see anything even if your did the previous step
3. You should, unless you're adding child components to it, provide a sizing hint, so that the layout managers have something to work with.

For example:

import java.awt.Color;
import java.awt.Dimension;
import java.awt.EventQueue;
import java.awt.GradientPaint;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.RenderingHints;
import javax.swing.JFrame;
import javax.swing.JPanel;

public class Test {

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

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

    class TestPanel extends JPanel {

        @Override
        public Dimension getPreferredSize() {
            return new Dimension(200, 200);
        }

        @Override
        protected void paintComponent(Graphics g) {
            super.paintComponent(g);
            Graphics2D g2d = (Graphics2D) g.create();
            g2d.setRenderingHint(RenderingHints.KEY_RENDERING, RenderingHints.VALUE_RENDER_QUALITY);
            int w = getWidth();
            int h = getHeight();
            Color color1 = Color.BLUE;
            Color color2 = Color.GREEN;
            GradientPaint gp = new GradientPaint(0, 0, color1, 0, h, color2);
            g2d.setPaint(gp);
            g2d.fillRect(0, 0, w, h);
            g2d.dispose();
        }
    }
}