我是 Java 圖形的新手，我正在嘗試繪制一個矩形。該矩形填充了由 3 個滑塊設置的顏色。問題是該正方形未打印在屏幕上。誰能幫我？import java.awt.Color;import java.awt.FlowLayout;import java.awt.Frame;import java.awt.Graphics;import java.awt.GridLayout;import java.awt.event.ActionEvent;import java.awt.event.ActionListener;import java.awt.event.WindowEvent;import java.awt.event.WindowListener;import javax.swing.JFrame;import javax.swing.JLabel;import javax.swing.JPanel;import javax.swing.JSlider;import javax.swing.JTextField;import javax.swing.event.ChangeEvent;import javax.swing.event.ChangeListener;/* * Program description: Write a Java application which includes 3 sliders which enables  *                      the user to set the color  *                      of a 100x100 square. The square will be drawn using a  *                      specific component (Canvas, etc.). * Author: Hornai Vlad * Date: 24.05.2019 */public class Square extends JPanel implements ChangeListener{     JPanel panel;     JSlider r,g,b;     JLabel rl,gl,bl;     int red,green,blue;    Square()    {        //set the layout        setLayout(new FlowLayout());        //the panel        panel = new JPanel();        //bounds of the panel        panel.setBounds(20,40,300,300);        rl = new JLabel("r");        rl.setBounds(10, 50, 15, 15);        //the first slider        r = new JSlider(0,255,0);        r.setPaintTrack(true);         r.setPaintTicks(true);         r.setPaintLabels(true);           }        我嘗試了幾個選項，這個對我來說有邏輯，我不知道顯示正方形的原因是什么。一開始我認為面板不夠大，所以我把它弄大了，但結果是一樣的。