java - Gui Swing 绘图对象未显示

Question

我需要帮助在某个面板上绘制/绘制对象。我想在“drawPanel”JPanel 中绘制东西。有人可以看一下这个并告诉我我错过了什么吗？我将不胜感激，如果我遗漏了一些明显的东西，我很抱歉。

当我运行该程序时，没有绘制任何内容。

package redball;

import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.EventQueue;
import java.awt.Graphics;

import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.border.EmptyBorder;
import javax.swing.JLabel;

public class drawTest extends JFrame {

    private JPanel contentPane;

    /**
     * Launch the application.
     */
    public static void main(String[] args) {
        EventQueue.invokeLater(new Runnable() {
            public void run() {
                try {
                    drawTest frame = new drawTest();
                    frame.setVisible(true);
                } catch (Exception e) {
                    e.printStackTrace();
                }
            }
        });
    }

    /**
     * Create the frame.
     */
    public drawTest() {
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        setBounds(100, 100, 450, 300);
        contentPane = new JPanel();
        contentPane.setBorder(new EmptyBorder(5, 5, 5, 5));
        contentPane.setLayout(new BorderLayout(0, 0));
        setContentPane(contentPane);


        JPanel leftPanel = new JPanel();
        contentPane.add(leftPanel, BorderLayout.WEST);

        JLabel lblNewLabel = new JLabel("some stuff");
        leftPanel.add(lblNewLabel);

        JPanel drawPanel = new JPanel(); //this is where I want to draw stuff
        contentPane.add(drawPanel, BorderLayout.CENTER);


    }

    protected void paintComponent(Graphics g) {
        super.paintComponents(g); 

        g.setColor(Color.black);
        g.fillRect(25, 500, 100, 20);
    }

}

Answer 1

一般提示(前两个有助于解决此问题):

1. 在任何更改的方法上使用@Override 表示法! (该代码试图重写不存在的方法。即使它是在面板中实现的，它也会调用错误的 super 方法。)
2. 自定义绘制的组件应建议首选尺寸。
3. 请学习常见的 Java 命名法(命名约定 - 例如 EachWordUpperCaseClass、firstWordLowerCaseMethod()、firstWordLowerCaseAttribute，除非它是 UPPER_CASE_CONSTANT)并一致地使用它。

这是实现上述建议的结果，并在代码中的注释中提到了更多提示。

import java.awt.*;
import javax.swing.*;
import javax.swing.border.*;

public class DrawTest extends JFrame {

    private JPanel contentPane;

    public DrawTest() {
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        // instead pack() once components are added
        // setBounds(100, 100, 450, 300); 
        contentPane = new JPanel();
        contentPane.setBorder(new EmptyBorder(5, 5, 5, 5));
        contentPane.setLayout(new BorderLayout(0, 0));
        setContentPane(contentPane);

        JPanel leftPanel = new JPanel();
        leftPanel.setBackground(Color.CYAN);
        contentPane.add(leftPanel, BorderLayout.LINE_START);

        JLabel lblNewLabel = new JLabel("some stuff");
        leftPanel.add(lblNewLabel);

        JPanel drawPanel = new JPanel() {

            Dimension preferredSize = new Dimension(300, 100);

            @Override
            protected void paintComponent(Graphics g) {
                super.paintComponent(g);

                g.setColor(Color.black);
                // 500? That's much deeper than the panel shows on-screen!
                //g.fillRect(25, 500, 100, 20);
                g.fillRect(25, 50, 100, 20);
            }

            @Override
            public Dimension getPreferredSize() {
                Dimension d = super.getPreferredSize();
                if (d.width<50 || d.height<50) {
                    return preferredSize;
                } else {
                    System.out.println("d: " + d);
                    return d;
                }
            }
        };
        drawPanel.setBackground(Color.YELLOW);
        contentPane.add(drawPanel, BorderLayout.CENTER);

        pack();
    }

    public static void main(String[] args) {
        EventQueue.invokeLater(new Runnable() {
            public void run() {
                try {
                    DrawTest frame = new DrawTest();
                    frame.setVisible(true);
                } catch (Exception e) {
                    e.printStackTrace();
                }
            }
        });
    }
}