即使我在上面画画也空白的JPanel

Question

从我前几天开始编写的一个简单程序继续，我有一个新的查询。我希望程序在面板中间画一个黑色圆圈。稍后我将使用按钮来移动圆圈，但我还没有在那个阶段。程序运行没有错误，我得到一个白色面板，下面有我的按钮，但白色面板中间没有黑色圆圈。我搜索了一些以前推荐使用paintComponent 的帖子，我已经这样做了，但是我遗漏了一些东西，因为它没有按我的预期工作，并且repaint() 也不起作用。感谢您收到任何提示。

import java.awt.Color;
import java.awt.Container;
import java.awt.Dimension;
import java.awt.FlowLayout;
import java.awt.Graphics;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.util.Random;

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

public class MovingArrows extends JFrame implements ActionListener {

    private JButton buttonUp, buttonDown, buttonLeft, buttonRight;
    private JPanel panel;
    private int xCircleCentre, yCircleCentre;

    final int xCircleCentreStarting = 250, yCircleCentreStarting = 250;
    final int RADIUS = 20;

    public static void main(String[] args) {
        // TODO Auto-generated method stub

        MovingArrows frame = new MovingArrows();
        frame.setSize(550, 600);

        frame.setVisible(true);
        frame.createGUI();
        // frame.repaint();

    }

    private void createGUI() {
        setDefaultCloseOperation(EXIT_ON_CLOSE);
        Container window = getContentPane();
        window.setLayout(new FlowLayout());

        panel = new JPanel();
        panel.setPreferredSize(new Dimension(500, 500));
        panel.setBackground(Color.white);

        window.add(panel);

        buttonUp = new JButton("Up");
        buttonDown = new JButton("Down");
        buttonLeft = new JButton("Left");
        buttonRight = new JButton("Right");
        window.add(buttonUp);
        window.add(buttonDown);
        window.add(buttonLeft);
        window.add(buttonRight);
        buttonUp.addActionListener(this);
        buttonDown.addActionListener(this);
        buttonLeft.addActionListener(this);
        buttonRight.addActionListener(this);
        // panel.repaint();

        Graphics paper = panel.getGraphics();

        paintComponent(paper);

    }

    public void paintComponent(Graphics g) {
        g.setColor(Color.black);
        g.fillOval(xCircleCentreStarting - RADIUS, yCircleCentreStarting
                - RADIUS, RADIUS * 2, RADIUS * 2);

    }

    @Override
    public void actionPerformed(ActionEvent event) {

        Graphics paper = panel.getGraphics();
        paper.setColor(Color.black);
        paper.fillOval(xCircleCentreStarting - RADIUS, yCircleCentreStarting
                - RADIUS, RADIUS * 2, RADIUS * 2);

    }
}

Answer 1

1) 你尝试在没有paintComponent() 的JFrame 上绘画，这是错误的。例如，您需要在 JPanel 的覆盖 paintComponent() 方法中进行自定义绘画。阅读有关custom paintings 的更多信息。

2)在你的actionPerformed()方法中，重画椭圆，你只需要调用repaint()方法，就可以了，你不需要自己获取Graphics实例来绘制。

我已更改您的代码，请检查：

import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.Container;
import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

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

public class MovingArrows extends JFrame implements ActionListener {

    private JButton buttonUp, buttonDown, buttonLeft, buttonRight;
    private int xCircleCentre, yCircleCentre;

    final int xCircleCentreStarting = 250, yCircleCentreStarting = 250;
    final int RADIUS = 20;

    public static void main(String[] args) {

        MovingArrows  frame = new MovingArrows();
        frame.createGUI();
        frame.pack();
        frame.setVisible(true);

    }

    private void createGUI() {
        setDefaultCloseOperation(EXIT_ON_CLOSE);
        JPanel btnPanel = new JPanel();
        Container window = getContentPane();
        DrawHere drawHere = new DrawHere();
        drawHere.setPreferredSize(new Dimension(400,400));
        window.add(drawHere);
        window.add(btnPanel, BorderLayout.SOUTH);
        buttonUp = new JButton("Up");
        buttonDown = new JButton("Down");
        buttonLeft = new JButton("Left");
        buttonRight = new JButton("Right");
        btnPanel.add(buttonUp);
        btnPanel.add(buttonDown);
        btnPanel.add(buttonLeft);
        btnPanel.add(buttonRight);
        buttonUp.addActionListener(this);
        buttonDown.addActionListener(this);
        buttonLeft.addActionListener(this);
        buttonRight.addActionListener(this);
    }

    @Override
    public void actionPerformed(ActionEvent event) {
        repaint();
    }

    class DrawHere extends JPanel {

        public DrawHere(){
            setBackground(Color.WHITE);
        }


        @Override
        public void paintComponent(Graphics g) {
            super.paintComponent(g);
            g.setColor(Color.black);
            g.fillOval(xCircleCentreStarting - RADIUS, yCircleCentreStarting
                    - RADIUS, RADIUS * 2, RADIUS * 2);

        }
    }
}

它看起来像：