即使當我畫它時，也是空白的JPanel

Question

繼續從一個簡單的程序開始寫一天，我有一個新的查詢。我希望程序在面板中間畫一個黑色圓圈。稍後我將使用按鈕來移動圓圈，但我還沒有在那個舞臺上。該程序運行時沒有錯誤，我用下面的按鈕得到一個白色面板，但在白色面板中間沒有黑色圓圈。我搜索了一些以前推薦使用paintComponent的帖子，我已經完成了，但我錯過了一些東西，因爲它不像我期望的那樣工作，並且repaint（）也不起作用。任何提示感激地收到。

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）您嘗試繪製JFrame，它沒有paintComponent()，這是錯誤的。例如，您需要在經過改寫的paintComponent()方法JPanel中進行自定義繪畫。閱讀更多關於custom paintings。在actionPerformed()方法

2），重新繪製橢圓形的，你只需要調用repaint()方法，即盡一切，你不需要自己得到Graphics實例和油漆。

我已經改變了你的代碼，檢查：

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); 

     } 
    } 
} 

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