java - 无法看到添加到 JPanel 的图形

Question

我打算用二手实现一个时钟，但我无法正确显示秒针。

我的代码如下所示，ClockFrame 新创建了 SecondHand 并将面板传递给 SecondHand。 SecondHand 预计会获得秒数并自行更新。我认为 g2.drawImage(bImage, 0, 0, null);显示我的位图，但根本不显示。我真的很困惑。提前致谢。

时钟.java

import java.awt.Color;
import javax.swing.JFrame;
import javax.swing.JPanel;
public class Clock {
  public static void main(String[] a) {
      ClockFrame c=new ClockFrame();
  }
}

二手.java

import java.awt.*;

import javax.imageio.ImageIO;
import javax.swing.*;
import java.util.Calendar;
import java.io.*;
import java.awt.image.*;

 public class SecondHand extends JComponent implements Runnable{

    private JPanel fpanel;
    private BufferedImage bImage;
    private Thread secThread;
    private Point center;
    double radPerSec = Math.PI/648000;
    public SecondHand(String path,Point p,Object o){
    try {
        bImage = ImageIO.read(new File(path));
    } catch (IOException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }
    //setLocation((int)p.getX(),(int)p.getY());
    setLocation(0,0);
    center=p;
    ((JPanel)o).add(this);
    fpanel=(JPanel)o;

    secThread=new Thread(this);
    secThread.start();

}

public void run() {
   while(true) {           
          try{
                Thread.sleep(1000);
                Graphics2D g2 = (Graphics2D) getGraphics();
                g2.drawImage(bImage, 0, 0, null);
             //fpanel.repaint();

           } catch (InterruptedException e) {

             e.printStackTrace();
           }
    }

}


}

时钟框架.java

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

public class ClockFrame extends JFrame {

public ClockFrame(){
    JPanel panel = new JPanel();
    setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    add(panel);
    setSize(1000, 1000);
    panel.setSize(getWidth(), getHeight());
    panel.setLayout(null);//!important
    setVisible(true);
    setResizable(false);
    panel.setBackground(Color.BLACK);
    SecondHand sHand=new SecondHand("./res/icon/sechand.png",new   Point(450,300),panel);


  }
}

Answer 1

SecondHand 组件没有出现有两个原因:

1. 它未添加到面板。
2. 面板没有布局，因此它会以 0 x 0 的大小显示。

如评论中所述，请勿使用 getGraphics()，而是重写 paintComponent(Graphics) 方法。要触发该方法，请使用基于 Swing 的 Timer 来调用 repaint()。 为了更快地获得更好的帮助，请发布 Minimal, Complete, and Verifiable example如下所示。获取示例图像的一种方法是热链接到 this Q&A 中看到的图像。 。下面我只是创建一个新的缓冲图像。

我在源代码中看到的其他问题。每个 JComponent 都是一个 ImageObserver，因此在绘制图像时使用它而不是 null。仔细查看其他代码注释以获得其他提示。

import java.awt.*;
import java.awt.image.BufferedImage;
import javax.swing.*;

public class ClockFrame extends JFrame {

    public ClockFrame() {
        JPanel panel = new JPanel();
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        add(panel);
        setSize(1000, 1000);
        panel.setSize(getWidth(), getHeight());
        // this is NOT the way to go
        //panel.setLayout(null);//!important
        panel.setLayout(new GridLayout());
        setResizable(false);
        panel.setBackground(Color.RED);
        SecondHand sHand = new SecondHand("./res/icon/sechand.png", new Point(450, 300), panel);
        panel.add(sHand); // THIS is impoertant
        // this should be done after all components are added!
        setVisible(true);
    }

    public static void main(String[] a) {
        ClockFrame c = new ClockFrame();
    }
}

class SecondHand extends JComponent implements Runnable {

    private JPanel fpanel;
    private BufferedImage bImage;
    private Thread secThread;
    private Point center;
    double radPerSec = Math.PI / 648000;

    public SecondHand(String path, Point p, Object o) {
        try {
            bImage
                    = new BufferedImage(900, 600, BufferedImage.TYPE_INT_RGB);
            //ImageIO.read(new File(path));
        } catch (Exception e) {
            e.printStackTrace();
        }
        //setLocation((int)p.getX(),(int)p.getY());
        setLocation(0, 0);
        center = p;
        ((JPanel) o).add(this);
        fpanel = (JPanel) o;

        secThread = new Thread(this);
        secThread.start();
    }

    public void run() {
        while (true) {
            try {
                Thread.sleep(1000);
                Graphics2D g2 = (Graphics2D) getGraphics();
                g2.drawImage(bImage, 0, 0, null);
                //fpanel.repaint();

            } catch (InterruptedException e) {

                e.printStackTrace();
            }
        }
    }
}