将图像显示到 JFrame 时出现 java.lang.NullPointerException

Question

我尝试编写一个只显示图像的程序，一切正常，但控制台显示此警告... 图像显示正常，但我想知道什么是控制台警告以及如何解决它 这是我的主要类(class)......

    public class main extends JFrame{
     Image ryu;
     imagen objRyu;
     public main(){
            super("Imagen1");
            this.setSize(500,500);
            this.setVisible(true);
            this.setResizable(true);
            this.setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
            objRyu = new imagen(ryu,"images/Ryu.png",100,100);
            repaint();

        }



    public static void main(String[] args) {
        new main();

    }
    public void paint(Graphics g){
        Graphics g2 = (Graphics2D)g;
         g2.setColor(Color.gray);
         g2.fillRect(0, 0, 500, 500);
         g2.drawImage(objRyu.getImagen(), 100, 50, objRyu.getAncho(), objRyu.getAlto(), null);


    }

}

我的图像类

 public class imagen {
    InputStream imgStream;
    private Image imagen;
    private int ancho;
    private int alto;

    public imagen(Image imagen, String ruta,int ancho, int alto){
        this.imagen = imagen;
        this.ancho = ancho;
        this.alto = alto;

        try{
            imgStream = imagen.class.getResourceAsStream(ruta);
            this.imagen = ImageIO.read(imgStream);
        }catch(IOException e){
            e.printStackTrace();
        }


    }

    public int getAncho() {
        return ancho;
    }

    public int getAlto() {
        return alto;
    }

    public Image getImagen() {
        return imagen;
    }

和控制台日志...

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at main.paint(main.java:34)
at javax.swing.RepaintManager$3.run(Unknown Source)
at javax.swing.RepaintManager$3.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
at javax.swing.RepaintManager.paintDirtyRegions(Unknown Source)
at javax.swing.RepaintManager.paintDirtyRegions(Unknown Source)
at javax.swing.RepaintManager.prePaintDirtyRegions(Unknown Source)
at javax.swing.RepaintManager.access$1100(Unknown Source)
at javax.swing.RepaintManager$ProcessingRunnable.run(Unknown Source)
at java.awt.event.InvocationEvent.dispatch(Unknown Source)
at java.awt.EventQueue.dispatchEventImpl(Unknown Source)
at java.awt.EventQueue.access$200(Unknown Source)
at java.awt.EventQueue$3.run(Unknown Source)
at java.awt.EventQueue$3.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
at java.awt.EventQueue.dispatchEvent(Unknown Source)
at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source)
at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.run(Unknown Source)
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at main.paint(main.java:34)
at javax.swing.RepaintManager$3.run(Unknown Source)
at javax.swing.RepaintManager$3.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
at javax.swing.RepaintManager.paintDirtyRegions(Unknown Source)
at javax.swing.RepaintManager.paintDirtyRegions(Unknown Source)
at javax.swing.RepaintManager.prePaintDirtyRegions(Unknown Source)
at javax.swing.RepaintManager.access$1100(Unknown Source)
at javax.swing.RepaintManager$ProcessingRunnable.run(Unknown Source)
at java.awt.event.InvocationEvent.dispatch(Unknown Source)
at java.awt.EventQueue.dispatchEventImpl(Unknown Source)
at java.awt.EventQueue.access$200(Unknown Source)
at java.awt.EventQueue$3.run(Unknown Source)
at java.awt.EventQueue$3.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
at java.awt.EventQueue.dispatchEvent(Unknown Source)
at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source)
at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.run(Unknown Source)

Answer 1

覆盖的paint() 方法在调用setVisible(true) 方法后立即被调用，此时对象objRyu 是空。

---

一些要点:

1. 不要直接在JFrame上绘制，而是使用JPanel等容器添加到JFrame中。

2. 不要重写 paint() 方法，而是使用 JPanel 的 paintComponent() 方法。

   @Overrie
   public void paintComponent(Graphics g) {
        super.paintComponent(g);
        //your custom painting here
   }
   

3. 在添加所有组件后最后调用JFrame.setVisible()。

4. 使用 SwingUtilities.invokeLater() 确保 EDT正确初始化。

   阅读更多

   • Why to use SwingUtilities.invokeLater in main method?

   • SwingUtilities.invokeLater

5. 除非您正在修改现有逻辑，否则不要扩展任何类。

   阅读更多

   • > Composition over inheritance

---

阅读更多

• Painting in AWT and Swing

• paintComponent() vs paint() and JPanel vs Canvas in a paintbrush-type GUI