将图像显示到JFrame中的java.lang.NullPointerException

Question

我试图做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在这里我的主要课程......

    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imagen课

 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

paint()在调用setVisible(true)方法之后立即调用重写的方法,此时对象objRyu是null.

一些要点:

1. 不要直接涂上JFrame而是使用像这样的容器JPanel并将其添加进去JFrame.

2. 而不是重写paint()方法使用paintComponent()方法JPanel.

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

3. JFrame.setVisible()添加完所有组件后最后调用.

4. 用于SwingUtilities.invokeLater()确保正确初始化EDT.

   阅读更多

   • 为什么在main方法中使用SwingUtilities.invokeLater？

   • 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

   • 继承的构成

阅读更多

• 在AWT和Swing中绘画

• paintComponent()与paint()和JPanel vs Canvas在画笔类型的GUI中