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

Question

I tried to do a program that just show an image, everything goes right but the console shows this warning... The image shows fine but i wanna know what is the console warning and how to solve it Here my main 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);


    }

}

my imagen class

 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;
    }

and the console log...

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

The overridden paint() method is called immediately after calling setVisible(true) method and at that point object objRyu is null .

---

Some Points:

1. Don't directly paint on JFrame instead use a container such as JPanel and add it in JFrame .

2. Instead of overriding paint() method use paintComponent() method for JPanel .

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

3. Call JFrame.setVisible() in the end after adding all the component.

4. Use SwingUtilities.invokeLater() to make sure that EDT is initialized properly.

   Read more

   • Why to use SwingUtilities.invokeLater in main method?

   • SwingUtilities.invokeLater

5. Don't extend any class until and unless you are modifying the existing logic.

   Read more

   • Composition over inheritance

---

Read more

• Painting in AWT and Swing

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

Answer 2

The error indicates that objRyu is null. As soon as any Swing component becomes visible, it is painted and later refreshed again.

//make this happen early
            objRyu = new imagen(ryu,"images/Ryu.png",100,100);
// this should be last
            this.setVisible(true);

Answer 3

I got it , just change the object call after the constructor

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



    }