为什么getGraphics在ActionPerformed方法中起作用？

Question

Why it is possible to draw with getGraphics() when called inside method ActionPerformed of Interface ActionListener but not within a method called from constructor or other method. Here is the code i made. Why the calls of the method "dibujar" is ignored inside "empezar" and constructor but not inside the ActionListener method?

import javax.swing.*;
import java.awt.Dimension;
import java.awt.Color;
import java.awt.Graphics;
import java.awt.event.ActionListener;
import java.awt.event.ActionEvent;

public class VentanaGrafica extends JFrame{


public static void main(String[] args){
SwingUtilities.invokeLater(new Runnable(){
@Override
    public void run(){
        new VentanaGrafica().setVisible(true);
    }
});
}


public VentanaGrafica(){    
    empezar();  
    dibujar();
}



private void empezar(){
setTitle("Graficar con Jpanel");
setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
setMinimumSize(new Dimension(600,600));
setResizable(false);
panel = new JPanel();
panel.setBackground(Color.blue);
panel.setSize(new Dimension(400,400));
boton = new JButton("Este boton");
boton.setFocusable(false);
panel.add(boton);
add(panel);
boton.addActionListener(new ActionListener(){
    @Override
    public void actionPerformed(ActionEvent e){
        dibujar();
    }
});
pack(); 
dibujar();

}

private void dibujar(){
gc = panel.getGraphics();
gc.setColor(Color.red);
gc.fillRect(200,0,120,80);
}
JPanel panel;
Graphics gc;
JButton boton;
}

I have read that the repaint() method is called every sometime to repaint and it means that dibujar() is not ignored but repaint() have erased its work but why it doesnt happen when the call is made inside the ActionEvent? it's somekind implicit to do some kind of loop or inhibit the repaint() call?

Answer 1

I think that this is caused because in the first case new VentanaGrafica().setVisible(true); is called after dibujar() which means that once you make the frame visible the panel will be repainted and the effects of the dibujar() will be erased and in the second case, the panel isn't repainted after you call dibujar() .

If you want to do some custom painting, you will have to override the paintComponent method and add some logic to make the component's state consistent.

Here is an example:

public class CustomPanel extends JPanel{
    Color color = Color.WHITE;
    protected void paintComponent(Graphics g){
        super.paintComponent(g);
        g.setColor(color);
        g.fillRect(200,0,120,80);
    }
    public void changeColor(Color color){
        this.color = color;
    }
}

And here is how you can use this panel:

CustomPanel panel = new CustomPanel();
panel.changeColor(Color.RED);
panel.repaint();