Can't get Java2D to render a simple line to a Graphics object

Question

Because I want to invoke different classes's drawing method and draw different graphs on the JPanel so I need to take the drawing plate(JPanel or something) as an argument, passing it to my drawing method.(But I don't know if I can do that or not.... The case here is another try...)

Here is my part of the implementation.

I create a class class_diagram as follows:

public class class_diagram extends Object
{

private final int width = 60;  
private final int height = 80; 
private final int first_separate_line_distance= 30; 
private final int second_separate_line_distance= 55;
private int left_up_x = 0;
private int left_up_y = 0; 

public void setLeft_up(int left_up_x,int left_up_y) 
{
    this.left_up_x = left_up_x;
    this.left_up_y = left_up_y;
}


//private Graphics to_draw ;
//private JPanel place_to_draw; 

public class_diagram()
{
     // instance variable "point to" the reference which was passed in.  
}

@Override
//the parameters stands for the left-up point's coordinate. 
public void draw(Graphics to_draw) {
    // TODO Auto-generated method stub

    System.out.println("Call draw method?\n");
    to_draw.setColor(Color.BLACK);
    to_draw.drawLine(31, 41, 131, 768);     
}

}

The above the is the class definition and its' drawing method.

And in the another class:

I call the draw method, and it indeed be invoked, because System.out.println("Call draw method?\n"); in that draw method shows the message to me.

Nevertheless!!! The drawing on my JPanel... It wore me out. Because I have tried at least 4-5 methods....

import java.awt.BorderLayout;

public class UML_Editor_13 extends JFrame {

private Edit_panel canvas = new Edit_panel();

public static void main(String[] args) {                    

UML_Editor_13 frame = new UML_Editor_13();

frame.setVisible(true);

Graphics m= frame.canvas.getGraphics();                 

Object n = new class_diagram();

n.draw(m);

}
}

1. Please somebody tell me why this line "Graphics m= frame.canvas.getGraphics();" doesn't work... If m references to the canvas, why

   to_draw.setColor(Color.BLACK); to_draw.drawLine(31, 41, 131, 768); //didn't work...?

2. Any other method to satisfy my requirements:

" invoke different classes's drawing method and draw different graphs on the JPanel so I need to take the drawing plate(JPanel or something) as an argument, passing it to my drawing method."

Answer 1

You should override the panel's paintComponent(Graphics g) method. In the method call super.paintComponent(g) and then your draw() method.