JAVA无法从另一个类中绘制到JFrame

Question

I am aware this is my error. My question is why isn't this working what am i missing i can call this is i put it a method instead of a class so i am assuming theirs something wrong with the third class?

Class 1:

package assignment.pkg1.java;

import java.awt.Color;
import javax.swing.JFrame;

public class JVMVeiwer  {

/**
 * @param args the command line arguments
 */
public static void main(String[] args) {
    final int FRAME_WIDTH = 1000; // Frame Width
    final int FRAME_HEIGHT = 800; // Frame Height
    JFrame frame = new JFrame();

    frame.setSize(FRAME_WIDTH, FRAME_HEIGHT); //Sets Frame Size
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.setTitle("JVM Diagram");// Sets the Title       
    JVMComponent component = new JVMComponent();
    frame.setBackground(Color.WHITE);
    frame.add(component); // adds the diagram to the JFrame       
    frame.setVisible(true); // Makes the frame visible
}

}

Class 2:

package assignment.pkg1.java;

import java.awt.*;
import javax.swing.JComponent;


public class JVMComponent extends  JComponent { 

@Override
public void  paintComponent(Graphics g) {      
    super.paintComponent(g);        
    Graphics2D g2 = (Graphics2D) g; // recover the graphic  
    JVMDiagram diagram = new JVMDiagram(); // creates an instance of JVM Diagram
    diagram.draw(g2);
  }
}

Class 3 this is the one ii cant use o paint to the jframe:

package assignment.pkg1.java;

import java.awt.Color;
import java.awt.Graphics2D;
import javax.swing.JComponent;

public class JVMDiagram {
// Constructor
public JVMDiagram() {

}
// Draw method for shape
public  void draw(Graphics2D g2) {
// Detailed instructions to draw shape
    int x = getWidth();
    int y = getHeight();
    int temp, temp2;
    int width = x / 2;
    int height = x / 2;
    x = (x - width) / 2;
    y= (y - height) / 2; 

    g2.setColor(Color.RED);
    g2.drawOval(x, y, width, height);
    g2.drawRect(x, y, width, height);
    g2.setColor(Color.RED);
    g2.drawLine(x, y, width + x, height + y);
    g2.drawRoundRect(x, y, width, height, y, y);

    g2.drawLine(x + width, y, x, height + y);
}

}

Answer 1

Your problem is that you're misusing inheritance. Your JVMDiagram is extending JVMComponent and it shouldn't. Yes, you gain JVMComponent's getWidth() and getHeight() method, but they don't mean anything since JVMDiagram isn't being added to the GUI as a component, shouldn't be added as a component, and it has 0 height and width (print them out).

Rethink your design, don't use inheritance for this. Instead pass in values from one object to the other if needed. For instance, create a JVMDiagram field within JVMComponent and initialize it. Pass in the width and height with the Graphics2D in the JVMDiagram draw method in JVMComponent's paintComponent method.

Side issue: never call repaint() from within a painting method or from code that is called from within a painting method.