Java: Accessing BufferedImage to be rendered only has an effect inside class

I am trying to build a little library, to use on other projects, that helps to set up a frame with an image that is redrawn X times per second.

It consists of two classes:

package displayKit;

public class Runtime 
{
	public Display display;
	static double lastTime;
	static double thisTime;
	public static double deltaT;
	static double lastShortTime;
	public static Thread thread;
	public int FPS = 50;
	public static boolean running = false;
	
	public Runtime(int w, int h)
	{
		display = new Display(w, h);
		lastTime = System.currentTimeMillis();
		lastShortTime = System.currentTimeMillis();
		running = true;
		
		while(running)
		{
			OnUpdate();
		}
	}
	
	void OnUpdate()
	{
		deltaT = (System.currentTimeMillis()-lastShortTime)/1000;
		lastShortTime = System.currentTimeMillis();
		
		while(AssessTime(FPS))
		{
			if(display != null)
			{
                //Where I've been putting the code that works
				display.repaint();
			}
		}
	}
	
	public static boolean AssessTime(int FPS)
	{
		...irrelevant...
	}
}

And:

package displayKit;

import java.awt.Color;
import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.image.BufferedImage;
import java.util.Random;

import javax.swing.JFrame;
import javax.swing.JPanel;

public class Display extends JPanel
{
	private static final long serialVersionUID = 1L;
	public static int width;
	public static int height;
	public static Dimension dimension;
	
	public static JFrame frame;
	public static JPanel panel;
	
	public BufferedImage image;
	public Runtime runtime;
	
	public Display(int w, int h)
	{
		width = w;
		height = h;
		dimension = new Dimension(width, height);
		frame = buildJFrame(w, h);
	}
	
	public JFrame buildJFrame(int w, int h)
	{
		JFrame frame = new JFrame();
		frame.setSize(dimension);
		frame.setResizable(false);
		frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
		frame.setLocationRelativeTo(null);
		panel = this;
		panel.setPreferredSize(dimension);
		panel.setSize(dimension);
		panel.setMaximumSize(dimension);
		panel.setMinimumSize(dimension);
		frame.add(panel);
		frame.setVisible(true);
		frame.pack();
		image = new BufferedImage(width,height,BufferedImage.TYPE_INT_RGB);
		return frame;
	}
	
	public void paintComponent(Graphics g)
	{
		g.setColor(Color.black);
		g.fillRect(0, 0, width, height);
		g.drawImage(image, 0, 0, null);
		g.dispose();
	} 
	
	public static BufferedImage ColorNoise(int w, int h)
    {
        ...irrelevant...
    }
}

I'm trying to create an instance of Runtime elsewhere (in another package) and then modify Display's variable 'image' such that the image rendered may be changed dynamically and simply in that context. I have a function that makes a noise-filled buffered image for testing and when I set Display's 'image' to this in OnUpdate() it works...

However, when I try and do this elsewhere it doesn't work i.e. (pseudocode):

public void main(String args[])
{
    Runtime runtime = new Runtime(...)
    while(true)
    {
        runtime.display.image = (randomly generated image)
    }
}

When I have the code in OnUpdate() as indicated in the first class it works and updates with noise every frame but when I try to do this elsewhere no error is thrown the JFrame just stays black. I feel like I'm structuring my code wrong but I don't know how else to make it flexible enough to use for several different projects.

Thanks for your help and sorry for my idiocy!

Well, I assume JFrame won't draw any image until you call repaint explicitly.

It works in OnUpdate because you call it explicitly there and you give some time to the component to repaint itself between loop iterations.

deltaT = (System.currentTimeMillis()-lastShortTime)/1000;
        lastShortTime = System.currentTimeMillis();

while(AssessTime(FPS)) {
    . . . 
    display.repaint();
}

If you just do while loop - your component will not have enough time to redraw itself.

Try to do something like that to validate my assumption:

while(true) {
    Thread.sleep(50);
    runtime.display.image = (randomly generated image)
    runtime.display.repaint();
}