weird JFrame problems - jframe

Question

I am trying to get a box to move in a JFrame by repainting it, but for some reason i cannot figure out it will not work. I know its probably something really stupid but here are my classes: (also sorry for the formatting its a pain in the ass...) It gives me an error on the frame.add(square) line.
import javax.swing.JFrame;
import javax.swing.WindowConstants;
public class Runner extends JFrame{
final static int FRAME_WIDTH = 1000;
final static int FRAME_HEIGHT = 600;
final static int BOX_WIDTH = 50;
final static int BOX_HEIGHT = 50;
public static void main (String[] args){
JFrame frame = new JFrame();
frame.setSize(FRAME_WIDTH, FRAME_HEIGHT);
frame.setTitle("Animation");
frame.setDefaultCloseOperation(WindowConstants.DISPOSE_ON_CLOSE);
frame.setVisible(true);
Component square = new Component();
Dimensions.setBoxDimensions(BOX_WIDTH, BOX_HEIGHT);
frame.add(square);
frame.setVisible(true);
for (int i = 0; i < 100; i++){
Dimensions.setPosition(i,i);
square.repaint();
frame.setVisible(true);
}
}
}
import java.awt.Color;
import java.awt.Graphics;
import java.awt.Graphics2D;
import javax.swing.JFrame;
public class Component extends JFrame{
public void paintComponent (Graphics g){
Graphics2D g2 = (Graphics2D) g;
g2.fillRect(Dimensions.xPos, Dimensions.yPos, Dimensions.boxWidth, Dimensions.boxHeight);
}
}
public class Dimensions {
public static int boxHeight = 50;
public static int boxWidth = 50;
public static int xPos = 0;
public static int yPos = 0;
public static void setBoxDimensions(int width, int height){
boxHeight = height;
boxWidth = width;
}
public static void setPosition(int x, int y){
xPos = x;
yPos = y;
}
}

Answer 1

Since your component class is a JFrame you cannot add teh JFrame to a JFrame. You may want to add a JPanel to the JFrame and paint on the JPanel instead.
Or you may be intending to extend java.awt.Component instead?
import javax.swing.JPanel;
public class MyComponent extends JPanel{
public void paintComponent (Graphics g){
Graphics2D g2 = (Graphics2D) g;
g2.fillRect(Dimensions.xPos, Dimensions.yPos, Dimensions.boxWidth, Dimensions.boxHeight);
}
}
And when you want a Component you can do this:
JPanel square = new MyComponent();

Answer 2

It's better to override the "paint" method in your JFrame. It's much easier to do.
#override
public void paint(Graphics g){
super();
/*
put the code to draw your shape of shapes here
*/
}
You can call repaint method of your JFrame whenever you want to update your JFrame.