when does componentResized get called?

Question

hello i'm new to java programming and when does a componentResized get called? does it get called when your program first starts up? because in my code and i didn't add the "rect" in the center of the screen, until the window got resized. but whenever i start my program, the "rect" is added to the middle of the screen. i'm sorry if i sound vague but thank u so much for taking a look at this. hope u have a great day!

import java.awt.event.ComponentEvent;
import java.awt.event.ComponentListener;
import acm.graphics.GCanvas;
import acm.graphics.GRect;

public class MyCanvas extends GCanvas implements ComponentListener {

    public MyCanvas() {
        addComponentListener(this);
        rect = new GRect(WIDTH, HEIGHT);
        rect.setFilled(true);
    }

    public void update() {
        removeAll();
        add(rect, (getWidth() - WIDTH) / 2, (getHeight() - HEIGHT) / 2);
    }

    public void componentResized(ComponentEvent e) {
        add(rect, 20, 20);
        update();
    }

    public void componentMoved(ComponentEvent e) {}
    public void componentShown(ComponentEvent e) {}
    public void componentHidden(ComponentEvent e) {}

    private int WIDTH = 30;
    private int HEIGHT = 30;

    private GRect rect;
}