我写了一个程序，希望它通过使用getWidth（）和getHeight（）方法显示在屏幕中间，但它出现在左上角。所以我的问题是getWidth（）和getHeight（）方法不应该获得整个屏幕的宽度和高度吗？

/*
 * This program displays Pascal's Triangle for 8 rows.
 */

import acm.graphics.*;
import acm.program.*;


public class combination extends GraphicsProgram {
    public void run() {
        for(int i = 0; i < NROWS; i++) {
            for (int n = 0; n <= i; n++) {

        add(new GLabel(Combination(i,n), x(i,n), y(i))); //the     coordination and the amount of labels are related to i;

        }       
    }
}

//method that adds labels on the screen.//
private String Combination(int i, int n) { //this method returns a string which   would be added to the screen.//

    return (String.valueOf(i) +","+ String.valueOf(n)); 
    }


//method that set the y coordination for the label//
private double y(int i) {
    double Height = getHeight()/NROWS;
    return i * Height + 9;

}

//method that set the x coordination for the label//
private double x(int i, int n)  {
    double Orgnx = getWidth() / 2;
    double HalfBase = getHeight() / NROWS / Math.sqrt(3);
    double Base = HalfBase * 2;
    return Orgnx - i * HalfBase + n * Base;
    }


/*The program displays 8 rows of pairs of number.*/
private static final int NROWS = 8;