求原因？import java.util.Scanner;public class Test { // public static void () static class Dian { int x; int y; public int getX() { return x; } public void setX(int x) { this.x = x; } public int getY() { return y; } public void setY(int y) { this.y = y; } } public static void main(String[] args) { // TODO Auto-generated method stub Dian da = null; da.setX(1); System.out.println(da.x); }}