在java中使用图形类时出现swing NullPointerException错误

Question

我对java完全陌生。我正试图编写一个简单的java程序，将输入的坐标和矩形的高度、宽度绘制出来。但我得到了NullPointerException错误。这是我的代码：

import javax.swing.JFrame;
import java.util.Scanner;
import java.awt.*;
public class shape extends JFrame{
public int x,y,width, height;
public void setxyhw(int f, int g, int h, int i){
  this.x = f;
  this.y = g;
  this.width =h;
  this.height = i;
 }

public shape(){
setTitle("frame1");
setSize(960,960);
setVisible(true);
setDefaultCloseOperation(EXIT_ON_CLOSE);
}
public void paint(Graphics g){

g.drawRect(x,y,height,width);

}


public static void main(String[] args) {
    int a,b,c,d ;
    Scanner in = new Scanner(System.in);
    System.out.println("Input Co-ordinates and widh & height for rectangle");
    a= in.nextInt();
    b=in.nextInt();
    c=in.nextInt();
    d=in.nextInt();

    shape s=new shape();

    s.setxyhw(a,b,c,d);

    s.paint(null);
}

}