我目前正在尝试使用Java作为大学项目来创建井字游戏。到目前为止，我所做的只是创建了gameBoard方法的框架代码。我已经遇到一个错误，说：

java.lang.NullPointerException
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
    at java.lang.reflect.Method.invoke(Unknown Source)
    at edu.rice.cs.drjava.model.compiler.JavacCompiler.runCommand(JavacCompiler.java:272)

我也不确定如何总体上完成该项目。我的基本想法是使用gameBoard方法在每个回合上打印棋盘。我试图将变量设置为全局变量，以便在玩家移动时可以更改它们。如何做到这一点，以及如何解决我遇到的错误？任何帮助将不胜感激，这是我当前的代码：

import javax.swing.JOptionPane;

public class Assignment5
{

      public static String top1 = " ";
      public static String top2 = " ";
      public static String top3 = " ";
      public static String mid1 = " ";
      public static String mid2 = " ";
      public static String mid3 = " ";
      public static String bot1 = " ";
      public static String bot2 = " ";
      public static String bot3 = " ";

  //Generate the game board
  public static void gameBoard()
  {
    System.out.println("+-----------+");
    System.out.println("| " + top1 + " | " + top2 + " | " + top3 + " |");
    System.out.println("+-----------+");
    System.out.println("| " + mid1 + " | " + mid2 + " | " + mid3 + " |");
    System.out.println("+-----------+");
    System.out.println("| " + bot1 + " | " + bot2 + " | " + bot3 + " |");
    System.out.println("+-----------+");
  }

  public void main(String args[])
  {
    gameBoard();
  }
}