NullPointerException错误:如何修复此问题？

Question

我试图通过Java图形程序制作一个简单的四个tic-tac-toe板的图像.我的程序编译并可以运行,但不会生成四个板的图像.我的交互窗格中出现此错误(我使用DrJava进行编码):

Welcome to DrJava.  Working directory is C:\Users\Daniel\Desktop\CS Programs
> run TicTacToeBoards
java.lang.NullPointerException
    at TicTacToeBoards.drawTicTacToeBoard(TicTacToeBoards.java:38)
    at TicTacToeBoards.<init>(TicTacToeBoards.java:18)
    at TicTacToeBoards.main(TicTacToeBoards.java:52)
    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)

这是我试图运行的程序:

import java.awt.Color;

public class TicTacToeBoards {

  private NsccWindow win;

  public TicTacToeBoards() {

    NsccWindow win;
    win = new NsccWindow(10, 10, 330, 300);
    win.setTitle("Tic-Tac-Toe Boards");
    drawTicTacToeBoard(40, 40);
    drawTicTacToeBoard(40, 170);
    drawTicTacToeBoard(170, 40);
    drawTicTacToeBoard(170, 170);

  }  

  public void drawTicTacToeBoard(int x, int y) {

    NsccLine vertLineL;
    NsccLine vertLineR;
    NsccLine horizLineU;
    NsccLine horizLineD;

    vertLineL = new NsccLine((x + 30), y, (x + 30), (y + 90));
    vertLineR = new NsccLine((x + 60), y, (x + 60), (y + 90));
    horizLineU = new NsccLine(x, (y + 30), (x + 90), (y + 30));
    horizLineD = new NsccLine(x, (y + 60), (x + 90), (y + 30));


    win.add(vertLineL);
    win.add(vertLineR);
    win.add(horizLineU);
    win.add(horizLineD);


    win.repaint();




  }

  public static void main(String[] args) {
    TicTacToeBoards test = new
    TicTacToeBoards();
  }
}

我是Java编程的新手,我一直试图通过DrJava中的调试找到错误,但没有用.有人可以帮忙吗？:)

Answer 1

从构造函数中删除此行:

NsccWindow win;

你已经有了win字段,所以也没有必要在构造函数中声明它.

在构造函数中声明它时,隐藏实例字段并实例化仅限于构造函数范围的变量.因此,当您尝试访问内部的实例字段时drawTicTacToeBoard,实际上就是这样null.