NullPointerExceptionとなるが、nullになる理由がわからない

###実現したいこと
stone型の二次元配列に値を設定したい(初期化).
###問題点
メンバ変数で宣言するためにBoardの中でStone[][] stone=new Stone[8][8];と宣言した。しかし存在しないと言われたが理由がわからない。

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException: Cannot invoke "Stone.setObverse(int)" because "this.stone[<local8>][<local9>]" is null
        at Board.paint(Reversi.java:77)
        at Reversi.paintComponent(Reversi.java:110)
        at java.desktop/javax.swing.JComponent.paint(JComponent.java:1074)
        at java.desktop/javax.swing.JComponent.paintChildren(JComponent.java:907)
        at java.desktop/javax.swing.JComponent.paint(JComponent.java:1083)
        at java.desktop/javax.swing.JComponent.paintChildren(JComponent.java:907)
        at java.desktop/javax.swing.JComponent.paint(JComponent.java:1083)
        at java.desktop/javax.swing.JLayeredPane.paint(JLayeredPane.java:586)
        at java.desktop/javax.swing.JComponent.paintChildren(JComponent.java:907)
        at java.desktop/javax.swing.JComponent.paintToOffscreen(JComponent.java:5271)
        at java.desktop/javax.swing.RepaintManager$PaintManager.paintDoubleBufferedFPScales(RepaintManager.java:1707)
        at java.desktop/javax.swing.RepaintManager$PaintManager.paintDoubleBuffered(RepaintManager.java:1616)
        at java.desktop/javax.swing.RepaintManager$PaintManager.paint(RepaintManager.java:1556)
        at java.desktop/javax.swing.RepaintManager.paint(RepaintManager.java:1323)
        at java.desktop/javax.swing.JComponent.paint(JComponent.java:1060)
        at java.desktop/java.awt.GraphicsCallback$PaintCallback.run(GraphicsCallback.java:39)
        at java.desktop/sun.awt.SunGraphicsCallback.runOneComponent(SunGraphicsCallback.java:75)
        at java.desktop/sun.awt.SunGraphicsCallback.runComponents(SunGraphicsCallback.java:112)
        at java.desktop/java.awt.Container.paint(Container.java:2003)
        at java.desktop/java.awt.Window.paint(Window.java:3949)
        at java.desktop/javax.swing.RepaintManager$4.run(RepaintManager.java:876)
        at java.desktop/javax.swing.RepaintManager$4.run(RepaintManager.java:848)
        at java.base/java.security.AccessController.doPrivileged(AccessController.java:391)
        at java.base/java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:85)
        at java.desktop/javax.swing.RepaintManager.paintDirtyRegions(RepaintManager.java:848)
        at java.desktop/javax.swing.RepaintManager.paintDirtyRegions(RepaintManager.java:823)
        at java.desktop/javax.swing.RepaintManager.prePaintDirtyRegions(RepaintManager.java:772)
        at java.desktop/javax.swing.RepaintManager$ProcessingRunnable.run(RepaintManager.java:1884)
        at java.desktop/java.awt.event.InvocationEvent.dispatch(InvocationEvent.java:316)
        at java.desktop/java.awt.EventQueue.dispatchEventImpl(EventQueue.java:770)
        at java.desktop/java.awt.EventQueue$4.run(EventQueue.java:721)
        at java.desktop/java.awt.EventQueue$4.run(EventQueue.java:715)
        at java.base/java.security.AccessController.doPrivileged(AccessController.java:391)
        at java.base/java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:85)
        at java.desktop/java.awt.EventQueue.dispatchEvent(EventQueue.java:740)
        at java.desktop/java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:203)
        at java.desktop/java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:124)
        at java.desktop/java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:113)
        at java.desktop/java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:109)
        at java.desktop/java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:101)
        at java.desktop/java.awt.EventDispatchThread.run(EventDispatchThread.java:90)

###コード

java
1import java.awt.*;
2import javax.swing.*;
3
4//石に対応するクラス
5class Stone{
6    public final static int black=1; //黒
7    public final static int white=2; //白
8    private int obverse; //表面の色
9    //コンストラクタ
10    Stone(){
11        obverse=0; //非配置にする
12    }
13
14    //表面の色を設定する
15    void setObverse(int color){
16        if(color==black || color==white){
17            obverse=color;
18        }else{
19            System.out.println("黒か白でなければいけません");
20        }
21    }
22
23    //表面の色で中心p,半径radの円を塗りつぶす
24    void paint(Graphics g,Point p,int rad){
25        /*省略*/
26    }
27}
28
29class Board{
30    Stone[][] stone=new Stone[8][8];
31
32    //コンストラクタ
33    Board(){
34    }
35
36    //画面描画
37    void paint(Graphics g,int unit_size){
38        int rad=32;
39
40        /*省略*/
41
42        int px=120,py=120;
43        for(int i=0;i<8;i++){
44            for(int j=0;j<8;j++){
45                stone[i][j].setObverse(1);
46            }
47        }
48        int b=1,w=2;
49        stone[3][3].setObverse(b);
50        stone[3][4].setObverse(w);
51        stone[4][3].setObverse(w);
52        stone[4][4].setObverse(b);
53        for(int i=0;i<8;i++){
54            for(int j=0;j<8;j++){
55                Point p=new Point();
56                p.x=px; //中心座標を定義
57                p.y=py;
58                stone[i][j].paint(g,p,rad);
59                py=py+80;
60            }
61            px=px+80;
62        }
63    }
64}
65
66
67public class Reversi extends JPanel{
68    public final static int UNIT_SIZE=80;
69    Board board =new Board();
70
71    //コンストラクタ
72    public Reversi(){
73        setPreferredSize(new Dimension(800,800));
74    }
75
76    //画面描画
77    public void paintComponent(Graphics g){
78        board.paint(g, UNIT_SIZE); //Boardクラスのpaintメソッドを呼び出す
79    }
80
81    //起動
82    public static void main(String[] args){
83        JFrame f=new JFrame();
84        f.getContentPane().setLayout(new FlowLayout());
85        f.getContentPane().add(new Reversi());
86        f.pack();
87        f.setResizable(false);
88        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
89        f.setVisible(true);
90    }
91
92
93}