getApplicationContextのnull object reference

AndroidStudio
1public class MainActivity extends AppCompatActivity {
2
3    private TextView textView;
4    private EditText editTextKey, editTextValue;
5    private KakeiboDatabase kDB;
6    private SQLiteDatabase db;
7    
8
9    public String readData(String date) {
10        if(kDB == null){
11            kDB = new KakeiboDatabase(getApplicationContext());
12        }
13
14        if(db == null){
15            db = kDB.getReadableDatabase();
16        }
17
18        Cursor cursor = db.query(
19                "kakeibodb",
20                new String[] { "SUM(price)" },
21                "date=" + date,
22                null,
23                null,
24                null,
25                null
26        );
27
28        cursor.moveToFirst();
29
30        String price = cursor.getString(0);
31        
32        cursor.close();
33
34        return price;
35    }
36}

AndroidStudio
1public class KakeiboDatabase extends SQLiteOpenHelper {
2
3
4    // データーベースのバージョン
5    private static final int DATABASE_VERSION = 1;
6
7    // データーベース名
8    private static final String DATABASE_NAME = "KakeiboDB.db";
9    private static final String TABLE_NAME = "kakeibodb";
10    private static final String PRIMARY_KEY = "date";
11    private static final String COLUMN_CATEGORY = "category";
12    private static final String COLUMN_PRICE= "price";
13    private static final String COLUMN_MEMO = "memo";
14    private static final String _ID = "_id";
15
16
17    private KakeiboDatabase kDB;
18    private SQLiteDatabase db;
19
20
21    private static final String SQL_CREATE_ENTRIES =
22            "CREATE TABLE " + TABLE_NAME + " (" +
23                    _ID + " INTEGER," +
24                    PRIMARY_KEY + " TEXT," +
25                    COLUMN_CATEGORY + " TEXT," +
26                    COLUMN_PRICE + " INTEGER," +
27                    COLUMN_MEMO + " TEXT," +
28                    "PRIMARY KEY(" + _ID + "," + PRIMARY_KEY + "))";
29
30    private static final String SQL_DELETE_ENTRIES =
31            "DROP TABLE IF EXISTS " + TABLE_NAME;
32
33
34    KakeiboDatabase(Context context) {
35        super(context, DATABASE_NAME, null, DATABASE_VERSION);
36
37    }
38
39    @Override
40    public void onCreate(SQLiteDatabase db) {
41
42        // テーブル作成
43        // SQLiteファイルがなければSQLiteファイルが作成される
44        db.execSQL(
45                SQL_CREATE_ENTRIES
46        );
47
48        Log.d("debug", "onCreate(SQLiteDatabase db)");
49
50    }
51
52    @Override
53    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
54        // アップデートの判別
55        db.execSQL(
56                SQL_DELETE_ENTRIES
57        );
58        onCreate(db);
59    }
60
61    public void onDowngrade(SQLiteDatabase db, int oldVersion, int newVersion) {
62        onUpgrade(db, oldVersion, newVersion);
63    }
64}

AndroidStudio
1public class CalendarAdapter extends BaseAdapter {
2        private List<Date> dateArray = new ArrayList();
3        private Context mContext;
4        private DateManager mDateManager;
5        private LayoutInflater mLayoutInflater;
6        private String price;
7        private SQLiteDatabase db;
8        private KakeiboDatabase kDB;
9
10        //カスタムセルを拡張したらここでWigetを定義
11        private static class ViewHolder {
12            public TextView dateText;
13        }
14
15        public CalendarAdapter(Context context){
16            mContext = context;
17            mLayoutInflater = LayoutInflater.from(mContext);
18            mDateManager = new DateManager();
19            dateArray = mDateManager.getDays();
20        }
21
22        @Override
23        public int getCount() {
24            return dateArray.size();
25        }
26
27        @Override
28        public View getView(int position, View convertView, ViewGroup parent) {
29            ViewHolder holder;
30            if (convertView == null) {
31                convertView = mLayoutInflater.inflate(R.layout.calendar_cell, null);
32                holder = new ViewHolder();
33                holder.dateText = convertView.findViewById(R.id.dateText);
34                convertView.setTag(holder);
35            } else {
36                holder = (ViewHolder)convertView.getTag();
37            }
38
39            //セルのサイズを指定
40            float dp = mContext.getResources().getDisplayMetrics().density;
41            AbsListView.LayoutParams params = new AbsListView.LayoutParams(parent.getWidth()/7 - (int)dp, (parent.getHeight() - (int)dp * mDateManager.getWeeks() ) / mDateManager.getWeeks());
42            convertView.setLayoutParams(params);
43
44            //その月日の金額をまとめる
45            SimpleDateFormat dateF = new SimpleDateFormat("yyyy-MM-dd", Locale.JAPAN);
46            MainActivity main = new MainActivity();
47            price = main.readData(dateF.format(dateArray.get(position)));
48
49            //日付とデータベースの金額を表示させる
50            SimpleDateFormat dateFormat = new SimpleDateFormat("d", Locale.JAPAN);
51            holder.dateText.setText(dateFormat.format(dateArray.get(position)) + "\n" + price);
52
53            //当月以外のセルをグレーアウト
54            if (mDateManager.isCurrentMonth(dateArray.get(position))){
55                convertView.setBackgroundColor(Color.WHITE);
56            }else {
57                convertView.setBackgroundColor(Color.LTGRAY);
58            }
59
60            //日曜日を赤、土曜日を青に
61            int colorId;
62            switch (mDateManager.getDayOfWeek(dateArray.get(position))){
63                case 1:
64                    colorId = Color.RED;
65                    break;
66                case 7:
67                    colorId = Color.BLUE;
68                    break;
69
70                default:
71                    colorId = Color.BLACK;
72                    break;
73            }
74            holder.dateText.setTextColor(colorId);
75
76            return convertView;
77        }
78
79        @Override
80        public long getItemId(int position) {
81            return 0;
82        }
83
84        @Override
85        public Object getItem(int position) {
86            return null;
87        }
88
89        //表示月を取得
90        public String getTitle(){
91            SimpleDateFormat format = new SimpleDateFormat("yyyy.MM", Locale.JAPAN);
92            return format.format(mDateManager.mCalendar.getTime());
93        }
94
95        //翌月表示
96        public void nextMonth(){
97            mDateManager.nextMonth();
98            dateArray = mDateManager.getDays();
99            this.notifyDataSetChanged();
100        }
101
102        //前月表示
103        public void prevMonth(){
104            mDateManager.prevMonth();
105            dateArray = mDateManager.getDays();
106            this.notifyDataSetChanged();
107    }
108}
109

このMainActivityにあるreadDataを外部から呼び出したときに
【java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.Context android.content.Context.getApplicationContext()' on a null object reference
at com.example.a162105.kakeibo.MainActivity.readData(MainActivity.java:33)】
というエラー文が出ます。

(MainActivity.java:33) は　kDB = new KakeiboDatabase(getApplicationContext()); のところです。

readDataの呼び出し部分は上から三番目のソース内の「//その月日の金額をまとめる」というコメントがついてる部分です。

調べてもいまいちわからなかったのでよろしければご助言お願いいたします。

行動規範の内容に同意します

回答1件

ベストアンサー

例えば下記のようなコードで実行すると、やはりNullPointerExceptionで落ちます。

java
1public class MainActivity extends AppCompatActivity {
2
3    private Context mContext = getApplicationContext();
4
5    @Override
6    protected void onCreate(Bundle savedInstanceState) {
7        super.onCreate(savedInstanceState);
8        setContentView(R.layout.activity_main);
9    }
10}