我是一名开发 Android 应用程序的初级程序员，所以我真的需要你的帮助。我知道我的问题很长，但如果你们能回答我的问题，我将不胜感激

我正在制作一个从表格中的多个单词中随机选择一个单词的应用。

因此，我用 SQLite 创建了我的数据库。我试过这些东西。

我制作了 WordDBHelper 以使用 SQLiteOpenHelper
我制作了 WordDBRecord 以将记录放入表中
我制作了 WordActivty 以打开数据库并查看通过 TextView.setText() 从表格中选择的单词。

这些是我的代码

文字 Activity

public class WordActivity extends AppCompatActivity {

    private WordDBRecord wordDBRecord;
    String category = wordDBRecord.category;
    String word = wordDBRecord.word;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_word);
        TextView t1 = (TextView) findViewById(R.id.textViewCategory);
        TextView t2 = (TextView) findViewById(R.id.textViewWord);

        wordDBRecord = new WordDBRecord(this);
        wordDBRecord.open();
        wordDBRecord.DBSearch("KeyWordDB");
        t1.setText(category);
        t2.setText(word);
        wordDBRecord.close();
        }
    }

WordDBHelper(使用 SQLite Open Helper)

    import static android.content.ContentValues.TAG;

    public class WordDBHelper extends SQLiteOpenHelper {
         static final String TABLE_NAME = "KeyWordDB";

    public WordDBHelper(Context context, String name, SQLiteDatabase.CursorFactory factory, int version) {
        super(context, name, factory, version);
        Log.d(TAG, "DataBaseHelper");
    }

    @Override
    public void onCreate(SQLiteDatabase sqLiteDatabase) {
        Log.d(TAG, "Table Create");

        String createQuery = "CREATE TABLE IF NOT EXISTS " + TABLE_NAME +
                "( ID INTEGER PRIMARY KEY AUTOINCREMENT, " +
                "CATEGORY TEXT NOT NULL, " +
                "WORD TEXT NOT NULL);";
        sqLiteDatabase.execSQL(createQuery);
    }

    @Override
    public void onUpgrade(SQLiteDatabase sqLiteDatabase, int i, int i1) {
        Log.d(TAG, "Table onUpgrade");
        String createQuery = "DROP TABLE IF EXISTS " + TABLE_NAME + ";";
        sqLiteDatabase.execSQL(createQuery);
        }
    }

3.WordDBRecord(放记录类)

    import static android.content.ContentValues.TAG;

    public class WordDBRecord {
    public String id;
    public String category;
    public String word;
    private Context context;
    private WordDBHelper dbHelper;
    private SQLiteDatabase database;

    public WordDBRecord(Context c) {
        context = c;
    }

    public WordDBRecord open() throws SQLException {
        dbHelper = new WordDBHelper(context, "DB", null, 1);
        database = dbHelper.getWritableDatabase();

          dbInsert("KeyWordDB", "Movie", "Harry Potter");

        return this;
    }

    public void DBSearch(String tableName) {
        Cursor cursor = null;
        try {
            cursor = database.query(tableName, null, null, null, null, null, null);
            if (cursor != null) {
                while (cursor.moveToNext()) {
                    id = cursor.getString(cursor.getColumnIndex("ID"));
                    category = cursor.getString(cursor.getColumnIndex("CATEGORY"));
                    word = cursor.getString(cursor.getColumnIndex("WORD"));
                }
            }
        } finally {
            if (cursor != null) {
                cursor.close();
            }
        }
    }

    public void close() {
        database.close();
        dbHelper.close();
    }

    public void dbInsert(String tableName, String category, String word) {
        Log.d(TAG, "Insert Data ");

        ContentValues contentValues = new ContentValues();

        contentValues.put("CATEGORY", category);
        contentValues.put("WORD", word);

        long id = database.insert(tableName, null, contentValues);
        Log.d(TAG, "id: " + id);
    }

    }

所以我运行了我的应用程序，但未能打开应用程序。单击我的应用程序时，我的应用程序停止了。我收到了那些 Logcats

    2019-12-15 11:37:59.005 22791-22791/org.techtown.worddbpractice E/AndroidRuntime: FATAL EXCEPTION: main
    Process: org.techtown.worddbpractice, PID: 22791
    java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{org.techtown.worddbpractice/org.techtown.worddbpractice.WordActivity}: java.lang.NullPointerException: Attempt to read from field 'java.lang.String org.techtown.worddbpractice.WordDBRecord.category' on a null object reference
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2849)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3045)
        at android.app.ActivityThread.-wrap14(ActivityThread.java)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1642)
        at android.os.Handler.dispatchMessage(Handler.java:102)
        at android.os.Looper.loop(Looper.java:154)
        at android.app.ActivityThread.main(ActivityThread.java:6776)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1496)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1386)
     Caused by: java.lang.NullPointerException: Attempt to read from field 'java.lang.String org.techtown.worddbpractice.WordDBRecord.category' on a null object reference
        at org.techtown.worddbpractice.WordActivity.<init>(WordActivity.java:17)
        at java.lang.Class.newInstance(Native Method)
        at android.app.Instrumentation.newActivity(Instrumentation.java:1086)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2839)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3045) 
        at android.app.ActivityThread.-wrap14(ActivityThread.java) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1642) 
        at android.os.Handler.dispatchMessage(Handler.java:102) 
        at android.os.Looper.loop(Looper.java:154) 
        at android.app.ActivityThread.main(ActivityThread.java:6776) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1496) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1386)

我真的很想解决这些问题，但我不知道如何... 如果有人回答我的超长问题，我将非常感激。谢谢

最佳答案

说明

您面临的问题是，当您使用 private WordDBRecord wordDBRecord; 这会将变量 wordDBRecord 声明为 WordDBRecord 类型，这不是 primative键入一个 Object。

声明一个不是原始类型的变量会导致该变量为null(即不存在对象的实例(尚未创建对象))。

在上面标题为 默认值 部分的链接中，它包括:-

因此，当遇到下一行 String category = wordDBRecord.category; 时，no object wordDBRecord 而不是 pointer to该对象设置为 null 以表明这一点。因此，当尝试从对象中获取类别时，您会得到空指针异常(您无法从无到有)。

如果遇到下一行 String word = wordDBRecord.word;，将导致相同的问题。

如何修复(空指针异常)

修复很简单，在实例化 wordDBRecord 之前，不要尝试使用 wordDBRecord 来设置类别和词的值。实例化由行 wordDBRecord = new WordDBRecord(this); 完成。

因此，您只想声明 category 和 word 变量。例如

public class WordActivity extends AppCompatActivity {

    private WordDBRecord wordDBRecord; //<<<<< only declares wordDBrecord so it is null
    String category; //<<<<< CHANGED TO ONLY DECLARE category
    String word; //<<<<< CHANGED TO ONLY DECALRE word

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        TextView t1 = (TextView) findViewById(R.id.textViewCategory);
        TextView t2 = (TextView) findViewById(R.id.textViewWord);

        wordDBRecord = new WordDBRecord(this); //<<<<< instantiates wordDBRecord so it is now not null
        wordDBRecord.open();
        wordDBRecord.DBSearch("KeyWordDB");
        t1.setText(category);
        t2.setText(word);
        wordDBRecord.close();
    }
}

但是 - 完成修复

应用修复后，您将不会在 TextView 中看到电影和哈利波特。

这是因为当上面的代码运行时，它声明了类别和单词，但它们从未被设置(它们是空的，但 setText 方法会对此进行检查并绕过空指针异常)。您需要设置值，例如在 wordDBRecord.DBSearch("KeyWordDB"); 行之后使用:-

        category = wordRecordDB.category;
        word = wordRecordDB.word;
        t1.setText(category);
        t2.setText(word);

因为您可以直接从 wordDBRecord 对象中获取值。甚至不需要具有可变类别和单词。因此，您可以使用更紧凑的:-

public class WordActivity extends AppCompatActivity {

    private WordDBRecord wordDBRecord; //<<<<< only declares wordDBrecord so it is null

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        TextView t1 = (TextView) findViewById(R.id.textViewCategory);
        TextView t2 = (TextView) findViewById(R.id.textViewWord);

        wordDBRecord = new WordDBRecord(this); //<<<<< instantiates wordDBRecord so it is now not null
        wordDBRecord.open();
        wordDBRecord.DBSearch("KeyWordDB");
        t1.setText(wordDBRecord.category);
        t2.setText(wordDBRecord.word);
        wordDBRecord.close();
    }
}

使用上面的结果:-

日志包含:-

2019-12-16 10:14:37.773 D/Constraints: DataBaseHelper
2019-12-16 10:14:37.789 D/Constraints: Table Create
2019-12-16 10:14:37.794 D/Constraints: Insert Data 
2019-12-16 10:14:37.798 D/Constraints: id: 1

附加

在类似的主题上，SQliteDatabase 查询方法永远不会返回空的 Cursor。不需要在检索它之前将 Cursor 设置为 null 并在检索它之后检查它是否为 null。

因此，您的 DBSearch 方法可能是:-

public void DBSearch(String tableName) {
    Cursor cursor = database.query(tableName,null,null,null,null,null,null);
    while (cursor.moveToNext()) {
        id = cursor.getString(cursor.getColumnIndex("ID"));
        category = cursor.getString(cursor.getColumnIndex("CATEGORY"));
        word = cursor.getString(cursor.getColumnIndex("WORD"));
    }
    cursor.close();
}

关于android - 如何修复我的 Android SQLite 数据库？，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/59340848/

android - 如何修复我的 Android SQLite 数据库？

说明

如何修复(空指针异常)

但是 - 完成修复

附加

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