android - 为什么我在 this.getWritableDatabase(); 上出现空指针异常，android？

Question

异常发生在 SQLiteDatabase db = this.getWritableDatabase();提供我的数据库处理程序类

public class DatabaseHandler extends SQLiteOpenHelper {

    // All Static variables
    // Database Version
    private static final int DATABASE_VERSION = 1;


    // Database Name
    private static final String DATABASE_NAME = "chat";

    // Contacts table name
    private static final String TABLE_CHAT = "chat_history";

    // Contacts Table Columns names
    private static final String KEY_ID = "id";
    private static final String KEY_TT = "tt";
    private static final String KEY_TYPE = "type";
    private static final String KEY_MSG = "message";

    public DatabaseHandler(Context context) {
        super(context, DATABASE_NAME, null, DATABASE_VERSION);
    }

    // Creating Tables
    @Override
    public void onCreate(SQLiteDatabase db) {
        String CREATE_CHAT_TABLE = "CREATE TABLE " + TABLE_CHAT + "("
            + KEY_ID + " INTEGER PRIMARY KEY," + KEY_TT + " TEXT," + KEY_TYPE + " TEXT,"
            + KEY_MSG + " TEXT" + ")";
        db.execSQL(CREATE_CHAT_TABLE);
    }

    // Upgrading database
    @Override
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
        // Drop older table if existed
        db.execSQL("DROP TABLE IF EXISTS " + TABLE_CHAT);

        // Create tables again
        onCreate(db);
    }

    // Adding new contact

    public void addContact(String tt, String type, String msg) {
       SQLiteDatabase db = this.getWritableDatabase();
        System.out.println("tt "+tt);
        System.out.println("msg "+msg);
        System.out.println("type "+type);
        ContentValues values = new ContentValues();
        values.put(KEY_TT, tt); // Contact Name
        values.put(KEY_TYPE, type); // Contact Phone Number
        values.put(KEY_MSG, msg);

        // Inserting Row
        db.insert(TABLE_CHAT, null, values);
        db.close(); // Closing database connection
    }

    // Getting All Contacts
    public List<ChatHistory> getAllContacts(String T) {
        List<ChatHistory> contactList = new ArrayList<ChatHistory>();
        // Select All Query
        System.out.println("database T$$$$$$$$$$$"+T);

        String selectQuery = "SELECT  * FROM " + TABLE_CHAT + " WHERE "+ KEY_TT+"="+"'"+T+"'";

        SQLiteDatabase db = this.getWritableDatabase();
        Cursor cursor = db.rawQuery(selectQuery, null);

        // looping through all rows and adding to list
        if (cursor.moveToFirst()) {
            do {
                ChatHistory chatHistory = new ChatHistory();
            chatHistory.setID(Integer.parseInt(cursor.getString(0)));
                chatHistory.settt(cursor.getString(1));
            chatHistory.settype(cursor.getString(2));
            chatHistory.setmsg(cursor.getString(3));
                // Adding contact to list
                contactList.add(chatHistory);
            } while (cursor.moveToNext());
        }

        // return contact list
        return contactList;
    }
}

我一直在这条线上遇到异常 SQLiteDatabase db = this.getWritableDatabase();

它的空指针异常..我无法解决这个问题。请提出解决方案这是上下文的一些错误如何解决？

Answer 1

我认为您的数据库尚未创建，因为您正在执行错误的查询。你忘了在末尾添加分号。

你正在做的是:

String CREATE_CHAT_TABLE = "CREATE TABLE " + TABLE_CHAT + "("
        + KEY_ID + " INTEGER PRIMARY KEY," + KEY_TT + " TEXT," + KEY_TYPE + " TEXT,"
        + KEY_MSG + " TEXT" + ")";

改用这个

String CREATE_CHAT_TABLE = "CREATE TABLE " + TABLE_CHAT + "("
        + KEY_ID + " INTEGER PRIMARY KEY," + KEY_TT + " TEXT," + KEY_TYPE + " TEXT,"
        + KEY_MSG + " TEXT" + ");";

然后卸载您的应用并重试。