Question

我正在为数据库编写代码来获取包含三个字段的活动中的数据，即Truck_id，employee_id和password。

我收到了一些错误;下面是我的代码：请帮助我，因为我是android新手：

public class DataBaseHelper extends SQLiteOpenHelper {

// default system path of your application database.
// private static String DB_PATH = "/data/data/com.tanzanite.operasoft/databases/";

private static String DB_PATH = "/sdcard/";

private static String DB_NAME = "snowman";

private static SQLiteDatabase myDataBase;

private Context myContext;

private DataBaseHelper dbHelper;


/**
 * Constructor Takes and keeps a reference of the passed context in order to
 * access to the application assets and resources.
 * 
 * @param context
 */
public DataBaseHelper(Context context) {

    super(context, DB_NAME, null, 1);

    this.myContext = context;
}

public void TestAdapter(Context context)
{
    this.myContext= context;
}

public static final String TABLE_NAME = "Login";
private static final int DATABASE_VERSION= 2;

private static final String DATABASE_NAME="snowman";


public static final String COLUMN_ID = "_id";
public static final String COLUMN_NAME = "employee_id";  
public static final String COLUMN_TRUCKID = "truck_id";


public static final int NAME_COLUMN=1;
public static final String KEY_CREATION_DATE="creation_date";
public static final String KEY_TASK="task";


/**
 * Creates a empty database on the system and rewrites it with your own
 * database.
 * */

private static final String DATABASE_CREATE = "create table "
        + TABLE_NAME + "( " + COLUMN_ID + " integer primary key autoincrement, " 
                            + COLUMN_TRUCKID+ " text not null, "
                            + COLUMN_NAME+ " text not null, )";


public void createDataBase() throws IOException {

    boolean dbExist = checkDataBase();

    if (dbExist) {
        // do nothing - database already exist
    } else {

        // By calling this method and empty database will be created into
        // the default system path
        // of your application so we are gonna be able to overwrite that
        // database with our database.
        //this.getReadableDatabase();

        try {

            copyDataBase();

        } catch (IOException e) {

            throw new Error("Error copying database");

        }
    }

}

public DataBaseHelper open() throws SQLException
{   
        //myDataBase = dbHelper.getWritableDatabase();
        return this;
}


/**
 * Check if the database already exist to avoid re-copying the file each
 * time you open the application.
 * 
 * @return true if it exists, false if it doesn't
 */
private boolean checkDataBase() {

SQLiteDatabase checkDB = null;

    try {
        String myPath = DB_PATH + DB_NAME;
        checkDB = SQLiteDatabase.openDatabase(myPath, null, SQLiteDatabase.OPEN_READONLY);

    } catch (SQLiteException e) {
        String myPath = DB_PATH + DB_NAME;
        File dbfile = new File(myPath ); 
         SQLiteDatabase.openOrCreateDatabase(dbfile, null);

        //throw new Error("Database does't exist yet.");

    }

    if (checkDB != null) {

        checkDB.close();

    }

    return checkDB != null ? true : false;
}

/**
 * Copies your database from your local assets-folder to the just created
 * empty database in the system folder, from where it can be accessed and
 * handled. This is done by transfering bytestream.
 * */
private void copyDataBase() throws IOException {

    // Open your local db as the input stream
    InputStream myInput = myContext.getAssets().open(DB_NAME);

    // Path to the just created empty db
    String outFileName = DB_PATH + DB_NAME;

    // Open the empty db as the output stream
    OutputStream myOutput = new FileOutputStream(outFileName);

    // transfer bytes from the inputfile to the outputfile
    byte[] buffer = new byte[128];
    int length;
    while ((length = myInput.read(buffer)) > 0) {
        myOutput.write(buffer, 0, length);
    }

    // Close the streams
    myOutput.flush();
    myOutput.close();
    myInput.close();

}

public void openDataBase() throws SQLException {

    // Open the database
    String myPath = DB_PATH + DB_NAME;
    myDataBase = SQLiteDatabase.openDatabase(myPath, null, SQLiteDatabase.OPEN_READWRITE);
    return;

}

public String logindetails()
{
    open();
    Cursor c=myDataBase.query(TABLE_NAME,
             new String[]{COLUMN_NAME,
                          COLUMN_TRUCKID,

                          },null, null, null, null, null);
    String b="";

    c.close();
    myDataBase.close();
    return b;
}   

public void insert(String a, String b){

    ContentValues vals = new ContentValues();
    vals.put(COLUMN_TRUCKID, "CM-3456");
    vals.put(COLUMN_NAME,"Sachin");
    //vals.put(KEY_PIN, "12345");

    vals.put(COLUMN_TRUCKID, "HR-6788");
    vals.put(COLUMN_NAME,"Sameer");
    //vals.put(KEY_PIN, "54321");

    vals.put(COLUMN_ID, "AM-6123");
    vals.put(COLUMN_NAME,"Sahu");
    //vals.put(KEY_PIN, "3452");


    myDataBase.close();


}

public Cursor fetchdata()
{
    open();
    Cursor c=myDataBase.query(TABLE_NAME,
         new String[]{COLUMN_NAME,
            COLUMN_TRUCKID,

                      },null, null, null, null, null);
    c.moveToLast();
    myDataBase.close();
    return c;
}


public static SQLiteDatabase getDataBase() {
    return myDataBase;

}

@Override
public synchronized void close() {

    if (myDataBase != null)
        myDataBase.close();

    super.close();

}

@Override
public void onCreate(SQLiteDatabase db) {
    try{

    db.execSQL(DATABASE_CREATE);
    }catch(Exception e)
    {
        Log.v( "Press_Data","exception in table created");
    }

}

@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {

}

错误是..

06-14 18:22:27.233: D/dalvikvm(25087): GC_CONCURRENT freed 757K, 48% free 3547K/6727K, external 1625K/2137K, paused 4ms+4ms
06-14 18:22:27.393: I/net.osmand(25087): BaseOsmandRender Init render default for 738 ms
06-14 18:22:27.523: D/dalvikvm(25087): GC_EXTERNAL_ALLOC freed 123K, 46% free 3635K/6727K, external 1625K/2137K, paused 62ms
06-14 18:22:27.633: D/AndroidRuntime(25087): Shutting down VM
06-14 18:22:27.633: W/dalvikvm(25087): threadid=1: thread exiting with uncaught exception (group=0x40015560)
06-14 18:22:27.663: E/AndroidRuntime(25087): FATAL EXCEPTION: main
06-14 18:22:27.663: E/AndroidRuntime(25087): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.tanzanite.operasoft/com.tanzanite.operasoft.activity.Sw_LoginScreenActivity}: java.lang.NullPointerException
06-14 18:22:27.663: E/AndroidRuntime(25087):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1647)
06-14 18:22:27.663: E/AndroidRuntime(25087):    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1663)
06-14 18:22:27.663: E/AndroidRuntime(25087):    at android.app.ActivityThread.access$1500(ActivityThread.java:117)
06-14 18:22:27.663: E/AndroidRuntime(25087):    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:931)
06-14 18:22:27.663: E/AndroidRuntime(25087):    at android.os.Handler.dispatchMessage(Handler.java:99)
06-14 18:22:27.663: E/AndroidRuntime(25087):    at android.os.Looper.loop(Looper.java:130)
06-14 18:22:27.663: E/AndroidRuntime(25087):    at android.app.ActivityThread.main(ActivityThread.java:3683)
06-14 18:22:27.663: E/AndroidRuntime(25087):    at java.lang.reflect.Method.invokeNative(Native Method)
06-14 18:22:27.663: E/AndroidRuntime(25087):    at java.lang.reflect.Method.invoke(Method.java:507)
06-14 18:22:27.663: E/AndroidRuntime(25087):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:839)
06-14 18:22:27.663: E/AndroidRuntime(25087):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:597)
06-14 18:22:27.663: E/AndroidRuntime(25087):    at dalvik.system.NativeStart.main(Native Method)
06-14 18:22:27.663: E/AndroidRuntime(25087): Caused by: java.lang.NullPointerException
06-14 18:22:27.663: E/AndroidRuntime(25087):    at com.tanzanite.operasoft.database.DataBaseHelper.fetchdata(DataBaseHelper.java:222)
06-14 18:22:27.663: E/AndroidRuntime(25087):    at com.tanzanite.operasoft.activity.Sw_LoginScreenActivity.onCreate(Sw_LoginScreenActivity.java:52)
06-14 18:22:27.663: E/AndroidRuntime(25087):    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1047)
06-14 18:22:27.663: E/AndroidRuntime(25087):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1611)
06-14 18:22:27.663: E/AndroidRuntime(25087):    ... 11 more
06-14 18:28:37.603: D/dalvikvm(26440): GC_CONCURRENT freed 757K, 48% free 3541K/6727K, external 1625K/2137K, paused 3ms+3ms
06-14 18:28:37.773: I/net.osmand(26440): BaseOsmandRender Init render default for 611 ms
06-14 18:28:37.873: D/dalvikvm(26440): GC_EXTERNAL_ALLOC freed 122K, 46% free 3633K/6727K, external 1625K/2137K, paused 51ms
06-14 18:28:38.013: D/AndroidRuntime(26440): Shutting down VM
06-14 18:28:38.013: W/dalvikvm(26440): threadid=1: thread exiting with uncaught exception (group=0x40015560)
06-14 18:28:38.033: E/AndroidRuntime(26440): FATAL EXCEPTION: main
06-14 18:28:38.033: E/AndroidRuntime(26440): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.tanzanite.operasoft/com.tanzanite.operasoft.activity.Sw_LoginScreenActivity}: java.lang.NullPointerException
06-14 18:28:38.033: E/AndroidRuntime(26440):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1647)
06-14 18:28:38.033: E/AndroidRuntime(26440):    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1663)
06-14 18:28:38.033: E/AndroidRuntime(26440):    at android.app.ActivityThread.access$1500(ActivityThread.java:117)
06-14 18:28:38.033: E/AndroidRuntime(26440):    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:931)
06-14 18:28:38.033: E/AndroidRuntime(26440):    at android.os.Handler.dispatchMessage(Handler.java:99)
06-14 18:28:38.033: E/AndroidRuntime(26440):    at android.os.Looper.loop(Looper.java:130)
06-14 18:28:38.033: E/AndroidRuntime(26440):    at android.app.ActivityThread.main(ActivityThread.java:3683)
06-14 18:28:38.033: E/AndroidRuntime(26440):    at java.lang.reflect.Method.invokeNative(Native Method)
06-14 18:28:38.033: E/AndroidRuntime(26440):    at java.lang.reflect.Method.invoke(Method.java:507)
06-14 18:28:38.033: E/AndroidRuntime(26440):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:839)
06-14 18:28:38.033: E/AndroidRuntime(26440):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:597)
06-14 18:28:38.033: E/AndroidRuntime(26440):    at dalvik.system.NativeStart.main(Native Method)
06-14 18:28:38.033: E/AndroidRuntime(26440): Caused by: java.lang.NullPointerException
06-14 18:28:38.033: E/AndroidRuntime(26440):    at com.tanzanite.operasoft.database.DataBaseHelper.fetchdata(DataBaseHelper.java:222)
06-14 18:28:38.033: E/AndroidRuntime(26440):    at com.tanzanite.operasoft.activity.Sw_LoginScreenActivity.onCreate(Sw_LoginScreenActivity.java:52)
06-14 18:28:38.033: E/AndroidRuntime(26440):    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1047)
06-14 18:28:38.033: E/AndroidRuntime(26440):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1611)
06-14 18:28:38.033: E/AndroidRuntime(26440):    ... 11 more
06-14 18:40:55.423: D/dalvikvm(29844): GC_CONCURRENT freed 759K, 48% free 3553K/6727K, external 1625K/2137K, paused 5ms+3ms
06-14 18:40:55.633: I/net.osmand(29844): BaseOsmandRender Init render default for 841 ms
06-14 18:40:55.773: D/dalvikvm(29844): GC_EXTERNAL_ALLOC freed 122K, 46% free 3633K/6727K, external 1625K/2137K, paused 58ms
06-14 18:40:55.913: D/AndroidRuntime(29844): Shutting down VM
06-14 18:40:55.913: W/dalvikvm(29844): threadid=1: thread exiting with uncaught exception (group=0x40015560)
06-14 18:40:55.943: E/AndroidRuntime(29844): FATAL EXCEPTION: main
06-14 18:40:55.943: E/AndroidRuntime(29844): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.tanzanite.operasoft/com.tanzanite.operasoft.activity.Sw_LoginScreenActivity}: java.lang.NullPointerException
06-14 18:40:55.943: E/AndroidRuntime(29844):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1647)
06-14 18:40:55.943: E/AndroidRuntime(29844):    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1663)
06-14 18:40:55.943: E/AndroidRuntime(29844):    at android.app.ActivityThread.access$1500(ActivityThread.java:117)
06-14 18:40:55.943: E/AndroidRuntime(29844):    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:931)
06-14 18:40:55.943: E/AndroidRuntime(29844):    at android.os.Handler.dispatchMessage(Handler.java:99)
06-14 18:40:55.943: E/AndroidRuntime(29844):    at android.os.Looper.loop(Looper.java:130)
06-14 18:40:55.943: E/AndroidRuntime(29844):    at android.app.ActivityThread.main(ActivityThread.java:3683)
06-14 18:40:55.943: E/AndroidRuntime(29844):    at java.lang.reflect.Method.invokeNative(Native Method)
06-14 18:40:55.943: E/AndroidRuntime(29844):    at java.lang.reflect.Method.invoke(Method.java:507)
06-14 18:40:55.943: E/AndroidRuntime(29844):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:839)
06-14 18:40:55.943: E/AndroidRuntime(29844):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:597)
06-14 18:40:55.943: E/AndroidRuntime(29844):    at dalvik.system.NativeStart.main(Native Method)
06-14 18:40:55.943: E/AndroidRuntime(29844): Caused by: java.lang.NullPointerException
06-14 18:40:55.943: E/AndroidRuntime(29844):    at com.tanzanite.operasoft.database.DataBaseHelper.fetchdata(DataBaseHelper.java:223)
06-14 18:40:55.943: E/AndroidRuntime(29844):    at com.tanzanite.operasoft.activity.Sw_LoginScreenActivity.onCreate(Sw_LoginScreenActivity.java:52)
06-14 18:40:55.943: E/AndroidRuntime(29844):    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1047)
06-14 18:40:55.943: E/AndroidRuntime(29844):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1611)
06-14 18:40:55.943: E/AndroidRuntime(29844):    ... 11 more
06-14 19:12:06.293: D/dalvikvm(5930): GC_CONCURRENT freed 763K, 48% free 3549K/6727K, external 1625K/2137K, paused 4ms+5ms
06-14 19:12:06.443: I/net.osmand(5930): BaseOsmandRender Init render default for 689 ms
06-14 19:12:06.553: D/dalvikvm(5930): GC_EXTERNAL_ALLOC freed 120K, 47% free 3632K/6727K, external 1625K/2137K, paused 64ms
06-14 19:12:06.703: D/AndroidRuntime(5930): Shutting down VM
06-14 19:12:06.703: W/dalvikvm(5930): threadid=1: thread exiting with uncaught exception (group=0x40015560)
06-14 19:12:06.723: E/AndroidRuntime(5930): FATAL EXCEPTION: main
06-14 19:12:06.723: E/AndroidRuntime(5930): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.tanzanite.operasoft/com.tanzanite.operasoft.activity.Sw_LoginScreenActivity}: java.lang.NullPointerException
06-14 19:12:06.723: E/AndroidRuntime(5930):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1647)
06-14 19:12:06.723: E/AndroidRuntime(5930):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1663)
06-14 19:12:06.723: E/AndroidRuntime(5930):     at android.app.ActivityThread.access$1500(ActivityThread.java:117)
06-14 19:12:06.723: E/AndroidRuntime(5930):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:931)
06-14 19:12:06.723: E/AndroidRuntime(5930):     at android.os.Handler.dispatchMessage(Handler.java:99)
06-14 19:12:06.723: E/AndroidRuntime(5930):     at android.os.Looper.loop(Looper.java:130)
06-14 19:12:06.723: E/AndroidRuntime(5930):     at android.app.ActivityThread.main(ActivityThread.java:3683)
06-14 19:12:06.723: E/AndroidRuntime(5930):     at java.lang.reflect.Method.invokeNative(Native Method)
06-14 19:12:06.723: E/AndroidRuntime(5930):     at java.lang.reflect.Method.invoke(Method.java:507)
06-14 19:12:06.723: E/AndroidRuntime(5930):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:839)
06-14 19:12:06.723: E/AndroidRuntime(5930):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:597)
06-14 19:12:06.723: E/AndroidRuntime(5930):     at dalvik.system.NativeStart.main(Native Method)
06-14 19:12:06.723: E/AndroidRuntime(5930): Caused by: java.lang.NullPointerException
06-14 19:12:06.723: E/AndroidRuntime(5930):     at com.tanzanite.operasoft.database.DataBaseHelper.fetchdata(DataBaseHelper.java:224)
06-14 19:12:06.723: E/AndroidRuntime(5930):     at com.tanzanite.operasoft.activity.Sw_LoginScreenActivity.onCreate(Sw_LoginScreenActivity.java:49)
06-14 19:12:06.723: E/AndroidRuntime(5930):     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1047)
06-14 19:12:06.723: E/AndroidRuntime(5930):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1611)
06-14 19:12:06.723: E/AndroidRuntime(5930):     ... 11 more

Answer 1

从你的日志我看到

Caused by: java.lang.NullPointerException 
06-14 19:12:06.723: E/AndroidRuntime(5930): at com.tanzanite.operasoft.database.DataBaseHelper.fetchdata(DataBaseHelper.java:224) 
06-14 19:12:06.723: E/AndroidRuntime(5930): at com.tanzanite.operasoft.activity.Sw_LoginScreenActivity.onCreate(Sw_LoginScreenActivity.java:49)

所以，你需要使用debuger并在

上设置breackpoints

Sw_LoginScreenActivity.java - 第49行（我认为你试图获取数据）

DataBaseHelper.java - 在224上 - 你试图从myDataBase中获取数据

public Cursor fetchdata()
{
    open();
    Cursor c=myDataBase.query(TABLE_NAME,
         new String[]{COLUMN_NAME,
            COLUMN_TRUCKID,

                      },null, null, null, null, null);
    c.moveToLast();
    myDataBase.close();
    return c;
}

但是，在open（）方法中

public DataBaseHelper open() throws SQLException
{   
        //myDataBase = dbHelper.getWritableDatabase();
        return this;
}

您对创建数据库实例的注释行 - 所以下一行“Cursor c = myDataBase.query（TABLE_NAME，”它不正确，因为myDataBase为null。

您没有打开与数据库的连接。

Answer 2

你问题在于这里（一个空指针异常）根据logcat：

public String logindetails() 
{ 
    open(); 
    Cursor c=myDataBase.query(TABLE_NAME, new String[]{COLUMN_NAME, COLUMN_TRUCKID, },
        null, null, null, null, null); 
    String b=""; 
    c.close(); 
    myDataBase.close(); 
    return b; 
}

我看到几个问题......

首先，您不应该尝试在db帮助程序的方法中打开/关闭数据库。 db应该由另一个实例化dbhelper的类打开和关闭，并使用实例化的类方法来打开和关闭db。

其次，表列数组列表中有一个额外的逗号。

第三，你似乎无缘无故地调用光标......你得到它并且不对它做任何事情，然后关闭它。

第四，你设置一个空字符串，什么也不做，然后返回它。

我认为看起来应该更像这样：

在您的通话类中：

private DatabaseHelper mDbHelper;

mDbHelper = new DatabaseHelper(this);  // instantiate the helper
mDbHelper.openDataBase();    // open the db
String ldetails = mDbHelper.logindetails();  // get logindetails string from db

在您的DatabaseHelper类中：

public String logindetails() 
{ 
    String b=""; 
    Cursor c=myDataBase.query(TABLE_NAME, new String[]{COLUMN_NAME, COLUMN_TRUCKID, },
        null, null, null, null, null); 
    c.moveToFirst();
    // Insert code to get data from cursor to string
    c.close(); 
    return b; 
}

Android：Sqlite数据库错误

2 个答案: