无法比较数据库中是否存在用户名

Question

我是android新手。我想将数据库中的用户名与活动中输入的值进行比较，并获得NullPointerException（NPE）。

这是我的DBHElper活动

public class DBHelper extends SQLiteOpenHelper 
{
    private SQLiteDatabase db;
    public static final String KEY_ROWID = "_id";
    public static final String KEY_ROWID1="_id";
    public static final String KEY_FNAME = "firstname";
    public static final String KEY_LNAME = "lastname";
    public static final String KEY_GENDER = "gender";
    public static final String KEY_USER = "username";
    public static final String KEY_EMAIL = "email";
    public static final String KEY_CNAME="Cname"; 
    public static final String KEY_CPOST="Cpost"; 
    public static final String KEY_CCRITERIA="Ccriteria"; 
    public static final String KEY_CUSER="Cusername"; 
    public static final String KEY_CEMAIL="Cemail"; 
    public static final String KEY_CCONTACT="Ccontact"; 
    public static final String KEY_CCUTOFF="Ccutoff"; 

    public static final String KEY_STATUS="status";
    public static final String KEY_USERNAME="username";


    DBHelper DB = null;
    private static final String DATABASE_NAME = "Sam2.db";
    private static final int DATABASE_VERSION = 3;
    public static final String DATABASE_TABLE_NAME = "JobSeeker";
    public static final String Company_Table = "Comp";
    public static final String Login_Table = "Login";
    private static final String DATABASE_TABLE_CREATE =
        "CREATE TABLE " + DATABASE_TABLE_NAME + "(" +
        "_id INTEGER PRIMARY KEY AUTOINCREMENT,"+
        "firstname TEXT NOT NULL, lastname TEXT NOT NULL, gender TEXT NOT NULL, username TEXT NOT NULL UNIQUE, password TEXT NOT NULL, email TEXT NOT NULL);";
    private static final String DATABASE_TABLE_CREATE2=
        " create table " + Company_Table +
        " (_id integer primary key autoincrement," +
        " Cname TEXT NOT NULL, Cpost TEXT NOT NULL,Ccriteria TEXT NOT NULL,Cusername TEXT NOT NULL, Cpassword TEXT NOT NULL, Cemail TEXT NOT NULL,Ccontact TEXT NOT NULL,Ccutoff TEXT NOT NULL);" ;
    private static final String DATABASE_TABLE_CREATE3=
        "CREATE TABLE " + Login_Table  + "(" +
        "_id INTEGER PRIMARY KEY AUTOINCREMENT,"+
        "username TEXT NOT NULL UNIQUE, password TEXT NOT NULL, status TEXT NOT NULL);";

    public DBHelper(Context context) {
      super(context, DATABASE_NAME, null, DATABASE_VERSION);
      System.out.println("In constructor");
    }

    @Override
    public void onCreate(SQLiteDatabase db) {
        try{
            db.execSQL(DATABASE_TABLE_CREATE);
            db.execSQL(DATABASE_TABLE_CREATE2);
            db.execSQL(DATABASE_TABLE_CREATE3);
        }catch(Exception e){
            e.printStackTrace();
        }
    }

    @Override
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
        // TODO Auto-generated method stub
    }

    public Cursor rawQuery(String string, String[] strings) {
        // TODO Auto-generated method stub
        return null;
    }

    public void open() {
        getWritableDatabase(); 
    }

    public Cursor getDetails(String text) throws SQLException 
    {
        Cursor mCursor =
                db.query(true, DATABASE_TABLE_NAME, 
                  new String[]{KEY_ROWID, KEY_FNAME, KEY_LNAME, KEY_GENDER, KEY_USER, KEY_EMAIL}, 
                  KEY_USER + "=" + text, 
                  null, null, null, null, null);
        if (mCursor != null) 
        {
            mCursor.moveToFirst();
        }
        return mCursor;
    }

    public Cursor getCompDetails(String text) throws SQLException 
    {

        Cursor mCursor =
                db.query(true, Company_Table, 
                  new String[]{KEY_ROWID1, KEY_CNAME, KEY_CPOST, KEY_CCRITERIA, KEY_CUSER, KEY_EMAIL,KEY_CCONTACT,KEY_CCUTOFF}, 
                  KEY_CUSER + "=" + text, 
                  null, null, null, null, null);
        if (mCursor != null) 
        {
            mCursor.moveToFirst();
        }
        return mCursor;
    }

    public Cursor getLoginDetails(String text) throws SQLException 
    {
        Cursor mCursor =
                db.query(true, Login_Table, 
                  new String[]{KEY_ROWID1, KEY_USERNAME, KEY_STATUS}, 
                  KEY_USERNAME + "=" + text, 
              null, null, null, null, null);
        if (mCursor != null) 
        {
            mCursor.moveToFirst();
        }
        return mCursor;
    }

    public Boolean checkusername(String name) throws SQLException {
        Cursor mCursor = db.query(
                  Login_Table, 
                  new String[] { KEY_ROWID, KEY_USERNAME }, 
                  KEY_USERNAME + "=?", 
                  new String[] {name}, 
                  null, 
                  null, 
                  null);
        if (mCursor!=null) {
            mCursor.moveToFirst();
            return true;
        }
        return false;
    }
}

以下是执行比较的活动。

DB.open();
if (DB.checkusername(uname)) {
    invalid = true;
    Toast.makeText(getApplicationContext(), 
          "username alredy in use enter another", 
          Toast.LENGTH_SHORT).show();
}

此行显示错误

   Cursor mCursor = db.query(
              Login_Table, 
              new String[] { KEY_ROWID, KEY_USERNAME }, 
              KEY_USERNAME + "=" + name, 
              null, 
              null, 
              null, 
              null);

这里是logcat

07-07 23:52:56.813: W/dalvikvm(536): threadid=1: thread exiting with uncaught exception (group=0x4001d800)
07-07 23:52:56.823: E/AndroidRuntime(536): FATAL EXCEPTION: main
07-07 23:52:56.823: E/AndroidRuntime(536): java.lang.NullPointerException
07-07 23:52:56.823: E/AndroidRuntime(536):  at com.google.data.DBHelper.checkusername(DBHelper.java:164)
07-07 23:52:56.823: E/AndroidRuntime(536):  at com.google.data.Registration.onClick(Registration.java:211)
07-07 23:52:56.823: E/AndroidRuntime(536):  at android.view.View.performClick(View.java:2408)
07-07 23:52:56.823: E/AndroidRuntime(536):  at android.view.View$PerformClick.run(View.java:8816)
07-07 23:52:56.823: E/AndroidRuntime(536):  at android.os.Handler.handleCallback(Handler.java:587)
07-07 23:52:56.823: E/AndroidRuntime(536):  at android.os.Handler.dispatchMessage(Handler.java:92)
07-07 23:52:56.823: E/AndroidRuntime(536):  at android.os.Looper.loop(Looper.java:123)
07-07 23:52:56.823: E/AndroidRuntime(536):  at android.app.ActivityThread.main(ActivityThread.java:4627)
07-07 23:52:56.823: E/AndroidRuntime(536):  at java.lang.reflect.Method.invokeNative(Native Method)
07-07 23:52:56.823: E/AndroidRuntime(536):  at java.lang.reflect.Method.invoke(Method.java:521)
07-07 23:52:56.823: E/AndroidRuntime(536):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:868)
07-07 23:52:56.823: E/AndroidRuntime(536):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:626)
07-07 23:52:56.823: E/AndroidRuntime(536):  at dalvik.system.NativeStart.main(Native Method)

Answer 1

用户名的类型为字符串，因此您需要将其与' name '

进行比较

试试这个：

db.rawQuery("SELECT * FROM "+ Login_Table + " WHERE "+ 
KEY_USERNAME + "='" + name + "'";

修改 提供完整代码后。

rawQuery应该可以工作，但是db（SQLiteDatabase）没有被初始化，所以它总是保持为NULL。

Answer 2

尝试此操作，而不是使用=，请使用LIKE

Cursor mCursor = db.query(
              Login_Table, 
              new String[] { KEY_ROWID, KEY_USERNAME }, 
              KEY_USERNAME + " LIKE '" + name + "'", 
              null, 
              null, 
              null, 
              null);

编辑：实际上，我的人工编译器失败了，是db在方法checkusername中实例化了吗？如果不是，则db为空，因此在尝试调用query时会发生NPE。

编辑＃2：是的，现在OP提供了更多信息，但事实证明变量的重要实例化已经丢失，例如：

db = new DBHelper(getApplicationContext());

然后它应该没有任何NPE错误。

Answer 3

编译字符串时使用查询参数：

Cursor mCursor = db.query(
          Login_Table, 
          new String[] { KEY_ROWID, KEY_USERNAME }, 
          KEY_USERNAME + "=?", 
          new String[] {name}, 
          null, 
          null, 
          null);