Question

I can’t understand why the query ends up throwing a NullPointerException.

My database contains two tables. The first one called TABLE_WORDS contains words; the second one called TABLE_TRANS contains the translations. One word can have many translations.

public static final String CREATE_TABLE_WORDS = "CREATE TABLE " + TABLE_WORDS + 
    " (" + WORDS_ID + " INTEGER PRIMARY KEY, " + WORDS_WORD + 
    " TEXT);";

public static final String CREATE_TABLE_TRANS = "CREATE TABLE " + TABLE_TRANS + 
    " (" + TRANS_ID + " INTEGER PRIMARY KEY, " + 
    TRANS_WORD + " TEXT, " + 
    TRANS_FOREIGN_ID_WORD + " INTEGER, " +
    "FOREIGN KEY (" + TRANS_FOREIGN_ID_WORD + ") REFERENCES " + TABLE_WORDS + "(" + WORDS_ID + "));";

I query translations by a word ID, and an NPE is thrown.

public Cursor queryTransesByWord(int idSend) {

    String query = "SELECT " + DbHelper.TABLE_WORDS + "." + DbHelper.WORDS_WORD + ", " +
        DbHelper.TABLE_TRANS + "." + DbHelper.TRANS_WORD + 
        " FROM " + DbHelper.TABLE_WORDS + 
        " INNER JOIN " + DbHelper.TABLE_TRANS + 
        " ON " + DbHelper.TABLE_WORDS + "." + DbHelper.WORDS_ID + "=" + 
        DbHelper.TABLE_TRANS + "." + DbHelper.TRANS_FOREIGN_ID_WORD + " WHERE " +
        DbHelper.TABLE_WORDS + "." + DbHelper.WORDS_ID + "="+ idSend; 

    Cursor cur = db.rawQuery(query, null); // NullPointer always throws here

    if (cur != null)
        cur.moveToFirst();
    else
        Log.e(Constants.LOG_TAG, Constants.DB_ADAPTER_CLASS_NAME_SEP + "queryTransesByWord cursor is null");
    return cur;
}

Answer 1

Well, given your code:

Cursor cur = db.rawQuery(query, null); // NullPointer always throws here

That suggests that db is null. There’s no other dereferencing operation in that statement – so unless the stack trace actually shows rawQuery in it somewhere, then db is null. You haven’t given any indication of where db is meant to be assigned a non-null value, but that’s the first thing to check…

As an aside, please don’t include values in your SQL like that – it’s a recipe for SQL injection attacks. Use parameterized SQL instead.