App Crashes On Startup Due To java.lang.IllegalArg

问题:

Whenever I start my app, I get a java.lang.IllegalArgumentException: column '_id' does not exist error in my LogCat. I have created the column '_id', but it still throws this. Here is my main .java:

package com.gantt.shoppinglist;

import android.app.Dialog;
import android.app.ListActivity;
import android.database.Cursor;
import android.os.Bundle;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.EditText;
import android.widget.ListView;
import android.widget.SimpleCursorAdapter;

public class ShoppingList extends ListActivity {

    private DataHelper DataHelper;
    /** Called when the activity is first created. */
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
        DataHelper = new DataHelper(this);

        Cursor c = (Cursor) DataHelper.selectAll();
        long id = c.getLong(c.getColumnIndex("_id"));
        startManagingCursor(c);

        ListView lv = (ListView) findViewById(android.R.id.list);

        String[] from = new String[] { com.gantt.shoppinglist.DataHelper.getDatabaseName() };
        int[] to = new int[] { android.R.id.text1 };

        SimpleCursorAdapter adapter = new SimpleCursorAdapter(this,
                android.R.layout.simple_list_item_1, c, from, to);       
        lv.setAdapter(adapter);

        Button button1main = (Button) findViewById(R.id.add);
        button1main.setOnClickListener(new OnClickListener()  {
            @Override
            public void onClick(View v)  {
            final Dialog additem = new Dialog(ShoppingList.this);
            additem.setContentView(R.layout.maindialog);
            final EditText et = (EditText)additem.findViewById(R.id.edittext);
            additem.setTitle("Type your item");
            additem.setCancelable(true);
            et.setHint("Type the name of an item...");

            Button button = (Button) additem.findViewById(R.id.cancel);
            button.setOnClickListener(new OnClickListener()  {
                @Override
                public void onClick(View v)  {
                    additem.dismiss();
                }
            });
            additem.show();

            Button ok = (Button) additem.findViewById(R.id.ok);
            ok.setOnClickListener(new OnClickListener() {
                @Override
                public void onClick(View v) {
                    final String text = et.getText().toString();
                    additem.dismiss();
                    et.setText("");
                }
            });
       }
        });
    }
}

Here is my DataHelper class:

package com.gantt.shoppinglist;

import java.util.ArrayList;
import java.util.List;

import android.content.Context;
import android.database.Cursor;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteOpenHelper;
import android.database.sqlite.SQLiteStatement;
import android.util.Log;

public class DataHelper {

       private static final String DATABASE_NAME = "items.db";
       private static final int DATABASE_VERSION = 1;
       private static final String TABLE_NAME = "table1";
       public static final String KEY_ROWID = "_id";

       private Context context;
       private SQLiteDatabase db;

       private SQLiteStatement insertStmt;
       private static final String INSERT = "insert into " 
          + TABLE_NAME + "(name) values (?)";

       public DataHelper(Context context) {
          this.context = context;
          OpenHelper openHelper = new OpenHelper(this.context);
          this.db = openHelper.getWritableDatabase();
          this.insertStmt = this.db.compileStatement(INSERT);
       }

       public long insert(String name) {
          this.insertStmt.bindString(1, name);
          return this.insertStmt.executeInsert();
       }

       public void deleteAll() {
          this.db.delete(TABLE_NAME, null, null);
       }

       public Cursor selectAll() {
          List<String> list = new ArrayList<String>();
          Cursor cursor = this.db.query(TABLE_NAME, new String[] { "name" }, 
            null, null, null, null, "name desc");
          if (cursor.moveToFirst()) {
             do {
                list.add(cursor.getString(0)); 
             } while (cursor.moveToNext());
          }
          if (cursor != null && !cursor.isClosed()) {
             cursor.close();
          }
          return cursor;
       }

       public static String getDatabaseName() {
        return DATABASE_NAME;
    }

    private static class OpenHelper extends SQLiteOpenHelper {

          OpenHelper(Context context) {
             super(context, getDatabaseName(), null, DATABASE_VERSION);
          }

          @Override
          public void onCreate(SQLiteDatabase db) {
              db.execSQL("CREATE TABLE " + TABLE_NAME + "(id INTEGER PRIMARY KEY, name TEXT");

          }

          @Override
          public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
             Log.w("Example", "Upgrading database, this will drop tables and recreate.");
             db.execSQL("DROP TABLE IF EXISTS " + TABLE_NAME);
             onCreate(db);
          }
       }
    }

回答1:

I had a similar issue - I think it's a case of having to 'select' (or 'select as') something called _id because the SimpleCursorAdapter needs it.

From the documentation:

Handling content URI IDs

By convention, providers offer access to a single row in a table by accepting a content URI with an ID value for the row at the end of the URI. Also by convention, providers match the ID value to the table's _ID column, and perform the requested access against the row that matches.

This convention facilitates a common design pattern for apps accessing a provider. The app does a query against the provider and displays the resulting Cursor in a ListView using a CursorAdapter. The definition of CursorAdapter requires one of the columns in the Cursor to be _ID.

In my case, I have an autonumber column in my table called 'oid' so I altered my SELECT command to be (for example)...

SELECT oid as _id, name, number FROM mytable

This cured the problem for me.

EDIT to show more extensive code...

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.channel_selector);

    GridView channel_selector_grid = (GridView) findViewById(R.id.channel_grid);
    sca = getGuideAdapter();
    channel_selector_grid.setAdapter(sca);
}

public SimpleCursorAdapter getGuideAdapter() {
    SimpleCursorAdapter adapter = null;
    SQLiteDatabase db = SQLiteDatabaseHelper.getReadableDatabase();
    Cursor cursor = db.rawQuery("SELECT DISTINCT oid as _id, name, number FROM CHAN_TABLE ORDER BY number", null);
    if (cursor.moveToFirst()) {
        String[] columnNames = { "name" };
        int[] resIds = { R.id.channel_name };
        adapter = new SimpleCursorAdapter(this, R.layout.channel_selector_item, cursor, columnNames, resIds);
    }
    return adapter; 
}

回答2:

Um not sure if you've just pasted it in wrong but:

 DataHelper = new DataHelper(this);
 Cursor c = (Cursor) DataHelper.selectAll();

is wrong. You need to declare it to an object and call your methods on the initialised DataBase object:

 DataHelper dataHelper = new DataHelper(this);
 Cursor c = (Cursor) dataHelper.selectAll();

Oh my bad you've just declared your variable name with an Uppercase D , this isn't recommended nor standard java coding style.

http://java.about.com/od/javasyntax/a/nameconventions.htm