问ListView在安卓应用程序中没有显示任何东西

EN

package com.gobtron.database_test;

import android.database.Cursor;
import android.database.DatabaseUtils;
import android.support.v7.app.ActionBarActivity;
import android.os.Bundle;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.widget.ListView;
import android.widget.SimpleCursorAdapter;


public class MainActivity extends ActionBarActivity {

    DBAdapter myDb;



    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        openDB();
        populateListView();
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.menu_main, menu);
        return true;
    }

    private void openDB() {
        myDb = new DBAdapter(this);
        myDb.open();
    }

    public void onClick_ViewData (View v){
        openDB();
        populateListView();
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        int id = item.getItemId();

        //noinspection SimplifiableIfStatement
        if (id == R.id.action_settings) {
            return true;
        }

        return super.onOptionsItemSelected(item);
    }


    private void populateListView() {
        Cursor cursor = myDb.getAllRows();
       // DatabaseUtils.dumpCursor(cursor);
        String[] fromFieldNames = new String[] {DBAdapter.KEY_ROWID, DBAdapter.KEY_NOM};
        int[] toViewIDs = new int[] {R.id.textView2, R.id.textView3};
        SimpleCursorAdapter myCursorAdapter;
        myCursorAdapter = new SimpleCursorAdapter(getBaseContext(), R.layout.item_layout, cursor, fromFieldNames, toViewIDs, cursor.getCount());
        ListView myList = (ListView) findViewById(R.id.listView);
        myList.setAdapter(myCursorAdapter);

    }
}

package com.gobtron.database_test;

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

public class DBAdapter {

    private static final String TAG = "DBAdapter"; //used for logging database version changes

    // Field Names:
    public static final String KEY_ROWID = "_id";
    public static final String KEY_MORPH = "'morph'";
    public static final String KEY_LONGFRONDE = "'long_fronde'";
    public static final String KEY_FORMELIMBE = "'forme_limbe'";
    public static final String KEY_DISPOSFRONDE = "'dispos_fronde'";
    public static final String KEY_DESC = "'description'";
    public static final String KEY_DESCHOIX = "'desc_choix'";
    public static final String KEY_DESCHOIX2 = "'desc_choix2'";
    public static final String KEY_NOM = "'nom'";


    public static final String[] ALL_KEYS ={KEY_ROWID, KEY_MORPH, KEY_LONGFRONDE, KEY_FORMELIMBE, KEY_DISPOSFRONDE, KEY_DESC, KEY_DESCHOIX, KEY_DESCHOIX2, KEY_NOM};
    public static final String[] ALL_KEYS2 = {"_id", "'long_fronde'"};

    // Column Numbers for each Field Name:
    public static final int COL_ROWID = 0;
    public static final int COL_MORPH = 1;
    public static final int COL_LONGFRONDE = 2;
    public static final int COL_FORMELIMBE = 3;
    public static final int COL_DISPOSFRONDE = 4;
    public static final int COL_DESC = 5;
    public static final int COL_DESCHOIX = 6;
    public static final int COL_DESCHOIX2 = 7;
    public static final int COL_NOM = 8;

    // DataBase info:
    public static final String DATABASE_NAME = "fougeres_db";
    public static final String DATABASE_TABLE = "mono_dimo";
    public static final int DATABASE_VERSION = 3; // The version number must be incremented each time a change to DB structure occurs.

    //SQL statement to create database
    private static final String DATABASE_CREATE_SQL =
            "CREATE TABLE " + DATABASE_TABLE
            + " (" + KEY_ROWID + " INTEGER PRIMARY KEY AUTOINCREMENT, "
            + KEY_NOM + " TEXT NOT NULL, "
            + KEY_DESC + " TEXT"
            + ");";

    private final Context context;
    private DatabaseHelper myDBHelper;
    private SQLiteDatabase db;


    public DBAdapter(Context ctx) {
        this.context = ctx;
        myDBHelper = new DatabaseHelper(context);
    }

    // Open the database connection.
    public DBAdapter open() {

        db = myDBHelper.getWritableDatabase();
        return this;
    }

    // Close the database connection.
    public void close() {
        myDBHelper.close();
    }

    // Add a new set of values to be inserted into the database.
    public long insertRow(String task, String date) {
        ContentValues initialValues = new ContentValues();
        initialValues.put(KEY_NOM, task);
        initialValues.put(KEY_MORPH, date);

        // Insert the data into the database.
        return db.insert(DATABASE_TABLE, null, initialValues);
    }

    // Delete a row from the database, by rowId (primary key)
    public boolean deleteRow(long rowId) {
        String where = KEY_ROWID + "=" + rowId;
        return db.delete(DATABASE_TABLE, where, null) != 0;
    }

    public void deleteAll() {
        Cursor c = getAllRows();
        long rowId = c.getColumnIndexOrThrow(KEY_ROWID);
        if (c.moveToFirst()) {
            do {
                deleteRow(c.getLong((int) rowId));              
            } while (c.moveToNext());
        }
        c.close();
    }

    // Return all data in the database.
    public Cursor getAllRows() {
        String where = null;
        Cursor c =  db.query(DATABASE_TABLE, ALL_KEYS, where, null, null, null, null);
        DatabaseUtils.dumpCursor(c);
        if (c != null) {
            c.moveToFirst();
        }
        return c;

    }

    // Get a specific row (by rowId)
    public Cursor getRow(long rowId) {
        String where = KEY_ROWID + "=" + rowId;
        Cursor c =  db.query(true, DATABASE_TABLE, ALL_KEYS, 
                        where, null, null, null, null, null);
        if (c != null) {
            c.moveToFirst();
        }
        return c;
    }

    // Change an existing row to be equal to new data.
    public boolean updateRow(long rowId, String task, String date) {
        String where = KEY_ROWID + "=" + rowId;
        ContentValues newValues = new ContentValues();
        newValues.put(KEY_NOM, task);
        newValues.put(KEY_MORPH, date);
        // Insert it into the database.
        return db.update(DATABASE_TABLE, newValues, where, null) != 0;
    }


    private static class DatabaseHelper extends SQLiteOpenHelper
    {
        DatabaseHelper(Context context) {
            super(context, DATABASE_NAME, null, DATABASE_VERSION);
        }

        @Override
        public void onCreate(SQLiteDatabase _db) {
            _db.execSQL(DATABASE_CREATE_SQL);
        }

        @Override
        public void onUpgrade(SQLiteDatabase _db, int oldVersion, int newVersion) {
            Log.w(TAG, "Upgrading application's database from version " + oldVersion
                    + " to " + newVersion + ", which will destroy all old data!");

            // Destroy old database:
            _db.execSQL("DROP TABLE IF EXISTS " + DATABASE_TABLE);

            // Recreate new database:
            onCreate(_db);
        }
    }


}