java - Helloworld 数据库 - android 与 sqlite 的连接

Question

我是 Android 的新开发者。我只需要编写一个简单的 hello-world 应用程序，它可以在屏幕上显示有关应用程序所连接的数据库的一些数据。这是我的尝试，但它在“选择或显示”部分崩溃了。它由两个文件组成:Main 和 DataBaseHelper。

主要内容:

package com.SQLearning;

import android.app.Activity;
import android.content.Context;
import android.os.Bundle;
import android.widget.TextView;

public class HelloTestSQLActivity extends Activity {
    DatabaseHelper dbHelper;

    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        //setContentView(R.layout.main);
        dbHelper = new DatabaseHelper(this);
        int seats = dbHelper.getSeats("2");
        TextView tv = new TextView(this);
        setContentView(tv);
        tv.setText( "My first Android App: This is a Hello World Test..." +
                    "Database connection in progress..." +
                    "Table 1 has "+seats);

        dbHelper.close();
    }
}

数据库助手:

package com.SQLearning;

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

public class DatabaseHelper extends SQLiteOpenHelper {

    static final String dbName="demoDB";
    static final String tablesTable="Tables";
    static final String colNum="TableNum";
    static final String colSeats="NumSeats";
    private final Context myContext; 

    static final String viewTables="ViewTables";

    public DatabaseHelper(Context context) {
        super(context, dbName, null,2); 
        this.myContext = context;
    }

    public void onCreate(SQLiteDatabase db) {
        // TODO Auto-generated method stub

        db = getWritableDatabase();

        db.execSQL("CREATE TABLE "+tablesTable+" ("+colNum+ " INTEGER PRIMARY KEY , "+
                                                  colSeats+ " INTEGER)");


        db.execSQL("CREATE VIEW "+viewTables+
                   " AS SELECT "+tablesTable+"."+colNum+" AS _id,"+
                   " "+tablesTable+"."+colSeats+","+
                   " FROM "+tablesTable);

        //Inserts pre-defined departments
        insertTableRecords();
    }

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

          db.execSQL("DROP TABLE IF EXISTS "+tablesTable);
          db.execSQL("DROP VIEW IF EXISTS "+viewTables);
          onCreate(db);
     }

    public void insertTableRecords() {
        SQLiteDatabase db=this.getWritableDatabase();
        ContentValues cv=new ContentValues();

        cv.put(colNum, 1);
        cv.put(colSeats, 3);
        db.insert(tablesTable, colNum, cv);

        cv.put(colNum, 2);
        cv.put(colSeats, 6);
        db.insert(tablesTable, colNum, cv);

        cv.put(colNum, 3);
        cv.put(colSeats, 5);
        db.insert(tablesTable, colNum, cv);

        cv.put(colNum, 4);
        cv.put(colSeats, 2);
        db.insert(tablesTable, colNum, cv);

        db.close();
    }

    public int getSeats(String tablenum)
    {
        SQLiteDatabase db=this.getReadableDatabase();

        String[] args = new String[] {"1"};
        Cursor cur = db.rawQuery(" SELECT "+colSeats+" FROM "+tablesTable+" WHERE "+colNum+"=? ", args);

        cur.moveToFirst();
        db.close();
        //return cur.getInt(cur.getColumnIndex(colSeats));
        return cur.getInt(0);

     }
 }

提前致谢

Answer 1

您关闭了数据库连接，然后尝试从游标中读取一些内容。光标指向您的数据库，如果您同时关闭连接，它将失败。

替换:

   db.close();
   return cur.getInt(0);

与:

  int value= cur.getInt(0);
  db.close();
  return value;

还要经常查看日志(Eclipse 中的 DDMS 选项卡)，您可以在那里看到异常的描述。

顺便说一句。你不应该直接使用列索引，代码不是很可读。而不是 cur.getInt(0) 使用 cur.getInt(cur.getColumnIndex(colSeats))...