Question

 -------This is my Database activity here i m creating database ----------     

     public void Database_Create(){

    try{
        db = openOrCreateDatabase(Environment.getExternalStorageDirectory()+"/TestExample/Test",MODE_PRIVATE,null);

    }catch (Exception e){

        e.printStackTrace();
    }

}

------这是我的MainActivity ------------------

    try{

        Database dbs = new Database();
        dbs.Database_Create();
        Toast.makeText(dbs, "Data Ok", Toast.LENGTH_SHORT).show();


    }   catch (Exception e){
        e.printStackTrace();
    }

我在Database Activity中创建数据库，我想在MainActivity中调用Database_create（）方法如何调用？我收到一个错误db object is null，我已经在manifest.xml文件中给出了权限

Answer 1

首先看一下这个doc，

import android.app.Activity;
import android.database.Cursor;
import android.database.sqlite.SQLiteDatabase;
import android.os.Bundle;
import android.util.Log;
import android.widget.TextView;
public class DBTest extends Activity {
 /** Called when the activity is first created. */
 @Override
 public void onCreate(Bundle savedInstanceState) {
  super.onCreate(savedInstanceState);

  SQLiteDatabase myDB= null;
  String TableName = "myTable";

  String Data="";

  /* Create a Database. */
  try {
   myDB = this.openOrCreateDatabase("DatabaseName", MODE_PRIVATE, null);

   /* Create a Table in the Database. */
   myDB.execSQL("CREATE TABLE IF NOT EXISTS "
     + TableName
     + " (Field1 VARCHAR, Field2 INT(3));");

   /* Insert data to a Table*/
   myDB.execSQL("INSERT INTO "
     + TableName
     + " (Field1, Field2)"
     + " VALUES ('Saranga', 22);");

   /*retrieve data from database */
   Cursor c = myDB.rawQuery("SELECT * FROM " + TableName , null);

   int Column1 = c.getColumnIndex("Field1");
   int Column2 = c.getColumnIndex("Field2");

   // Check if our result was valid.
   c.moveToFirst();
   if (c != null) {
    // Loop through all Results
    do {
     String Name = c.getString(Column1);
     int Age = c.getInt(Column2);
     Data =Data +Name+"/"+Age+"\n";
    }while(c.moveToNext());
   }
   TextView tv = new TextView(this);
   tv.setText(Data);
   setContentView(tv);
  }
  catch(Exception e) {
   Log.e("Error", "Error", e);
  } finally {
   if (myDB != null)
    myDB.close();
  }
 }
}

Answer 2

您可以像这样创建数据库：

private SQLiteDatabase db;

private SQLiteDatabase db = openOrCreateDatabase(Environment.getExternalStorageDirectory()+"/TestExample/Test.db", Context.MODE_PRIVATE, null);

Answer 3

这是在android sqlite中创建SQLite数据库的完整示例。我们需要使用SQLiteOpenHelper扩展一个类。并且需要创建一个构造函数来传递活动中的参数。

public class FlightsDatabase extends SQLiteOpenHelper {
    private Context context;
    public FlightsDatabase(Context context) {
        super(context,"My_Datas",null,1);
        this.context=context;
    }

然后，我们需要声明数据库名称，当您希望在发布应用程序后对数据库进行一些更改时，可以使用2-3个选项来创建数据库和升级数据库版本。然后，我们将表创建为：

@Override
    public void onCreate(SQLiteDatabase db) {

        db.execSQL("CREATE TABLE Flight (id INTEGER PRIMARY KEY AUTOINCREMENT,sr TEXT,today_date TEXT,imei TEXT,too TEXT,fromm TEXT,dept_date TEXT,status1 TEXT,file_url TEXT,plane_pic TEXT,plane_name TEXT,idd TEXT)");
    }

然后您可以在数据库上应用搜索，检查数据和删除操作的操作。

数据库在Android中创建

3 个答案: