我想使用来自不同类的数据库，我该如何在 JAVA 中做到这一点？ (AndroidStudio/Java)

Question

Im struggling with the following problem:

I have to classes, MainClass and SecondClass.

In the MainClass I create a database (mDatabase) and I am inserting data to the database.

public class MainActivity extends AppCompatActivity{
public SQLDatabase dbHelper; //created class to initialize my database
public SQLiteDatabase mDatabase;


public void onCreate(Bundle savedInstanceState){
        dbHelper = new SQLDatabase(this);
        mDatabase = dbHelper.getWritableDatabase();
        mDatabase.insert(....);
}

Now I want to display the data from the database (mDatabase) in a different Activity. So, I have to use the variable mDatabase from another class, because I am using a recyclerView in a different Activity, this is why:

public class SecondActivity{

    public void onCreate(){
        recyclerView=findViewById(R.id.recyclerView);
        recyclerView.setLayoutManager(new LinearLayoutManager(this));
        mAdapter = new AdapterClass(this, getAllItems());
        recyclerView.setAdapter(mAdapter);
    }

    public Cursor getAllItems(){

    return mDatabase.query(....);
    }


}

I want to have one Activity where I type in data for the database, and I want to have a second Activity where I can display the inserted data.

Do you know how I can use the mDatabase from two classes?

Answer 1

Open a new database connection in the other activity.

Variables and objects cannot be shared between activities unless they are static (and compatible with that use case: objects that need an Activity Context will cause problems and leaks if made static), or Serializable and you provide it as a parameter.

You can also consider Fragments : that will allow to create the mDatabase connection in your containing Activity and reuse it from your Fragments , or place the Database connection at the Application Level, if the database connection can be initialized with an Application context.

Answer 2

1. Create your own Application class.

    public class TestApplication extends Application { 

            public static SQLDatabase dbHelper; 
            public static SQLiteDatabase mDatabase;

            @Override
            public void onCreate() {
                super.onCreate();
                initDb();
            }

            public void initDb() {
                dbHelper = new SQLDatabase(this);
            mDatabase = dbHelper.getWritableDatabase();
            mDatabase.insert(....);
            }

            public static SQLiteDatabase getDb()  {
                return mDatabase;
            }
        }

2. Add you Application class to AndroidManifest under Application tag

   <application android:name="TestApplication"

3. Use it from your activities:

   TestApplication.getDb()

If it helped please mark as answered:)