Android-在Activity类和Adapter类之间传递数据

Question

I have 3 classes: LoginActivity,MapsActivity and MatchAdapter

The first 2 extends AppCompactActivity, the last one ArrayAdapter. When i make login (if correct, matching on mySQLiteDB) i used to get ID_contact of current user and pass it to MapsActivity with intent in such way:

On my LoginActivity:

String contact=databaseHelper.searchID_Contact(username,password);
Intent intent=new Intent(LoginActivity.this,MapsActivity.class);

            intent.putExtra("ID_CONTACT",contact);
            startActivity(intent);

On MapsActivity i can easily retrieve this data in such way:

public  String getId_contact(String conct){

    return conct;
}

 @Override
public void onMapReady(GoogleMap googleMap) {

String id_contact1=getIntent().getStringExtra("ID_CONTACT");
    String contact=getId_contact(id_contact1);
    Toast.makeText(MapsActivity.this, contact, Toast.LENGTH_LONG).show();
}

Till now everything works fine, it appears the id of the current user. My problem is to pass this data (with intent i don't know how) even to another class named MatchAdapter that extends ArrayAdapter.

I tried this way on MapsActivity:

public class MapsActivity extends AppCompatActivity implements ...{

public  String getId_contact(){

        String contact=getIntent().getStringExtra("ID_CONTACT");


        return contact;
    }

So on MatchAdaper trying to retrieve such way:

MapsActivity mapsActivity=new MapsActivity();
String text=mapsActivity.getId_contact().toString();

But nothing..i get NULLPOINTEREXCEPTION...Can someone help me?

Answer 1

那么，您可以通过以下方式从适配器访问活动方法，即从适配器的构造函数或所需的任何位置调用此方法。

((ActivityName)context).methodName();

Answer 2

When you create a new instance of MapsActivity, that isn't the same Activity instance you got when you called startActivity(). This is basically why you have a null pointer exception.

More importantly, you should never be manually creating Activity instances using "new". Generally the system creates Activity objects for you via mechanisms like startActivity(), and that is how you should obtain them.

Himanshu's suggestion can work, if your activity does happen to be "hosting" your adapter, but this isn't always guaranteed. A better approach is to pass the ID to your MatchAdapter directly, either in the constructor or as a direct setter function. At the least, you should perform a "instanceof" check to make sure your adapter context is really of type MapsActivity.

Answer 3

That's my MatchAdapter `package vincenzo.futsal4you;

import android.app.Activity;
import android.content.Context;
import android.content.Intent;
import android.support.annotation.Nullable;
import android.support.v7.app.AppCompatActivity;
import android.util.Log;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.ArrayAdapter;
import android.widget.Button;
import android.widget.RelativeLayout;
import android.widget.TextView;
import android.widget.Toast;

import java.util.ArrayList;
import java.util.List;
import java.util.regex.MatchResult;


public class MatchAdapter extends ArrayAdapter{

    List list=new ArrayList();




    String text1=null;




    MatchAdapter matchAdapter;
    static String id_contatto3="";
    String fatto3="";
    Player player=new Player();




    public MatchAdapter(Context context, int resource) {
        super(context, resource);
    }

    public void add(Match object) {
        list.add(object);
        super.add(object);
    }

    @Override
    public int getCount() {
        return super.getCount();
    }

    @Override
    public Object getItem(int position) {
        return super.getItem(position);
    }

    @Override
    public View getView(int position, View convertView, ViewGroup parent) {

        View row=convertView;
        final Context context = null;
        final MatchHolder matchHolder;
        final String cc=null;



        if (row==null){


            LayoutInflater layoutInflater=(LayoutInflater)this.getContext().getSystemService(Context.LAYOUT_INFLATER_SERVICE);
            row=layoutInflater.inflate(R.layout.display_match_row,parent,false);
            matchHolder=new MatchHolder();

            matchHolder.id_match=(TextView)row.findViewById(R.id.id_partita);
            matchHolder.id_contact=(TextView)row.findViewById(R.id.id_contatto);
            matchHolder.nome_partita=(TextView)row.findViewById(R.id.nome_partita);
            matchHolder.citta=(TextView)row.findViewById(R.id.citta);
            matchHolder.indirizzo=(TextView)row.findViewById(R.id.indirizzo);
            matchHolder.data=(TextView)row.findViewById(R.id.data);
            matchHolder.ora=(TextView)row.findViewById(R.id.ora);
//            matchHolder.id_contact=row.findViewById()
            matchHolder.join_us = (Button) row.findViewById(R.id.join_us);


            row.setTag(matchHolder);

        }
        else {
            matchHolder=(MatchHolder)row.getTag();
        }

        final Match match=(Match)getItem(position);

//        matchHolder.id_contact.setText(mapsActivity.getId_partita().toString());
        matchHolder.id_match.setText(match.getId().toString());
        matchHolder.nome_partita.setText(match.getName().toString());
        matchHolder.citta.setText(match.getCitta().toString());
        matchHolder.indirizzo.setText(match.getIndirizzo().toString());
        matchHolder.data.setText(match.getData().toString());
        matchHolder.ora.setText(match.getOra().toString());

//        assert ((MapsActivity) context) != null;
//        ((MapsActivity) context).getId_partita();

//        final String contact=matchHolder.getId_contatto();


        Log.e("BOOOOOO", matchHolder.getId_contatto2());
        final String fatto=matchHolder.getId_contatto2();
        fatto3=matchHolder.getId_contatto2();




        matchHolder.join_us.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
//
                String fatto2=matchHolder.getId_contatto2();
//                final String text3=mapsActivity.getId_partita(cc).toString();

                RelativeLayout rl = (RelativeLayout) v.getParent();
                RelativeLayout r2 = (RelativeLayout) v.getParent();
//                TextView tv = (TextView)rl.findViewById(R.id.nome_partita);
                TextView tv = (TextView) rl.findViewById(R.id.id_partita);
                TextView tv2 = (TextView) r2.findViewById(R.id.id_contatto);

                String id_partita = tv.getText().toString();
                String text2 = tv2.getText().toString();



                Toast.makeText(getContext(), id_partita, Toast.LENGTH_SHORT).show();
//                Toast.makeText(getContext(), matchHolder.setId_contatto(contact), Toast.LENGTH_SHORT).show();
                Toast.makeText(getContext(),matchHolder.getId_contatto2(),Toast.LENGTH_SHORT).show();


                player.setId_contatto(fatto3);
                player.setId_partita(id_partita);
//                databaseHelper=new DatabaseHelper(context);
//                databaseHelper.insertPlayer2(player);

                ((MapsActivity)context).getJoinPlayer(player);

                Toast pass1=Toast.makeText(getContext(), "One Row JOIN US created !", Toast.LENGTH_SHORT);
                pass1.show();


            }
        });

        return row;
    }

     static class MatchHolder{
        TextView id_match,nome_partita,citta,indirizzo,data,ora,id_contact;
        Button join_us;

        public MatchHolder(){}

        public String getId_contatto(String id_contatto) {
            return id_contatto;
        }
        public String getId_contatto2() {
            return id_contatto3;
        }

        public void setId_contatto(String id_contatto) {
            id_contatto3 = id_contatto;
        }


        public MatchHolder(String id_contatto){
            id_contatto3=id_contatto;

        }


    }
}

` So the problem was that i declared a String (id_contatto3) inside a static (inner) class (MatchHolder).Declaring it up to MatchAdapter i solve it somehow, but now i wanna call inside onClick a method that is inside another class (DatabaseHelper that extends SQLiteOpenhelper)..

I can't do ((DatabaseHelper)context).mymethod() So i've done the following "trick"...inside class MapsActivity where i've created a method (JoinPlayer) in such way:

public void JoinPlayer(Player player){
        databaseHelper.insertPlayer(player);
    }

where insertPlayer(Player) in DatabaseHelper is:

public void insertPlayer(Player player){
        try{
        db=this.getWritableDatabase();
        }
        catch(Exception e){
           Log.e("ERROR","ERROR");
        }
        ContentValues contentValues=new ContentValues();
        String query="select * from player";
        Cursor cursor=db.rawQuery(query,null);
        int count=cursor.getCount();

        contentValues.put(COLUMN_ID_PLAYER,count);
        contentValues.put(COLUMN_ID_MATCH_PLAYER,player.getId_partita());
        contentValues.put(COLUMN_ID_CONTACT_PLAYER,player.getId_contatto());

        db.insert(TABLE_PLAYER, null, contentValues);
        db.close();
    } 

But Android suggest me to add a null condition(if ((MapsActivity)context)!=null) than ((MapsActivity)context).JoinPlayer(player) but it advise me it will be Always null and that's exactly what I get... I think is the context the main problem but have no clue right now how to solve it. Any Idea?

Answer 4

Ok...found the solution...On MatchAdapter extends ArrayAdapter

DatabaseHelper databaseHelper=new DatabaseHelper(getContext()); ...than OnClick function....

databaseHelper.myfunction();