How to access data from SQLite in different activity than activity_main?

Question

I'm creating a basic app on Android Studio with SQLite. I want to show some data in a listview on an activity that's not the activity_main, but this listview always returns null and then the compiler crashes due to a NullPointerException.

This is an app with a drawer layout menu. I will post only the xml code of the activity where I want to show data (maf.xml), and the MainActivity.java (without package and imports). The objects BD and Lobito are created in other java files and work correctly because I tried it in another project.

Can anyone help me? Thanks.

--

maf.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical" android:layout_width="match_parent"
    android:layout_height="match_parent">

    <ListView
        android:id="@+id/listViewItems"
        android:layout_width="match_parent"
        android:layout_height="match_parent" />
</LinearLayout>

MainActivity.java

public class MainActivity extends AppCompatActivity {

    private AppBarConfiguration mAppBarConfiguration;

    ListView listViewItems;
    ArrayAdapter<String> adapter;
    ArrayList<String> arrayList;
    BD bd = new BD(this);

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

        listViewItems = findViewById(R.id.listViewItems);
        listAll();

        Toolbar toolbar = findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);
        FloatingActionButton fab = findViewById(R.id.fab);
        fab.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                Snackbar.make(view, "Replace with your own action", Snackbar.LENGTH_LONG)
                        .setAction("Action", null).show();
            }
        });
        DrawerLayout drawer = findViewById(R.id.drawer_layout);
        NavigationView navigationView = findViewById(R.id.nav_view);

        mAppBarConfiguration = new AppBarConfiguration.Builder(
                R.id.nav_home, R.id.bandos, R.id.maf,
                R.id.nav_tools, R.id.nav_share, R.id.maf)
                .setDrawerLayout(drawer)
                .build();
        NavController navController = Navigation.findNavController(this, R.id.nav_host_fragment);
        NavigationUI.setupActionBarWithNavController(this, navController, mAppBarConfiguration);
        NavigationUI.setupWithNavController(navigationView, navController);
    }

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

    @Override
    public boolean onSupportNavigateUp() {
        NavController navController = Navigation.findNavController(this, R.id.nav_host_fragment);
        return NavigationUI.navigateUp(navController, mAppBarConfiguration)
                || super.onSupportNavigateUp();
    }


    public void listAll() {
        List<Lobito> lobitos = bd.listaTodosLobitos();

        arrayList = new ArrayList<String>();

        adapter = new ArrayAdapter<String>(MainActivity.this,
                android.R.layout.simple_list_item_1, arrayList);

        listViewItems.setAdapter(adapter);

        for (Lobito l : lobitos) {

            arrayList.add(l.getId() + "-"  + l.getNome());
            adapter.notifyDataSetChanged();
        }
    }


}

Answer 1

You are finding listViewItems from the activity_main.xml file, while you have declared it in maf.xml. That's why you are getting error.

So just replace this line

setContentView(R.layout.activity_main);

with

setContentView(R.layout.maf);

Answer 2

I moved the code referent to maf.xml to a new file MAF.java, and changed to setContentView(R.layout.maf). However, looks like the code in MAF.java don't run. There's something wrong?

MAF.java

public class MAF extends AppCompatActivity {


    ListView listViewItems;
    ArrayAdapter<String> adapter;
    ArrayList<String> arrayList;
    BD bd = new BD(this);

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

        listViewItems = findViewById(R.id.listViewItems);
        listAll();

    }


    public void listAll() {
        List<Lobito> lobitos = bd.listaTodosLobitos();

        arrayList = new ArrayList<String>();

        adapter = new ArrayAdapter<String>(MAF.this,
                android.R.layout.simple_list_item_1, arrayList);

        listViewItems.setAdapter(adapter);

        for (Lobito l : lobitos) {
            //Log.d("Lista", "\nID: " + l.getId() + " Nome: " + l.getNome());

            arrayList.add(l.getId() + "-"  + l.getNome());
            adapter.notifyDataSetChanged();
        }
    }


}