如何从另一个类及其上下文中调用函数（Android，Java）

Question

I'm trying to reload a viewPager whenever the user pushes a button inside an Item from the viewPager. The relevant classes for this case are SetupActivity and Adapter . In the activity class I've already made the two functions to reload the viewPager:

public void deleteSession(String title) {
        dbHelper = new DatabaseHelper(this);
        ((DatabaseHelper) dbHelper).deleteRow(title);
    }

and

public void refreshAdapter () {

        dbHelper = new DatabaseHelper(this); 
        Cursor dataCursor = ((DatabaseHelper) dbHelper).getData();
        models = new ArrayList<>(); 

        while (dataCursor.moveToNext()) { 
            models.add(new Model(
                    dataCursor.getString(1),
                    dataCursor.getInt(2),
                    dataCursor.getInt(3),
                    dataCursor.getInt(4)
            ));
        }
        adapter = new Adapter(models, this);
        viewPager.setAdapter(adapter);
        viewPager.setCurrentItem(0, true);
    }

So basically, deleteSession() deletes a specific item from a database. refreshAdapter() reads the updated data from the database and sets a new adapter to the viewPager. These functions are working fine, if I call them from an OnClick event in the SetupActivity class. However, if I try to call them from an OnClick event in the Adapter class, the app crashes with the following errors:

java.lang.NullPointerException: Attempt to invoke virtual method 'java.io.File android.content.Context.getDatabasePath(java.lang.String)' on a null object reference
        at android.content.ContextWrapper.getDatabasePath(ContextWrapper.java:327)
        at android.database.sqlite.SQLiteOpenHelper.getDatabaseLocked(SQLiteOpenHelper.java:352)
        at android.database.sqlite.SQLiteOpenHelper.getReadableDatabase(SQLiteOpenHelper.java:322)
        at com.example.android.slink.DatabaseHelper.getData(DatabaseHelper.java:69)
        at com.example.android.slink.SetupActivity.refreshAdapter(SetupActivity.java:436)
        at com.example.android.slink.Adapter$3.onClick(Adapter.java:111)
        at android.view.View.performClick(View.java:6669)
        at android.view.View.performClickInternal(View.java:6638)
        at android.view.View.access$3100(View.java:789)
        at android.view.View$PerformClick.run(View.java:26145)
        at android.os.Handler.handleCallback(Handler.java:873)
        at android.os.Handler.dispatchMessage(Handler.java:99)
        at android.os.Looper.loop(Looper.java:193)
        at android.app.ActivityThread.main(ActivityThread.java:6863)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:537)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858)

Here's my SetupActiviy class (reduced to the relevant parts):

package com.example.android.slink;

public class SetupActivity extends AppCompatActivity {

    private SQLiteOpenHelper dbHelper;
    private SQLiteDatabase database;

    ViewPager viewPager;
    Adapter adapter;
    static ArrayList<Model> models;

    public void deleteSession(String title) {
        dbHelper = new DatabaseHelper(this);
        ((DatabaseHelper) dbHelper).deleteRow(title);
    }

    public void refreshAdapter () {
        dbHelper = new DatabaseHelper(this);
        Cursor dataCursor = ((DatabaseHelper) dbHelper).getData();
        models = new ArrayList<>();
        models.add(new Model("", 0,0,0));
        while (dataCursor.moveToNext()) {
            models.add(new Model(
                    dataCursor.getString(1),
                    dataCursor.getInt(2),
                    dataCursor.getInt(3),
                    dataCursor.getInt(4)
            ));
        }
        adapter = new Adapter(models, this);
        viewPager.setAdapter(adapter);
        viewPager.setCurrentItem(0, true);
    }

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

        editTextScreenTitle = findViewById(R.id.editTextScreenTitle);
        relativeLayoutCard = findViewById(R.id.relativeLayoutCard);
        viewPager = findViewById(R.id.viewPager);

        dbHelper = new DatabaseHelper(this);
        Cursor dataCursor = ((DatabaseHelper) dbHelper).getData();

        models = new ArrayList<>();
        models.add(new Model("", 0,0,0));
        while (dataCursor.moveToNext()) {
            models.add(new Model(
                    dataCursor.getString(1),
                    dataCursor.getInt(2),
                    dataCursor.getInt(3),
                    dataCursor.getInt(4)
            ));
        }
        adapter = new Adapter(models, this);
        viewPager.setAdapter(adapter);
    }
}

Here's my Adapter class (also reduced to the relevant parts):

package com.example.android.slink;

public class Adapter extends PagerAdapter {

    private List<Model> models;
    private LayoutInflater layoutInflater;
    private Context context;

    SetupActivity setupActivity = new SetupActivity();

    public Adapter(List<Model> models, Context context) {
        this.models = models;
        this.context = context;
    }

    @NonNull
    @Override
    public Object instantiateItem(@NonNull final ViewGroup container, final int position) {
        layoutInflater = LayoutInflater.from(context);
        final View view = layoutInflater.inflate(R.layout.item, container, false);

        TextView buttonDeleteCard = view.findViewById(R.id.textViewDeleteCard);

        buttonDeleteCard.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                setupActivity.deleteSession([...some title...]);
                setupActivity.refreshAdapter();
            }
        });

        container.addView(view, 0);
        return view;
    }
}

I think it has something to do whith the context. Do I have to pass the context from SetupActiity to the Adapter class and if so, how do I do that? Or is it not possible and I need to catch the OnClick event in the Adapter class from in the SetupActivity class? I'm new to java programming and would be very grateful if any of you could help me, solve this problem!

Answer 1

Change your Adapter like this:

public class Adapter extends PagerAdapter {

    private List<Model> models;
    private LayoutInflater layoutInflater;
    private Context context;

    private SetupActivity setupActivity;

    public Adapter(List<Model> models, Context context) {
        this.models = models;
        this.context = context;
        setupActivity = (SetupActivity) context;
    }

    @NonNull
    @Override
    public Object instantiateItem(@NonNull final ViewGroup container, final int position) {
        layoutInflater = LayoutInflater.from(context);
        final View view = layoutInflater.inflate(R.layout.item, container, false);

        TextView buttonDeleteCard = view.findViewById(R.id.textViewDeleteCard);

        buttonDeleteCard.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                setupActivity.deleteSession([...some title...]);
                setupActivity.refreshAdapter();
            }
        });

        container.addView(view, 0);
        return view;
    }
}

Why the error?

You pass the Context from SetupActivity to your Adapter , you save that reference in the field ( context ) but you mistakenly created a new instance of SetupActivity .

That's why you got the NullPointerException . The activity is your Context . It inherits from it. So just cast it in your constructor and use it later.