Android - How to add files of one extension to listview

Question

I am trying to make an app that finds all pdf in my phone, but the application crashes. I already got permission to write on storage but it didn't help. Here is my code:

package prototype.reader.com;
import android.app.*;
import android.os.*;
import android.content.*;
import android.view.*;
import java.util.*;
import java.io.*;
import android.widget.*;
public class lst extends Activity 
{
    ArrayList<String> bk;
        ArrayAdapter<String> ad = new ArrayAdapter<String>(this, android.R.layout.simple_list_item_1, bk);
private ListView lv;
protected void onCreate(Bundle savedinstancestate){
        super.onCreate(savedinstancestate);
        setContentView(R.layout.listpdfs);
        lv = findViewById(R.id.lpdf);
       File sdCard = Environment.getExternalStorageDirectory();
        recursiveFind(sdCard);
        lv.setAdapter(ad);
    }
    public void recursiveFind(File file) { 
        File[] f = file.listFiles();
        for (File ff : f) {
            if (ff.isDirectory()) recursiveFind(file);
            if (ff.isFile() &&     ff.getPath().endsWith(".pdf")) {
                bk.add(ff.toString());
            }
            }
        }
    }

Answer 1

Try this you declare adapter wrong before arraylist add data you set adapter use below code

    public class lst extends Activity {
    ArrayList<String> bk;
    ArrayAdapter<String> ad;
    private ListView lv;

    protected void onCreate(Bundle savedinstancestate) {
        super.onCreate(savedinstancestate);
        setContentView(R.layout.listpdfs);
        bk = new ArrayList<>();
        lv = findViewById(R.id.lpdf);
        File sdCard = Environment.getExternalStorageDirectory();
        recursiveFind(sdCard);
        ad = new ArrayAdapter<String>(this, android.R.layout.simple_list_item_1, bk);
        lv.setAdapter(ad);
    }

    public void recursiveFind(File file) {
        File[] f = file.listFiles();
        for (File ff : f) {

            if (ff.isFile() && ff.getPath().endsWith(".pdf")) {
                bk.add(ff.toString());
            }
        }
    }
}