如何从我的Android应用程序的所有目录中获取所有pdf文件

import android.Manifest;
import android.content.pm.PackageManager;
import android.os.Build;
import android.os.Bundle;
import android.os.Environment;
import androidx.annotation.NonNull;
import androidx.appcompat.app.AppCompatActivity;
import androidx.core.app.ActivityCompat;
import androidx.core.content.ContextCompat;
import java.io.File;
import java.util.ArrayList;
import java.util.List;

public class MainActivity extends AppCompatActivity {

    private static final int REQUEST_CODE = 100;
    private List<File> pdfFiles = new ArrayList<>();

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

        if (ContextCompat.checkSelfPermission(this, Manifest.permission.READ_EXTERNAL_STORAGE) != PackageManager.PERMISSION_GRANTED) {
            ActivityCompat.requestPermissions(this, new String[]{Manifest.permission.READ_EXTERNAL_STORAGE}, REQUEST_CODE);
        } else {
            findPdfFiles();
        }
    }

    @Override
    public void onRequestPermissionsResult(int requestCode, @NonNull String[] permissions, @NonNull int[] grantResults) {
        super.onRequestPermissionsResult(requestCode, permissions, grantResults);
        if (requestCode == REQUEST_CODE && grantResults.length > 0 && grantResults[0] == PackageManager.PERMISSION_GRANTED) {
            findPdfFiles();
        }
    }

    private void findPdfFiles() {
        // Check internal storage
        File internalDir = getFilesDir();
        findPdfInDirectory(internalDir);

        // Check external storage if available
        if (Environment.getExternalStorageState().equals(Environment.MEDIA_MOUNTED)) {
            File externalDir = Environment.getExternalStorageDirectory();
            findPdfInDirectory(externalDir);
        }
    }

    private void findPdfInDirectory(File directory) {
        if (directory != null && directory.isDirectory()) {
            File[] files = directory.listFiles();
            if (files != null) {
                for (File file : files) {
                    if (file.isDirectory()) {
                        findPdfInDirectory(file); // Recursively search subdirectories
                    } else if (file.getName().toLowerCase().endsWith(".pdf")) {
                        pdfFiles.add(file);
                    }
                }
            }
        }
    }
}