MainActivity.java

public class MainActivity extends AppCompatActivity {

    private static final String TAG = "MainActivity";

    private static final int PERMISSION_REQUEST_READ_EXTERNAL_STORAGE = 1;

    private Button openStorage;

    private SharedPreferences sharedPreferences;

    private boolean storagePermissionGranted;

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

        sharedPreferences = getApplicationContext().getSharedPreferences(Constants.MY_SHARED_PREFERENCES,
                MODE_PRIVATE);

        openStorage = findViewById(R.id.button_openStorage);

        openStorage.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                getExternalStoragePermission();

                //After asking permission is done
                if (storagePermissionGranted){
                    //Open Storage or whatever
                }
            }
        });
    }

    private void getExternalStoragePermission() {
        if (ContextCompat.checkSelfPermission(getApplicationContext(),
                Manifest.permission.READ_EXTERNAL_STORAGE) != PackageManager.PERMISSION_GRANTED) {
            //Permission not granted

            if (ActivityCompat.shouldShowRequestPermissionRationale(this,
                    Manifest.permission.READ_EXTERNAL_STORAGE)) {
                //Can ask user for permission

                ActivityCompat.requestPermissions(this, new String[]{Manifest.permission.READ_EXTERNAL_STORAGE},
                        PERMISSION_REQUEST_READ_EXTERNAL_STORAGE);

            } else {
                boolean userAskedPermissionBefore = sharedPreferences.getBoolean(Constants.USER_ASKED_STORAGE_PERMISSION_BEFORE,
                        false);

                if (userAskedPermissionBefore) {
                    //If User was asked permission before and denied
                    AlertDialog.Builder alertDialogBuilder = new AlertDialog.Builder(this);

                    alertDialogBuilder.setTitle("Permission needed");
                    alertDialogBuilder.setMessage("Storage permission needed for accessing photos");
                    alertDialogBuilder.setPositiveButton("Open Setting", new DialogInterface.OnClickListener() {
                        @Override
                        public void onClick(DialogInterface dialogInterface, int i) {
                            Intent intent = new Intent();
                            intent.setAction(Settings.ACTION_APPLICATION_DETAILS_SETTINGS);
                            Uri uri = Uri.fromParts("package", MainActivity.this.getPackageName(),
                                    null);
                            intent.setData(uri);
                            MainActivity.this.startActivity(intent);
                        }
                    });
                    alertDialogBuilder.setNegativeButton("Cancel", new DialogInterface.OnClickListener() {
                        @Override
                        public void onClick(DialogInterface dialogInterface, int i) {
                            Log.d(TAG, "onClick: Cancelling");
                        }
                    });

                    AlertDialog dialog = alertDialogBuilder.create();
                    dialog.show();
                }
                else {
                    //If user is asked permission for first time
                    ActivityCompat.requestPermissions(this, new String[]{Manifest.permission.READ_EXTERNAL_STORAGE},
                            PERMISSION_REQUEST_READ_EXTERNAL_STORAGE);

                    SharedPreferences.Editor editor = sharedPreferences.edit();
                    editor.putBoolean(Constants.USER_ASKED_STORAGE_PERMISSION_BEFORE, true);
                    editor.apply();
                }
            }

        } else {
            storagePermissionGranted = true;
        }

    }

    @Override
    public void onRequestPermissionsResult(int requestCode, @NonNull String[] permissions, @NonNull int[] grantResults) {
        storagePermissionGranted = false;
        switch (requestCode) {
            case PERMISSION_REQUEST_READ_EXTERNAL_STORAGE: {
                if (grantResults.length > 0 && grantResults[0] == PackageManager.PERMISSION_GRANTED) {
                    //Granted
                    storagePermissionGranted = true;

                } else {
                    //Denied
                }
            }
        }
    }

}