Checking whether WRITE_EXTERNAL_STORAGE permission is granted or not in run-time app.

Android - Check and Ask for Permission

// make sure that your Manifest file has the following line of code:
    <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />

// then in you Java code file:
    private static final int REQUEST_STORAGE_PERMISSION = 1;
    
    public void requestPermission() {

        // if the permission is NOT granted.
        if (ContextCompat.checkSelfPermission(this,
                Manifest.permission.WRITE_EXTERNAL_STORAGE) != PackageManager.PERMISSION_GRANTED) {

            // then request run-time permission
            ActivityCompat.requestPermissions(this,
                    new String[]{Manifest.permission.WRITE_EXTERNAL_STORAGE}, REQUEST_STORAGE_PERMISSION);
        }
        // else if it's granted
        else {
            // do what you want to do
        }
    }
    
    // This method is invoked for every call on requestPermissions(android.app.Activity, String[], int).
    https://developer.android.com/reference/android/support/v4/app/ActivityCompat.OnRequestPermissionsResultCallback
    
        @Override
    public void onRequestPermissionsResult(int requestCode, @NonNull String[] permissions,
            @NonNull int[] grantResults) {
        // Called when you request permission to read and write to external storage
        switch (requestCode) {
            case REQUEST_STORAGE_PERMISSION: {
                if (grantResults.length > 0
                        && grantResults[0] == PackageManager.PERMISSION_GRANTED) {
                    // If you get permission, launch the camera
                    launchCamera();
                } else {
                    // If you do not get permission, show a Toast
                    Toast.makeText(this, R.string.permission_denied, Toast.LENGTH_SHORT).show();
                }
                break;
            }
        }
    }