Android: Saving image to storage

Currently my application, takes a photo, and put's the data into a ImageView.

public void openCamera() {
        Intent takePictureIntent = new Intent(MediaStore.ACTION_IMAGE_CAPTURE);
        if (takePictureIntent.resolveActivity(getPackageManager()) != null) {
            startActivityForResult(takePictureIntent, REQUEST_IMAGE_CAPTURE);
        }
    }

    protected void onActivityResult(int requestCode, int resultCode, Intent data) {
        if (requestCode == REQUEST_IMAGE_CAPTURE && resultCode == RESULT_OK) {
            Bundle extras = data.getExtras();
            Bitmap photo = (Bitmap) data.getExtras().get("data");
            imageView.setImageBitmap(photo);

        }
    }

Now, i want it to do this: When the user clicks the button 'Save Button' I would like it to save to storage. How would i do this? I have my button set up with an OnClick listener.

public void save_btn() {

    }