Making an android image editor and i have an issue with handling my Bitmaps into variables

This is a part of my onActivity Result code. What i want to do is make a bitmap variable that i can modify everytime i press a button instead of modifying the ImageView(imagen) and not saving the changes that happen to the image.

When i try to change the btp_tmp variable inside the onClick code of the button it throws an error about btp_tmp being in an inner class and must be made final.

@Override protected void onActivityResult(int requestCode, int resultCode, Intent data) {

    Bitmap btp_img = null;
    InputStream in_stream;
     Bitmap btp_tmp = null;



    if (resultCode == Activity.RESULT_OK && requestCode == RCode)

    {
        //ARXIKO IMAGE
        try {
            if (btp_img != null) {
                btp_img.recycle();
            }
            in_stream = getContentResolver().openInputStream(
                    data.getData());
            btp_img = BitmapFactory.decodeStream(in_stream);
            in_stream.close();

            btp_tmp = btp_img;
            imagen.setImageBitmap(btp_img);

            //btn_seleccion.setText(getResources().getString(R.string.modifa));
        } catch (IOException e) {
            e.printStackTrace();
        }

        //NEGATIVE
        //final boolean test = false;
        final Bitmap finalBtp_img1 = btp_tmp;

        //if (test == false){}
        btp_tmp = btp_img;
        Neg_button.setOnClickListener(new View.OnClickListener() {
            @Override
                public void onClick(View v) {
                //imagen.setImageBitmap(invert(finalBtp_img));
                Negative neg = new Negative();
                imagen.setImageBitmap(neg.invert(finalBtp_img1));
                btp_tmp = neg.invert(finalBtp_img1);
            }
        });



        //UNDO
        final Bitmap finalBtp_imgUndo = btp_img;
        eraser.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                imagen.setImageBitmap(finalBtp_imgUndo);
            }
        });