SpannableStringBuilder is clearing formatting!?

[dipetca]

hello everybody
i'm learning to program in Java for Android. i'm trying to format text on a TextView using SpannableStringBuilder. but, for some reason, when the code gets to line 42, the first part of the text loses the formatting.
(button is a simple Button and textView is a simple TextView, both defined in activity_main.xml)

package com.example.myapplication;

import androidx.appcompat.app.AppCompatActivity;

import android.os.Bundle;
import android.text.SpannableStringBuilder;
import android.text.Spanned;
import android.text.style.RelativeSizeSpan;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;

public class MainActivity extends AppCompatActivity {
    boolean answer_ison;
    Button button;
    TextView textView;
    SpannableStringBuilder str;
    final RelativeSizeSpan title_span = new RelativeSizeSpan(1.6f);

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

        answer_ison = true;
        button = findViewById(R.id.button);
        textView = findViewById(R.id.textView);
        str = new SpannableStringBuilder();

        button.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                if (answer_ison) {
                    str.clear();
                    str.clearSpans();
                    str.append("Question:\n", title_span, Spanned.SPAN_EXCLUSIVE_EXCLUSIVE);
                    str.append("The question goes here :)");
                    button.setText("Show Answer");
                    answer_ison = false;
                }
                else {
                    str.append("\n\nAnswer:\n", title_span, Spanned.SPAN_EXCLUSIVE_EXCLUSIVE);
                    str.append("Aand the answer goes here");
                    button.setText("Next Question");
                    answer_ison = true;
                }
                textView.setText(str);
            }
        });

    }
}

first its like this (its what I expect):


but if I press the button, it seems to clear the the formatting of the first part:


any help is apreciated. thanks in advance!

 

[Deleted User]

It's probably a quirk of using the SPAN_EXCLUSIVE_EXCLUSIVE flag.
I'd probably do this by using two TextViews. Use one for the question - always visible. Make the other TextView contain the answer, and it's initially not visible. Clicking on the button will make it visible.