How to display words of different sizes, such as Office or function of Google Docs in Android

I try to do like the Office or Google Docs can enter a different word sizes this feature

Here is my code

input =(EditText)findViewById(R.id.Input_EditText); 
input.addTextChangedListener(new TextWatcher() 
{
    public void onTextChanged(CharSequence s, int start, int before, int count) {   
    }
    public void beforeTextChanged(CharSequence s, int start, int count, int after) {    
    }
    public void afterTextChanged(Editable s) 
    {
        int index = input.getSelectionEnd();
        String nowstr;

        if(index==0)
        {
            //do nothing
        }
        else
        {
            nowstr = s.toString();
            char nowstr_array[] = nowstr.toCharArray();
            show_input = String.valueOf(nowstr_array[index-1]);

            SpannableStringBuilder  spann = new SpannableStringBuilder(show_input);
            AbsoluteSizeSpan word_size = new AbsoluteSizeSpan(40,true);
            spann.setSpan(word_size, 0, 1, Spanned.SPAN_EXCLUSIVE_EXCLUSIVE);
            input.getText().insert(index, spann);
        }
    }
});

But this program will crash...

then,I try to do

Toast.makeText(MainActivity.this, spann, Toast.LENGTH_SHORT).show();
//input.getText().insert(index, spann);

But doing so can be displayed...

That is why?

public void beforeTextChanged(CharSequence s, int start, int count,
                int after) {
            beforeText = s.toString() ;
        }

        public void afterTextChanged(Editable s) {
            int index = input.getSelectionEnd();
            String nowstr;

            if (index == 0) {
                // do nothing
            } else {
                if (beforeText.length() < s.toString().length()) {
                    nowstr = s.toString();
                    char nowstr_array[] = nowstr.toCharArray();
                    String show_input = String.valueOf(nowstr_array[index - 1]);

                    SpannableStringBuilder spann = new SpannableStringBuilder(
                            show_input);
                    AbsoluteSizeSpan word_size = new AbsoluteSizeSpan(40, true);
                    spann.setSpan(word_size, 0, 1,
                            Spanned.SPAN_EXCLUSIVE_EXCLUSIVE);
                    input.removeTextChangedListener(this) ;
                    input.getText().replace(index-1, index, spann) ;
                    input.addTextChangedListener(this) ;

                }
            }
        }

    });

beforeText is a global variable