I found this article on how to use a Spannable or in a string resource file: <sup> or <sub> for superscript and subscript, respectively.

((TextView)findViewById(R.id.text)).setText(Html.fromHtml("X<sup>2</sup>"));

or

Common Tasks and How to Do Them in Android

Example:

equation = (TextView) findViewById(R.id.textView1);
SpannableStringBuilder cs = new SpannableStringBuilder("X3 + X2");
cs.setSpan(new SuperscriptSpan(), 1, 2, Spannable.SPAN_EXCLUSIVE_EXCLUSIVE);
cs.setSpan(new RelativeSizeSpan(0.75f), 1, 2, Spannable.SPAN_EXCLUSIVE_EXCLUSIVE);
cs.setSpan(new SuperscriptSpan(), 6, 7, Spannable.SPAN_EXCLUSIVE_EXCLUSIVE);
cs.setSpan(new RelativeSizeSpan(0.75f), 6, 7, Spannable.SPAN_EXCLUSIVE_EXCLUSIVE);
equation.setText(cs);

To all people asking, if you want to make it smaller besides of making super or subscript, you just need to add tag as well. EX:

"X <sup><small> 2 </small></sup>"

If you want to set the superscript from string.xml file try this:

string resource:

<string name="test_string">X&lt;sup&gt;3&lt;/sup&gt;</string>

if you want the superscript to be smaller:

<string name="test_string">X&lt;sup&gt;&lt;small&gt;3&lt;/small&gt;&lt;/sup&gt;</string>

Code:

textView.setText(Html.fromHtml("Anything you want to put here"+getString(R.string.test_string)));

((TextView)findViewById(R.id.text)).setText(Html.fromHtml("X<sup><small>2</small></sup>"));

(or) From String Resource File:

<string name="test_string">
<![CDATA[ X<sup><small>2</small></sup> ]]>
</string>

It bit late but following just work fine, use superscript as special character, I used spacial char here.

<string name="str">H₂</string>

In the strings.xml files, you can just use the HTML <sup>3</sup> tag. Work perfectly for me

EXAMPLE

<string name="turnoverRate">Turnover rate m<sup>3</sup>/m<sup>2</sup>/hour:</string>

They are called Unicode characters, and Android TextView supports them. Copy the super/sub-script you want from this Wiki: https://en.wikipedia.org/wiki/List_of_Unicode_characters#Superscripts_and_Subscripts

yourTextView.setText(Html.fromHtml("X<sup>2</sup>"));


This will be the result in you yourTextView =

X²

The HTML.fromHTML(String) was deprecated as of API 24. They say to use this one instead, which supports flags as a parameter. So to go off of the accepted answer:

TextView textView = ((TextView)findViewById(R.id.text));
textView.setText(Html.fromHtml("X<sup>2</sup>", Html.FROM_HTML_MODE_LEGACY));

And if you want code that considers pre-24 API's as well:

TextView textView = ((TextView)findViewById(R.id.text));
if (android.os.Build.VERSION.SDK_INT >= android.os.Build.VERSION_CODES.N) {
textView.setText(Html.fromHtml("X<sup>2</sup>", Html.FROM_HTML_MODE_LEGACY));
} else {
textView.setText(Html.fromHtml("X<sup>2</sup>"));
}

This answer was derived from: https://stackoverflow.com/a/37905107/4998704

The flags and other documentation can be found here: https://developer.android.com/reference/android/text/Html.html

The Accepted answer is deprecated now. So please go through this piece of code. I got this from some website. I forgot the name but anyway thanks for this good piece of working code.

     SpannableString styledString
= new SpannableString("Large\n\n"     // index 0 - 5
+ "Bold\n\n"          // index 7 - 11
+ "Underlined\n\n"    // index 13 - 23
+ "Italic\n\n"        // index 25 - 31
+ "Strikethrough\n\n" // index 33 - 46
+ "Colored\n\n"       // index 48 - 55
+ "Highlighted\n\n"   // index 57 - 68
+ "K Superscript\n\n" // "Superscript" index 72 - 83
+ "K Subscript\n\n"   // "Subscript" index 87 - 96
+ "Url\n\n"           //  index 98 - 101
+ "Clickable\n\n");   // index 103 - 112


// make the text twice as large
styledString.setSpan(new RelativeSizeSpan(2f), 0, 5, 0);


// make text bold
styledString.setSpan(new StyleSpan(Typeface.BOLD), 7, 11, 0);


// underline text
styledString.setSpan(new UnderlineSpan(), 13, 23, 0);


// make text italic
styledString.setSpan(new StyleSpan(Typeface.ITALIC), 25, 31, 0);


styledString.setSpan(new StrikethroughSpan(), 33, 46, 0);


// change text color
styledString.setSpan(new ForegroundColorSpan(Color.GREEN), 48, 55, 0);


// highlight text
styledString.setSpan(new BackgroundColorSpan(Color.CYAN), 57, 68, 0);


// superscript
styledString.setSpan(new SuperscriptSpan(), 72, 83, 0);
// make the superscript text smaller
styledString.setSpan(new RelativeSizeSpan(0.5f), 72, 83, 0);


// subscript
styledString.setSpan(new SubscriptSpan(), 87, 96, 0);
// make the subscript text smaller
styledString.setSpan(new RelativeSizeSpan(0.5f), 87, 96, 0);


// url
styledString.setSpan(new URLSpan("http://www.google.com"), 98, 101, 0);


// clickable text
ClickableSpan clickableSpan = new ClickableSpan() {


@Override
public void onClick(View widget) {
// We display a Toast. You could do anything you want here.
Toast.makeText(MainActivity.this, "Clicked", Toast.LENGTH_SHORT).show();


}
};


styledString.setSpan(clickableSpan, 103, 112, 0);




// Give the styled string to a TextView
spantext = (TextView) findViewById(R.id.spantext);




// this step is mandated for the url and clickable styles.
spantext.setMovementMethod(LinkMovementMethod.getInstance());


// make it neat
spantext.setGravity(Gravity.CENTER);
spantext.setBackgroundColor(Color.WHITE);


spantext.setText(styledString);

Note : Always put android:textAllCaps="false" of your spantext.

In the code just put this "\u00B2" Like this:

textView.setText("X\u00B2");

Android String Resource Superscript and Subscript for letters

You don't really have to use html document if any of the letters you want is represented here

For "a" copy and paste this "ᵃ"

You can copy and paste any of these Superscripts and Subscripts directly into your Android String Resource.

Example:

    <string name="word_with_superscript" translatable="false">Trademark ᵀᴹ</string>

Result:Trademark ᵀᴹ

Superscript and Subscript letters

Superscript capital ᴬ ᴮ ᴰ ᴱ ᴳ ᴴ ᴵ ᴶ ᴷ ᴸ ᴹ ᴺ ᴼ ᴾ ᴿ ᵀ ᵁ ⱽ ᵂ

Superscript minuscule ᵃ ᵇ ᶜ ᵈ ᵉ ᶠ ᵍ ʰ ⁱ ʲ ᵏ ˡ ᵐ ⁿ ᵒ ᵖ ʳ ˢ ᵗ ᵘ ᵛ ʷ ˣ ʸ ᶻ

Subscript minuscule ₐ ₑ ₕ ᵢ ⱼ ₖ ₗ ₘ ₙ ₒ ₚ ᵣ ₛ ₜ ᵤ ᵥ ₓ

Based on Gerardo's answer here I created this extension on Int

fun Int.toSuperScript(): String {
return when (this) {
0 -> "\u2070"
1 -> "\u00B9"
2 -> "\u00B2"
3 -> "\u00B3"
4 -> "\u2074"
5 -> "\u2075"
6 -> "\u2076"
7 -> "\u2077"
8 -> "\u2078"
9 -> "\u2079"
else -> ""
}

}