How to set underline text on textview?

You have to use SpannableString for that :

String mystring=new String("Hello.....");
SpannableString content = new SpannableString(mystring);
content.setSpan(new UnderlineSpan(), 0, mystring.length(), 0);
yourtextview.setText(content);

Update : You can refer my answer on Underling TextView's here in all possible ways.

Use this

tvHide.setPaintFlags(tvHide.getPaintFlags() | Paint.UNDERLINE_TEXT_FLAG);

Following are the some approaches for underlined text in android:

1st Approach

You can define your string in strings.xml

<string name="your_string"><u>Underlined text</u></string>

And use that string in your xml file

<TextView
            android:id="@+id/txt_underlined"
            android:layout_width="wrap_content"
            android:layout_height="match_parent"
            android:text="@string/your_string"/>

Or you can use that string in your Activity/Fragment

txtView.setText(R.string.your_string);

2nd Approach

To underline the text in TextView, you can use SpannableString

String text="Underlined Text";
SpannableString content = new SpannableString(text);
content.setSpan(new UnderlineSpan(), 0, text.length(), 0);
txtView.setText(content);

3rd Approach

You can make use of setPaintFlags method of TextView to underline the text of TextView.

txtView.setPaintFlags(mTextView.getPaintFlags() | Paint.UNDERLINE_TEXT_FLAG);
txtView.setText("Underlined Text");

4th Approach

Make use of Html.fromHtml(htmlString);

String htmlString="<u>Underlined Text</u>";
txtView.setText(Html.fromHtml(htmlString));

Or

txtView.setText(Html.fromHtml("<u>underlined</u> text"));

Note:

If you have added this line android:textAllCaps="true" in your layout, then none of the above will work. For that, you have to define your string in Caps and then any of the above approach.