SpannableStringBuilder to create String with multiple fonts/text sizes etc Example?

First Part Not Bold   BOLD  rest not bold

You can do this either as @Rajesh suggested or by this.

String normalBefore= "First Part Not Bold ";
String normalBOLD=  "BOLD ";
String normalAfter= "rest not bold";
String finalString= normalBefore+normalBOLD+normalAfter;
Spannable sb = new SpannableString( finalString );
sb.setSpan(new StyleSpan(android.graphics.Typeface.BOLD), finalString.indexOf(normalBOLD)+ normalBOLD.length(), Spannable.SPAN_EXCLUSIVE_EXCLUSIVE); //bold
sb.setSpan(new AbsoluteSizeSpan(intSize), finalString.indexOf(normalBOLD)+ normalBOLD.length(), Spannable.SPAN_EXCLUSIVE_EXCLUSIVE);//resize size

to show this in TextView

textview.setText(sb,  TextView.BufferType.SPANNABLE);

The accepted answer is fine (and I upvoted it), but it fails to use the SpannableStringBuilder as the submitter requested. As I had a case where the Builder made the most sense, here is the code for that (with a bonus use of also changing the color of the text if that is helpful to others). Note that you could also provide the initial string to the SpannableStringBuilder constructor, but I set it here to use "append" to be clear that you can append a lot before your desired "bold" text and then just record the start as shown. I would suspect that this is also faster code than the accepted answer.

SpannableStringBuilder longDescription = new SpannableStringBuilder();
longDescription.append("First Part Not Bold ");
int start = longDescription.length();
longDescription.append("BOLD");
longDescription.setSpan(new ForegroundColorSpan(0xFFCC5500), start, longDescription.length(), Spannable.SPAN_EXCLUSIVE_EXCLUSIVE);
longDescription.setSpan(new StyleSpan(android.graphics.Typeface.BOLD), start, longDescription.length(), Spannable.SPAN_EXCLUSIVE_EXCLUSIVE);
longDescription.append(" rest not bold");

If you are using Kotlin you can do the following using the android-ktx library

val s = SpannableStringBuilder()
        .append("First Part Not Bold ")
        .bold { append("BOLD") } 
        .append("Rest not bold")

The bold is an extension function on SpannableStringBuilder. You can see the documentation here for a list of operations you can use.

Another example:

val s = SpannableStringBuilder()
            .color(green, { append("Green text ") })
            .append("Normal text ")
            .scale(0.5, { append("Text at half size " })
            .backgroundColor(green, { append("Background green") })

Where green is a resolved RGB color.

It is even possible to nest spans so you end up with an embedded DSL:

bold { underline { italic { append("Bold and underlined") } } }

You will need the following in your app module level build.gradle for it to work:

repositories {
    google()
}

dependencies {
    implementation "androidx.core:core-ktx:1.2.0"
}