What is the purpose of these interfaces?

CharSequence is a standard Java interface representing a sequence of characters. String is the most commonly-used concrete implementation of CharSequence, followed by StringBuilder.

Spanned is a CharSequence with "spans" indicating formatting to apply to portions of the text, where those spans cannot be modified.

Spannable is a Spanned, adding in the ability to modify the spans (to add or remove formatting), but not to modify the text itself.

SpannedString is a concrete implementation of the Spanned interface.

SpannableString is a concrete implementation of the Spannable interface.

in which scenarios is it mostly common to use them?

When there is a method that returns one (e.g., getText() on an EditText) or when there is a method that takes one as a parameter (e.g., setText() on a TextView).

Your cited case of using Html.fromHtml() is perhaps the most common in conventional Android development, as a TextView with a Spanned is much lighter in weight than is a WebView. However, there are other use cases, such as:

• Highlighting search results

• Allowing users to enter in rich text, then using Html.toHtml() to persist that formatted text in an HTML rendition

In which cases is it best to avoid using them?

They are singularly awful at combating baldness, snow removal, heat pump repair, making a soufflé, etc.

:-)

Are there any obvious performance impacts to be considered when using any one of them?

Interfaces, by definition, do not have "performance impacts" -- they are merely a description of an API.

I am not aware that SpannableString is significantly slower than SpannedString at any particular operation. However, SpannableStringBuilder (which allows for manipulating the text in addition to the spans that format that text) may well be a bit slower than SpannableString or SpannedString for various things. Whether or not the performance differences are enough to matter will depend on usage, though.

String

A String is immutable (ie, the text can't change). It also doesn't have any spans associated with it. (Spans are ranges over the text that include styling information like color, highlighting, italics, links, etc.) So you can use a String when your text doesn't need to be changed and doesn't need any styling.

StringBuilder

A StringBuilder has mutable text, so you can modify it without creating a new object. However, it doesn't have any span information. It is just plain text. So use a StringBuilder when you need to change the text, but you don't care about styling it.

SpannedString

A SpannedString has immutable text (like a String) and immutable span information. It is a concrete implementation of the requirements defined by the Spanned interface. Use a SpannedString when your text has style but you don't need to change either the text or the style after it is created.

Note: There is no such thing as a SpannedStringBuilder because if the text changed then the span information would also very likely have to change.

SpannableString

A SpannableString has immutable text, but its span information is mutable. It is a concrete implementation of the requirements defined by the Spannable interface. Use a SpannableString when your text doesn't need to be changed but the styling does.

SpannableStringBuilder

A SpannableStringBuilder has both mutable text and span information. It is a concrete implementation of the requirements defined by the Spannable and Editable interfaces (among others). Use a SpannableStringBuilder when you will need to update the text and its style.

CharSequence

A CharSequence is an interface and not a concrete class. That means it just defines a list of rules to follow for any class that implements it. And all of the classes mentioned above implement it. So you can use a CharSequence when you want to generalize the type of object that you have for maximum flexibility. You can always downcast it to a String or SpannableStringBuilder or whatever later if you need to.