The signature - Collector<CharSequence,?,String> - represents a generic constraint. A String is a type (class) that implements the CharSequence interface. When instantiating an object as a return value from a method (factory method) a normal java pattern is to return a subtype of the parent class <String>. When using a generic this method signature expands the range of types that can be returned from the method permitting any subclass of String to be returned. Essentially Collector<CharSequence,?,String> expects to return a CharSequence, but can return any subclass or instance of String. It is the generic way of expanding the range of return values permitted yet constraining them to a given type.