Let's map a list of words using Java 8 streams:

List<String> words = Arrays.asList("A", "bbb", "CC", "dd");

// Map the words to UPPERCASE and join them with the ":" separator
String joined = words.stream()
        .collect(Collectors.joining(":")); // "A:BBB:CC:DD"

Example of mapping Java 8 streams using the Stream#map method.

Calling the map method of a stream returns another stream with the mapped values. In this example the words are mapped into their uppercase versions, and the resulting stream is converted to a string by collecting all of its items and concatenating/joining them with a joining collector, using the ":" separator.