java8_StringJoinerUsingCustomStreamCollector.java

List<String> names = Arrays.asList("one","two ","three");

// fancy builtin collector with delimiter
String s = names.stream().collect(Collectors.joining(", "));
System.out.println(s);
// "one, two , three"

// name elements does not get trimmed, so, need custom. 
// Let's see how we can get there with the custom collector collect(supplier,accumulator,multiplier)

// custom collector using method references (without delimiter)
StringBuilder sb = names.stream().collect(StringBuilder::new, // supplier
                                          StringBuilder::append, // accumulator
                                          StringBuilder::append); // multiplier
System.out.println(sb.toString());
// "onetwo three"

// Exact same as above using lambdas (without delimiter)
sb = names.stream().collect(() -> {return new StringBuilder();}, // supplier
                            (sb1,name) -> {sb1.append(name);}, // accumulator
                            (sb1,sb2) -> {sb1.append(sb2);}); // multiplier
System.out.println(sb.toString());
// "onetwo three"


// Exac collector using lambdas with the delimiter logic.
final String delim = ", ";
sb = names.stream().collect(() -> {return new StringBuilder();}, // supplier
                            (sb1,name) -> { // accumulator
                              if (sb1.length() > 0){
                                sb1.append(delim);
                              }
                              sb1.append(name);
                            },
                            (sb1,sb2) -> {sb1.append(sb2);}); // multiplier
System.out.println(sb.toString());
// "one, two , three"

// trim and lambda only as needed (only accumulator needs to have custom logic)
sb = names.stream().collect(StringBuilder::new, // supplier
                            (sb1,name) -> { // accumulator
                              if (sb1.length() > 0){
                                sb1.append(delim);
                              }
                              sb1.append(name.trim());
                            },
                            StringBuilder::append); // multiplier
System.out.println(sb.toString());
// "one, two, three"

// Conclusion:
// + Stream.collect API it much simpler than it looks, and very flexible.
// + Use builtin collector when possible.
// + Do not be afraid to do you custom collector.
// + Most of the time, custom code is just necessary on the accumulator.