// streams/OptionalFlatMap.java
// (c)2016 MindView LLC: see Copyright.txt
// We make no guarantees that this code is fit for any purpose.
// Visit http://OnJava8.com for more book information.
import java.util.*;
import java.util.stream.*;
import java.util.function.*;

class OptionalFlatMap {
  static String[] elements = { "12", "", "23", "45" };
  static Stream<String> testStream() {
    return Arrays.stream(elements);
  }
  static void test(String descr,
    Function<String, Optional<String>> func) {
    System.out.println(" ---( " + descr + " )---");
    for(int i = 0; i <= elements.length; i++) {
      System.out.println(
        testStream()
          .skip(i)
          .findFirst()
          .flatMap(func));
    }
  }
  public static void main(String[] args) {

    test("Add brackets", s -> Optional.of("[" + s + "]"));

    test("Increment", s -> {
      try {
        return Optional.of(Integer.parseInt(s) + 1 + "");
      } catch(NumberFormatException e) {
        return Optional.of(s);
      }
    });

    test("Replace",
      s -> Optional.of(s.replace("2", "9")));

    test("Take last digit",
      s -> Optional.of(s.length() > 0 ?
        s.charAt(s.length() - 1) + ""
        : s));
  }
}
/* Output:
 ---( Add brackets )---
Optional[[12]]
Optional[[]]
Optional[[23]]
Optional[[45]]
Optional.empty
 ---( Increment )---
Optional[13]
Optional[]
Optional[24]
Optional[46]
Optional.empty
 ---( Replace )---
Optional[19]
Optional[]
Optional[93]
Optional[45]
Optional.empty
 ---( Take last digit )---
Optional[2]
Optional[]
Optional[3]
Optional[5]
Optional.empty
*/