Java 8 付諸實踐

public class Trader {
  private final String name;
  private final String city;

  public Trader(String name, String city) {
    this.name = name;
    this.city = city;
  }

  public String getName() {
    return name;
  }

  public String getCity() {
    return city;
  }

  @Override
  public String toString() {
    return "Trader{" + "name='" + name + '\'' + ", city='" + city + '\'' + '}';
  }
}

public class Transaction {
  private final Trader trader;
  private final int year;
  private final int value;

  public Transaction(Trader trader, int year, int value) {
    this.trader = trader;
    this.year = year;
    this.value = value;
  }

  @Override
  public String toString() {
    return "Transaction{" + "trader=" + trader + ", year=" + year + ", value=" + value + '}';
  }

  public Trader getTrader() {
    return trader;
  }

  public int getYear() {
    return year;
  }

  public int getValue() {
    return value;
  }
}

public void test() {
    Trader raoul = new Trader("Raoul", "Cambridge");
    Trader mario = new Trader("Mario", "Milan");
    Trader alan = new Trader("Alan", "Cambridge");
    Trader brian = new Trader("Brian", "Cambridge");
    List<Transaction> transactions =
        Arrays.asList(
            new Transaction(brian, 2011, 300),
            new Transaction(raoul, 2012, 1000),
            new Transaction(raoul, 2011, 400),
            new Transaction(mario, 2012, 710),
            new Transaction(mario, 2012, 700),
            new Transaction(alan, 2012, 950));
    // (1) 找出2011年發生的所有交易，並按交易額排序（從低到高）
    transactions.stream()
        .filter(transaction -> transaction.getYear() == 2012)
        .sorted(Comparator.comparing(Transaction::getValue))
        .collect(Collectors.toList());

    // (2) 交易員都在哪些不同的城市工作過
    transactions.stream()
        .map(transaction -> transaction.getTrader().getCity())
        .distinct()
        .collect(Collectors.toList());

    // (3) 查找所以來自於劍橋的交易員，並按照姓名順序排序
    transactions.stream()
        .map(Transaction::getTrader)
        .filter(trader -> "Cambridge".equals(trader.getCity()))
        .distinct()
        .sorted(Comparator.comparing(Trader::getName))
        .collect(Collectors.toList());

    // (4) 返回所有交易員的姓名字符串，並按照字母排序
    transactions.stream()
        .map(transaction -> transaction.getTrader().getName())
        .distinct()
        .sorted()
        .collect(Collectors.joining());

    // (5) 打印生活在劍橋的交易員的所有交易額
    transactions.stream()
        .filter(transaction -> transaction.getTrader().getName().equals("Cambridge"))
        .forEach(transaction -> System.out.println(transaction.getValue()));

    // (6) 有沒有交易員是在米蘭工作的
    boolean milanBased =
        transactions.stream()
            .anyMatch(transaction -> transaction.getTrader().getName().equals("Milan"));

    // (7) 所有交易中，最高的交易額是多少
    Optional<Integer> highestValue =
        transactions.stream().map(Transaction::getValue).reduce(Integer::max);
    // (8) 找到交易額最小的交易
    transactions.stream()
        .reduce((a, b) -> a.getValue() > b.getValue() ? b : a)
        .ifPresent(transaction -> System.out.println(transaction.getValue()));
  }

 參考書籍：Java8實戰