Java 8 Stream

分组

Map<Integer, List<Apple>> groupBy = appleList.stream()
    .collect(Collectors.groupingBy(Apple::getId));

Map<Boolean, List<Person>> part = personList.stream()
        .collect(Collectors.partitioningBy(x -> x.getSalary() > 8000));

List 转 Map

Map<Integer, Apple> appleMap = appleList.stream()
    .collect(Collectors.toMap(Apple::getId, a -> a,(k1,k2)->k1));

过滤

List<Apple> filterList = appleList.stream()
    .filter(a -> a.getName().equals("香蕉")).collect(Collectors.toList());

求和

BigDecimal totalMoney = appleList.stream()
    .map(Apple::getMoney).reduce(BigDecimal.ZERO, BigDecimal::add);

最大值

Optional<Dish> maxDish = Dish.menu.stream().
      collect(Collectors.maxBy(Comparator.comparing(Dish::getCalories)));
maxDish.ifPresent(System.out::println);

最小值

Optional<Dish> minDish = Dish.menu.stream().
      collect(Collectors.minBy(Comparator.comparing(Dish::getCalories)));
minDish.ifPresent(System.out::println);

去重

List<Person> unique = appleList.stream().collect(
                collectingAndThen(
                        toCollection(() -> new TreeSet<>(comparingLong(Apple::getId))), ArrayList::new)
        );

原文

java8 快速实现List转map 、分组、过滤等操作