分組

//List 以ID分組 Map<Integer,List<Apple>>
Map<Integer, List<Apple>> groupBy = appleList.stream().collect(Collectors.groupingBy(Apple::getId));
System.err.println("groupBy:"+groupBy);
{1=[Apple{id=1, name='蘋果1', money=3.25, num=10}, Apple{id=1, name='蘋果2', money=1.35, num=20}], 2=[Apple{id=2, name='香蕉', money=2.89, num=30}], 3=[Apple{id=3, name='荔枝', money=9.99, num=40}]}

 

List ---> Map

/**
 * List -> Map
 * 需要注意的是：
 * toMap 如果集合物件有重複的key，會報錯Duplicate key ....
 *  apple1,apple12的id都為1。
 *  可以用 (k1,k2)->k1 來設定，如果有重複的key,則保留key1,捨棄key2
 */
Map<Integer, Apple> appleMap = appleList.stream().collect(Collectors.toMap(Apple::getId, a -> a,(k1,k2)->k1));

 

過濾

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);

最大值和最小值

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

 

去重

import static java.util.Comparator.comparingLong;
import static java.util.stream.Collectors.collectingAndThen;
import static java.util.stream.Collectors.toCollection;
 
// 根據id去重
     List<Person> unique = appleList.stream().collect(
                collectingAndThen(
                        toCollection(() -> new TreeSet<>(comparingLong(Apple::getId))), ArrayList::new)
        )