Java 8 – List分组GroupBy

转载自：http://blog.csdn.net/u013078669/article/details/52717142

1. 分组, 计数和排序

1.1 分组, 计数

    public static void main(String[] args) {

     //3 apple, 2 banana, others 1
     List items =
             Arrays.asList("apple", "apple", "banana",
                     "apple", "orange", "banana", "papaya");

     Maplong=""> result =
             items.stream().collect(
                     Collectors.groupingBy(
                             Function.identity(), Collectors.counting()
                     )
             );

     System.out.println(result);


 }
 /string>

输出

 {
     papaya=1, orange=1, banana=2, apple=3
 }

1.2 分组, 计数和排序

  public static void main(String[] args) {

         //3 apple, 2 banana, others 1
         List items =
                 Arrays.asList("apple", "apple", "banana",
                         "apple", "orange", "banana", "papaya");

         Maplong=""> result =
                 items.stream().collect(
                         Collectors.groupingBy(
                                 Function.identity(), Collectors.counting()
                         )
                 );

         Maplong=""> finalMap = new LinkedHashMap<>();

         //Sort a map and add to finalMap
         result.entrySet().stream()
                 .sorted(Map.Entry.long="">comparingByValue()
                         .reversed()).forEachOrdered(e -> finalMap.put(e.getKey(), e.getValue()));

         System.out.println(finalMap);


     }

输出:

 {
     apple=3, banana=2, papaya=1, orange=1
 }

2.用户自定义对象集合分组, 计数、排序和求和

 public static void main(String[] args) {

        //3 apple, 2 banana, others 1
        List items = Arrays.asList(
                new Item("apple", 10, new BigDecimal("9.99")),
                new Item("banana", 20, new BigDecimal("19.99")),
                new Item("orang", 10, new BigDecimal("29.99")),
                new Item("watermelon", 10, new BigDecimal("29.99")),
                new Item("papaya", 20, new BigDecimal("9.99")),
                new Item("apple", 10, new BigDecimal("9.99")),
                new Item("banana", 10, new BigDecimal("19.99")),
                new Item("apple", 20, new BigDecimal("9.99"))
        );

        Maplong=""> counting = items.stream().collect(
                Collectors.groupingBy(Item::getName, Collectors.counting()));

        System.out.println(counting);

        Map""> sum = items.stream().collect(
                Collectors.groupingBy(Item::getName, Collectors.summingInt(Item::getQty)));

        System.out.println(sum);

    }
 lt;/string>

输出

 //Group by + Count
 {
     papaya=1, banana=2, apple=3, orang=1, watermelon=1
 }

 //Group by + Sum qty
 {
     papaya=20, banana=30, apple=40, orang=10, watermelon=10
 }

 public static void main(String[] args) {

         //3 apple, 2 banana, others 1
         List items = Arrays.asList(
                 new Item("apple", 10, new BigDecimal("9.99")),
                 new Item("banana", 20, new BigDecimal("19.99")),
                 new Item("orang", 10, new BigDecimal("29.99")),
                 new Item("watermelon", 10, new BigDecimal("29.99")),
                 new Item("papaya", 20, new BigDecimal("9.99")),
                 new Item("apple", 10, new BigDecimal("9.99")),
                 new Item("banana", 10, new BigDecimal("19.99")),
                 new Item("apple", 20, new BigDecimal("9.99"))
                 );

         //group by price
         Map> groupByPriceMap =
             items.stream().collect(Collectors.groupingBy(Item::getPrice));

         System.out.println(groupByPriceMap);

         // group by price, uses 'mapping' to convert List to Set
         Map> result =
                 items.stream().collect(
                         Collectors.groupingBy(Item::getPrice,
                                 Collectors.mapping(Item::getName, Collectors.toSet())
                         )
                 );

         System.out.println(result);

     }

输出

 {
     19.99=[
             Item{name='banana', qty=20, price=19.99},
             Item{name='banana', qty=10, price=19.99}
         ],
     29.99=[
             Item{name='orang', qty=10, price=29.99},
             Item{name='watermelon', qty=10, price=29.99}
         ],
     9.99=[
             Item{name='apple', qty=10, price=9.99},
             Item{name='papaya', qty=20, price=9.99},
             Item{name='apple', qty=10, price=9.99},
             Item{name='apple', qty=20, price=9.99}
         ]
 }

 //group by + mapping to Set
 {
     19.99=[banana],
     29.99=[orang, watermelon],
     9.99=[papaya, apple]
 }