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"); Map result = items.stream().collect( Collectors.groupingBy( Function.identity(), Collectors.counting() ) ); System.out.println(result); }
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"); Map result = items.stream().collect( Collectors.groupingBy( Function.identity(), Collectors.counting() ) ); Map finalMap = new LinkedHashMap<>(); //Sort a map and add to finalMap result.entrySet().stream() .sorted(Map.Entry.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")) ); Map 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); }
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] }
