在这篇文章中，我们将向您展示如何使用java 8  Stream Collectors 对列表分组，计数，求和和排序。

1. Group By, Count and Sort

1.1 Group by a List and display the total count of it.（按列表分组，并显示其总数）

Java8Example1.java

package com.mkyong.java8; import java.util.Arrays; import java.util.List; import java.util.Map; import java.util.function.Function; import java.util.stream.Collectors; public class Java8Example1 { public static void main(String[] args) { //3 apple, 2 banana, others 1 List<String> items = Arrays.asList("apple", "apple", "banana", "apple", "orange", "banana", "papaya"); Map<String, Long> result = items.stream().collect( Collectors.groupingBy( Function.identity(), Collectors.counting() ) ); System.out.println(result); } }

output

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

1.2 Add sorting.（增加排序实现）

Java8Example2.java

package com.mkyong.java8; import java.util.Arrays; import java.util.LinkedHashMap; import java.util.List; import java.util.Map; import java.util.function.Function; import java.util.stream.Collectors; public class Java8Example2 { public static void main(String[] args) { //3 apple, 2 banana, others 1 List<String> items = Arrays.asList("apple", "apple", "banana", "apple", "orange", "banana", "papaya"); Map<String, Long> result = items.stream().collect( Collectors.groupingBy( Function.identity(), Collectors.counting() ) ); Map<String, Long> finalMap = new LinkedHashMap<>(); //Sort a map and add to finalMap result.entrySet().stream() .sorted(Map.Entry.<String, Long>comparingByValue() .reversed()).forEachOrdered(e -> finalMap.put(e.getKey(), e.getValue())); System.out.println(finalMap); } }

output

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

2. List Objects

Examples to ‘group by’ a list of user defined Objects.（通过“用户定义的对象”列表进行分组的示例。）

2.1 A Pojo.

Item.java

package com.mkyong.java8; import java.math.BigDecimal; public class Item { private String name; private int qty; private BigDecimal price; //constructors, getter/setters }

2.2 Group by the name + Count or Sum the Qty. （name + Count分组或者 对 Qty求和分组）

Java8Examples3.java

package com.mkyong.java8; import java.math.BigDecimal; import java.util.Arrays; import java.util.List; import java.util.Map; import java.util.stream.Collectors; public class Java8Examples3 { public static void main(String[] args) { //3 apple, 2 banana, others 1 List<Item> 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<String, Long> counting = items.stream().collect( Collectors.groupingBy(Item::getName, Collectors.counting())); System.out.println(counting); Map<String, Integer> sum = items.stream().collect( Collectors.groupingBy(Item::getName, Collectors.summingInt(Item::getQty))); System.out.println(sum); } }

output

//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
}

2.2  Price 分组 – Collectors.groupingBy and Collectors.mapping例子.

Java8Examples4.java

package com.mkyong.java8; import java.math.BigDecimal; import java.util.Arrays; import java.util.List; import java.util.Map; import java.util.Set; import java.util.stream.Collectors; public class Java8Examples4 { public static void main(String[] args) { //3 apple, 2 banana, others 1 List<Item> 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<BigDecimal, List<Item>> groupByPriceMap = items.stream().collect(Collectors.groupingBy(Item::getPrice)); System.out.println(groupByPriceMap); // group by price, uses 'mapping' to convert List<Item> to Set<String> Map<BigDecimal, Set<String>> result = items.stream().collect( Collectors.groupingBy(Item::getPrice, Collectors.mapping(Item::getName, Collectors.toSet()) ) ); System.out.println(result); } }

output

{
	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]
}

References

1. Java 8 Stream Collectors JavaDoc
2. Java – How to sort a Map
3. Stackoverflow – Sort a Map by values (Java)