Java 8 Stream Collectors groupingBy

展示如何使用Java 8 Stream Collectors进行分组，计数，总和和排序List

分组，计数和排序

注：输出结果中的中文均应为英文。

1.1分组List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

{
	番木瓜= 1，橙= 1，香蕉= 2，苹果= 3
}

1.2添加排序。

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

{
	苹果= 3，香蕉= 2，木瓜= 1，橙= 1
}

2.List Objects

“分组”用户定义的对象列表的示例。

2.1 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 按姓名+数字或数量组合。

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
{
	番木瓜= 1，香蕉= 2，苹果= 3，猩猩= 1，西瓜= 1
}

// Group by + Sum qty
{
	番木瓜= 20，香蕉= 30，苹果= 40，orang = 10，西瓜= 10
}

2.2按价格分组 - Collectors.groupingBy以Collectors.mapping示例为例。

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 +映射到Set
{
	19.99 = [香蕉] 
	29.99 = [orang，西瓜]， 
	9.99 = [番木瓜，苹果]
}