在这篇教程中，将向你展示如何使用Java 8 Stream的Collectors，来对一个List进行分组，计算个数，求和以及排序。

1. Group By, Count and Sort

1.1 对一个List进行分组，并显示每组的个数

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 加入排序

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&lt;&gt;();

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

        System.out.println(finalMap);


    }
}

output

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

2. List Objects

对用户定义的对象数组进行分组的例子。

2.1 一个简单java对象.

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 通过name属性分组，并显示分组个数或Qty属性的数量

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 通过Price分组 – 使用Collectors.groupingBy 和 Collectors.mapping 的例子.

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) ][Stackoverflow _ Sort a Map by values Java]

[Stackoverflow _ Sort a Map by values Java]: http://stackoverflow.com/questions/109383/sort-a-mapkey-value-by-values-java