分组

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

[java] view plain copy
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);  

}
/string>
输出
[text] view plain copy
{
papaya=1, orange=1, banana=2, apple=3
}
1.2 分组, 计数和排序

[java] view plain copy
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);  


}  

输出:
[text] view plain copy
{
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

[java] view plain copy
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);  

}
lt;/string>
输出
[text] view plain copy
//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
}
[java] view plain copy
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);  

}  

输出
[text] view plain copy
{
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]
}