Collector的使用

一、Collector的引入　　　　　　

1）Collector的聚合作用前面已經使用過，將list.stream後的一系列操作之後再返回list。

2）Collector的引入，通過需求：將綠色的Apple放在一個list，黃色的Apple放在一個list

代碼例子：

 1 package com.cy.java8;
 2 
 3 import java.util.*;
 4 import java.util.stream.Collectors;
 5 
 6 public class CollectorIntroduce {
 7 
 8     public static void main(String[] args) {
 9         List<Apple> list = Arrays.asList(new Apple("green", 150),
10                                             new Apple("yellow", 120),
11                                             new Apple("green", 170),
12                                             new Apple("green", 150),
13                                             new Apple("yellow", 120),
14                                             new Apple("green", 170) );
15 
16         //Collector的聚合作用
17         List<Apple> greenList = list.stream().filter(a -> a.getColor().equals("green")).collect(Collectors.toList());
18         System.out.println(greenList);
19 
20         Map<String, List<Apple>> result1 = groupByNormal(list);
21         System.out.println(result1);
22 
23         Map<String, List<Apple>> result2 = groupByFunction(list);
24         System.out.println(result2);
25 
26         //Collector的groupBy
27         Map<String, List<Apple>> result3 = groupByCollector(list);
28         System.out.println(result3);
29     }
30 
31 
32 
33     /**
34      * 需求：將綠色的放在一個list，黃色的放在一個list
35      * 以前的寫法
36      */
37     private static Map<String, List<Apple>> groupByNormal(List<Apple> apples){
38         Map<String, List<Apple>> map = new HashMap<>();
39 
40         for(Apple a : apples){
41             List<Apple> list = map.get(a.getColor());
42             if(list == null){
43                 list = new ArrayList<>();
44                 map.put(a.getColor(), list);
45             }
46             list.add(a);
47         }
48 
49         return map;
50     }
51 
52     /**
53      * 需求：將綠色的放在一個list，黃色的放在一個list
54      * 使用FunctionInterface的方法
55      * 雖然去掉了判斷null的操作，但是也還是非常囉嗦，不夠精簡
56      */
57     private static Map<String, List<Apple>> groupByFunction(List<Apple> apples){
58         Map<String, List<Apple>> map = new HashMap<>();
59 
60         apples.stream().forEach(a -> {
61             List<Apple> colorList = Optional.ofNullable(map.get(a.getColor())).orElseGet(() -> {
62                 List<Apple> list = new ArrayList<>();
63                 map.put(a.getColor(), list);
64                 return list;
65             });
66             colorList.add(a);
67         });
68 
69         return map;
70     }
71 
72     /**
73      * 需求：將綠色的放在一個list，黃色的放在一個list
74      * 使用Collector
75      */
76     private static Map<String, List<Apple>> groupByCollector(List<Apple> apples){
77         return apples.stream().collect(Collectors.groupingBy(Apple::getColor));
78     }
79 }

打印結果：

[Apple(color=green, weight=150), Apple(color=green, weight=170), Apple(color=green, weight=150), Apple(color=green, weight=170)]
{green=[Apple(color=green, weight=150), Apple(color=green, weight=170), Apple(color=green, weight=150), Apple(color=green, weight=170)], yellow=[Apple(color=yellow, weight=120), Apple(color=yellow, weight=120)]}
{green=[Apple(color=green, weight=150), Apple(color=green, weight=170), Apple(color=green, weight=150), Apple(color=green, weight=170)], yellow=[Apple(color=yellow, weight=120), Apple(color=yellow, weight=120)]}
{green=[Apple(color=green, weight=150), Apple(color=green, weight=170), Apple(color=green, weight=150), Apple(color=green, weight=170)], yellow=[Apple(color=yellow, weight=120), Apple(color=yellow, weight=120)]}

 

二、

 

 

 

 

 

 

 

 

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