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Java 8 Stream

1、关于Java8部分新特性介绍

Java8的新特性很多，在此就不一一介绍了，这里只说一下我自己在工作用用得比较多的几点：

1.1、Lambda表达式

Lambda允许把函数作为一个方法的参数（函数作为参数传递进方法中）

语法格式：

　　(parameters) -> expression 或者 (parameters) -> {statements;}

PS：

　　（1）如果参数只有一个，可以不加圆括号

　　（2）不需要声明参数类型

　　（3）如果只有一条语句，可以不加花括号

　　（4）如果只有一条语句，编译器会自动将值返回；如果多条的话，需要手动return

1.2、方法引用

方法引用通过方法的名字来指向一个方法

语法格式：

　　方法引用使用一对冒号 ::

　　构造方法引用：类::new

　　静态方法引用：类::静态方法

　　实例方法引用：类::实例方法或者对象::实例方法

1.3、Stream API

这个有点像Strom的处理方法（Spout和Blot），又有点像MapReduce（map和reduce）。用流的方式去处理，把一个集合元素转成一个一个的流，然后分别处理，最后再汇总。

1.4、接口中可以定义默认方法和静态方法

2、Stream API

 1     private List<CouponInfo> couponInfoList;
 2 
 3     private List<String> strList;
 4 
 5     private List<Integer> intList;
 6 
 7     @Before
 8     public void init() {
 9         CouponInfo couponInfo1 = new CouponInfo(123L, 10001, "5元现金券");
10         CouponInfo couponInfo2 = new CouponInfo(124L, 10001, "10元现金券");
11         CouponInfo couponInfo3 = new CouponInfo(125L, 10002, "全场9折");
12         CouponInfo couponInfo4 = new CouponInfo(126L, 10002, "全场8折");
13         CouponInfo couponInfo5 = new CouponInfo(127L, 10003, "全场7折");
14 
15         couponInfoList = new ArrayList<>();
16         couponInfoList.add(couponInfo1);
17         couponInfoList.add(couponInfo2);
18         couponInfoList.add(couponInfo3);
19         couponInfoList.add(couponInfo4);
20         couponInfoList.add(couponInfo5);
21 
22         couponInfoList = new ArrayList<>();
23         couponInfoList.add(couponInfo1);
24         couponInfoList.add(couponInfo2);
25         couponInfoList.add(couponInfo3);
26         couponInfoList.add(couponInfo4);
27         couponInfoList.add(couponInfo5);
28 
29         strList = Arrays.asList(new String[]{"A", "S", "D", "F", "X", "C", "Y", "H", "", null});
30 
31         intList = Arrays.asList(new Integer[]{1, 2, 3, 4, 5, 6, 6, 2, 3});
32     }

2.1、forEach

 1     /**
 2      * 迭代  forEach
 3      */
 4     @Test
 5     public void testForEach() {
 6         strList.stream().forEach(System.out::println);
 7         strList.stream().forEach(e->System.out.print(e));
 8         System.out.println();
 9         strList.forEach(System.out::print);
10     }

A
S
D
F
X
C
Y
H

null
ASDFXCYHnull
ASDFXCYHnull

2.2、filter

 1     /**
 2      * 过滤  filter
 3      */
 4     @Test
 5     public void testFilter() {
 6         List<String> list = strList.stream().filter(x-> StringUtils.isNotBlank(x)).collect(Collectors.toList());
 7         System.out.println(list);
 8         List<Integer> list2 = intList.stream().distinct().collect(Collectors.toList());
 9         System.out.println(list2);
10         List<CouponInfo> list3 = couponInfoList.stream().filter(x->x.getMerchantId() != 10001).collect(Collectors.toList());
11         System.out.println(list3);
12     }

[A, S, D, F, X, C, Y, H]
[1, 2, 3, 4, 5, 6]
[CouponInfo{id=125, merchantId=10002, couponName='全场9折'}, CouponInfo{id=126, merchantId=10002, couponName='全场8折'}, CouponInfo{id=127, merchantId=10003, couponName='全场7折'}]

2.3、limit

1     /**
2      * limit
3      */
4     @Test
5     public void testLimit() {
6         List<String> list = strList.stream().limit(3).collect(Collectors.toList());
7         System.out.println(list);
8     }

[A, S, D]

2.4、sorted

 1     /**
 2      * 排序  sorted
 3      */
 4     @Test
 5     public void testSorted() {
 6         List<Integer> list = intList.stream().sorted().collect(Collectors.toList());
 7         System.out.println(list);
 8         //  倒序
 9         List<Integer> list2 = intList.stream().sorted(Comparator.reverseOrder()).collect(Collectors.toList());
10         System.out.println(list2);
11 
12         List<String> list3 = strList.stream().sorted(Comparator.nullsLast(Comparator.naturalOrder())).collect(Collectors.toList());
13         List<String> list4 = strList.stream().sorted(Comparator.nullsLast(Comparator.reverseOrder())).collect(Collectors.toList());
14         System.out.println(list3);
15         System.out.println(list4);
16 
17         List<CouponInfo> list5 = couponInfoList.stream().sorted(Comparator.comparing(CouponInfo::getId)).collect(Collectors.toList());
18         List<CouponInfo> list6 = couponInfoList.stream().sorted(Comparator.comparing(CouponInfo::getId).reversed()).collect(Collectors.toList());
19         List<Long> list51 = list5.stream().map(e->e.getId()).collect(Collectors.toList());
20         List<Long> list61 = list6.stream().map(e->e.getId()).collect(Collectors.toList());
21         System.out.println(list51);
22         System.out.println(list61);
23     }

[1, 2, 2, 3, 3, 4, 5, 6, 6]
[6, 6, 5, 4, 3, 3, 2, 2, 1]
[, A, C, D, F, H, S, X, Y, null]
[Y, X, S, H, F, D, C, A, , null]
[123, 124, 125, 126, 127]
[127, 126, 125, 124, 123]

2.5、map

 1     /**
 2      * map
 3      * 对每个元素进行处理，相当于MapReduce中的map阶段
 4      * Collectors.mapping()类似
 5      */
 6     @Test
 7     public void testMap() {
 8         List<Integer> list = intList.stream().map(e->2*e).collect(Collectors.toList());
 9         System.out.println(list);
10     }

[2, 4, 6, 8, 10, 12, 12, 4, 6]

2.6、toMap

 1     /**
 2      * 转成Map<K,V>
 3      *
 4      * 特别注意，key不能重复，如果重复的话默认会报错，可以指定key重复的时候怎么处理
 5      *
 6      * 例如：Map<String, Student> studentIdToStudent = students.stream().collect(toMap(Student::getId, Functions.identity());
 7      */
 8     @Test
 9     public void testToMap() {
10         //  因为ID不重复，所以这里这么写没问题；但如果key换成CouponInfo::getMerchantId就有问题了
11         Map<Long, CouponInfo> map = couponInfoList.stream().collect(Collectors.toMap(CouponInfo::getId, Function.identity()));
12         //  这里重复的处理方式就是用后者覆盖前者
13         Map<Integer, CouponInfo> map2 = couponInfoList.stream().collect(Collectors.toMap(CouponInfo::getMerchantId, Function.identity(), (c1, c2)->c2));
14         Map<Integer, CouponInfo> map3 = couponInfoList.stream().collect(Collectors.toMap(CouponInfo::getMerchantId, Function.identity(),
15                 (c1, c2)->{if (c1.getId() > c2.getId()) {
16                     return c2;
17                 }else {
18                     return c1;
19                 }
20         }));
21         System.out.println(map);
22         System.out.println(map2);
23         System.out.println(map3);
24     }

{123=CouponInfo{id=123, merchantId=10001, couponName='5元现金券'}, 124=CouponInfo{id=124, merchantId=10001, couponName='10元现金券'}, 125=CouponInfo{id=125, merchantId=10002, couponName='全场9折'}, 126=CouponInfo{id=126, merchantId=10002, couponName='全场8折'}, 127=CouponInfo{id=127, merchantId=10003, couponName='全场7折'}}
{10001=CouponInfo{id=124, merchantId=10001, couponName='10元现金券'}, 10002=CouponInfo{id=126, merchantId=10002, couponName='全场8折'}, 10003=CouponInfo{id=127, merchantId=10003, couponName='全场7折'}}
{10001=CouponInfo{id=123, merchantId=10001, couponName='5元现金券'}, 10002=CouponInfo{id=125, merchantId=10002, couponName='全场9折'}, 10003=CouponInfo{id=127, merchantId=10003, couponName='全场7折'}}

2.6、groupingBy

 1     /**
 2      * 分组  groupingBy
 3      */
 4     @Test
 5     public void testGroupBy() {
 6         Map<Integer, List<CouponInfo>> map = couponInfoList.stream().collect(Collectors.groupingBy(CouponInfo::getMerchantId));
 7         Map<Integer, Long> map2 = couponInfoList.stream().collect(Collectors.groupingBy(CouponInfo::getMerchantId, Collectors.counting()));
 8         Map<Integer, Set<String>> map3 = couponInfoList.stream().collect(Collectors.groupingBy(CouponInfo::getMerchantId, Collectors.mapping(CouponInfo::getCouponName, Collectors.toSet())));
 9         System.out.println(map);
10         System.out.println(map2);
11         System.out.println(map3);
12     }

{10001=[CouponInfo{id=123, merchantId=10001, couponName='5元现金券'}, CouponInfo{id=124, merchantId=10001, couponName='10元现金券'}], 10002=[CouponInfo{id=125, merchantId=10002, couponName='全场9折'}, CouponInfo{id=126, merchantId=10002, couponName='全场8折'}], 10003=[CouponInfo{id=127, merchantId=10003, couponName='全场7折'}]}
{10001=2, 10002=2, 10003=1}
{10001=[10元现金券, 5元现金券], 10002=[全场9折, 全场8折], 10003=[全场7折]}

2.7、summary

 1     /**
 2      * 数值统计
 3      */
 4     @Test
 5     public void testSum() {
 6         IntSummaryStatistics summaryStatistics = intList.stream().mapToInt(x->x).summaryStatistics();
 7         System.out.println(summaryStatistics.getMax());
 8         System.out.println(summaryStatistics.getMin());
 9         System.out.println(summaryStatistics.getAverage());
10         System.out.println(summaryStatistics.getSum());
11     }

6
1
3.5555555555555554
32

3、完整代码

  1 package com.cjs.boot.demo;
  2 
  3 import com.cjs.boot.domain.entity.CouponInfo;
  4 import org.apache.commons.lang3.StringUtils;
  5 import org.junit.Before;
  6 import org.junit.Test;
  7 
  8 import java.util.*;
  9 import java.util.function.Function;
 10 import java.util.stream.Collectors;
 11 
 12 public class StreamDemoTest {
 13 
 14     private List<CouponInfo> couponInfoList;
 15 
 16     private List<String> strList;
 17 
 18     private List<Integer> intList;
 19 
 20     @Before
 21     public void init() {
 22         CouponInfo couponInfo1 = new CouponInfo(123L, 10001, "5元现金券");
 23         CouponInfo couponInfo2 = new CouponInfo(124L, 10001, "10元现金券");
 24         CouponInfo couponInfo3 = new CouponInfo(125L, 10002, "全场9折");
 25         CouponInfo couponInfo4 = new CouponInfo(126L, 10002, "全场8折");
 26         CouponInfo couponInfo5 = new CouponInfo(127L, 10003, "全场7折");
 27 
 28         couponInfoList = new ArrayList<>();
 29         couponInfoList.add(couponInfo1);
 30         couponInfoList.add(couponInfo2);
 31         couponInfoList.add(couponInfo3);
 32         couponInfoList.add(couponInfo4);
 33         couponInfoList.add(couponInfo5);
 34 
 35         couponInfoList = new ArrayList<>();
 36         couponInfoList.add(couponInfo1);
 37         couponInfoList.add(couponInfo2);
 38         couponInfoList.add(couponInfo3);
 39         couponInfoList.add(couponInfo4);
 40         couponInfoList.add(couponInfo5);
 41 
 42         strList = Arrays.asList(new String[]{"A", "S", "D", "F", "X", "C", "Y", "H", "", null});
 43 
 44         intList = Arrays.asList(new Integer[]{1, 2, 3, 4, 5, 6, 6, 2, 3});
 45     }
 46 
 47     /**
 48      * 迭代  forEach
 49      */
 50     @Test
 51     public void testForEach() {
 52         strList.stream().forEach(System.out::println);
 53         strList.stream().forEach(e->System.out.print(e));
 54         System.out.println();
 55         strList.forEach(System.out::print);
 56     }
 57 
 58     /**
 59      * 过滤  filter
 60      */
 61     @Test
 62     public void testFilter() {
 63         List<String> list = strList.stream().filter(x-> StringUtils.isNotBlank(x)).collect(Collectors.toList());
 64         System.out.println(list);
 65         List<Integer> list2 = intList.stream().distinct().collect(Collectors.toList());
 66         System.out.println(list2);
 67         List<CouponInfo> list3 = couponInfoList.stream().filter(x->x.getMerchantId() != 10001).collect(Collectors.toList());
 68         System.out.println(list3);
 69     }
 70 
 71     /**
 72      * limit
 73      */
 74     @Test
 75     public void testLimit() {
 76         List<String> list = strList.stream().limit(3).collect(Collectors.toList());
 77         System.out.println(list);
 78     }
 79 
 80     /**
 81      * 排序  sorted
 82      */
 83     @Test
 84     public void testSorted() {
 85         List<Integer> list = intList.stream().sorted().collect(Collectors.toList());
 86         System.out.println(list);
 87         //  倒序
 88         List<Integer> list2 = intList.stream().sorted(Comparator.reverseOrder()).collect(Collectors.toList());
 89         System.out.println(list2);
 90 
 91         List<String> list3 = strList.stream().sorted(Comparator.nullsLast(Comparator.naturalOrder())).collect(Collectors.toList());
 92         List<String> list4 = strList.stream().sorted(Comparator.nullsLast(Comparator.reverseOrder())).collect(Collectors.toList());
 93         System.out.println(list3);
 94         System.out.println(list4);
 95 
 96         List<CouponInfo> list5 = couponInfoList.stream().sorted(Comparator.comparing(CouponInfo::getId)).collect(Collectors.toList());
 97         List<CouponInfo> list6 = couponInfoList.stream().sorted(Comparator.comparing(CouponInfo::getId).reversed()).collect(Collectors.toList());
 98         List<Long> list51 = list5.stream().map(e->e.getId()).collect(Collectors.toList());
 99         List<Long> list61 = list6.stream().map(e->e.getId()).collect(Collectors.toList());
100         System.out.println(list51);
101         System.out.println(list61);
102     }
103 
104     /**
105      * map
106      * 对每个元素进行处理，相当于MapReduce中的map阶段
107      * Collectors.mapping()类似
108      */
109     @Test
110     public void testMap() {
111         List<Integer> list = intList.stream().map(e->2*e).collect(Collectors.toList());
112         System.out.println(list);
113     }
114 
115     /**
116      * 转成Map<K,V>
117      *
118      * 特别注意，key不能重复，如果重复的话默认会报错，可以指定key重复的时候怎么处理
119      *
120      * 例如：Map<String, Student> studentIdToStudent = students.stream().collect(toMap(Student::getId, Functions.identity());
121      */
122     @Test
123     public void testToMap() {
124         //  因为ID不重复，所以这里这么写没问题；但如果key换成CouponInfo::getMerchantId就有问题了
125         Map<Long, CouponInfo> map = couponInfoList.stream().collect(Collectors.toMap(CouponInfo::getId, Function.identity()));
126         //  这里重复的处理方式就是用后者覆盖前者
127         Map<Integer, CouponInfo> map2 = couponInfoList.stream().collect(Collectors.toMap(CouponInfo::getMerchantId, Function.identity(), (c1, c2)->c2));
128         Map<Integer, CouponInfo> map3 = couponInfoList.stream().collect(Collectors.toMap(CouponInfo::getMerchantId, Function.identity(),
129                 (c1, c2)->{if (c1.getId() > c2.getId()) {
130                     return c2;
131                 }else {
132                     return c1;
133                 }
134         }));
135         System.out.println(map);
136         System.out.println(map2);
137         System.out.println(map3);
138     }
139 
140     /**
141      * 分组  groupingBy
142      */
143     @Test
144     public void testGroupBy() {
145         Map<Integer, List<CouponInfo>> map = couponInfoList.stream().collect(Collectors.groupingBy(CouponInfo::getMerchantId));
146         Map<Integer, Long> map2 = couponInfoList.stream().collect(Collectors.groupingBy(CouponInfo::getMerchantId, Collectors.counting()));
147         Map<Integer, Set<String>> map3 = couponInfoList.stream().collect(Collectors.groupingBy(CouponInfo::getMerchantId, Collectors.mapping(CouponInfo::getCouponName, Collectors.toSet())));
148         System.out.println(map);
149         System.out.println(map2);
150         System.out.println(map3);
151     }
152 
153     /**
154      * 数值统计
155      */
156     @Test
157     public void testSum() {
158         IntSummaryStatistics summaryStatistics = intList.stream().mapToInt(x->x).summaryStatistics();
159         System.out.println(summaryStatistics.getMax());
160         System.out.println(summaryStatistics.getMin());
161         System.out.println(summaryStatistics.getAverage());
162         System.out.println(summaryStatistics.getSum());
163     }
164 
165 }

参考

http://ifeve.com/java-8-tutorial-2/

https://www.cnblogs.com/justcooooode/p/7701260.html

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原文地址：https://www.cnblogs.com/cjsblog/p/8992048.html