JDK8系列一：Stream流

0、Account.java

 1 package com.moon.bean;
 2 
 3 import lombok.AllArgsConstructor;
 4 import lombok.Data;
 5 import lombok.NoArgsConstructor;
 6 
 7 import java.io.Serializable;
 8 
 9 @Data
10 @AllArgsConstructor
11 @NoArgsConstructor
12 public class Account implements Serializable {
13     /**
14      * 身份证号
15      */
16     private String id;
17 
18     /**
19      * 银行卡号
20      */
21     private String bankCardNo;
22 
23     /**
24      * 用户名
25      */
26     private String username;
27 
28     /**
29      * 手机号码
30      */
31     private String mobile;
32 
33     /**
34      * 邮箱
35      */
36     private String email;
37 
38 }

 

1、搜集list集合中的身份证号和用户名，将其转换成Map<String,String>

 1 /**
 2  * 搜集list集合中的身份证号和用户名，将其转换成Map<String,String>
 3  */
 4 @Test
 5 public void convertList2Map1() {
 6     List<Account> accountList = new ArrayList<>();
 7     Account account1 = new Account("411856198009092331", "6220201", "刘备", "18513668898", "liubei@qq.com");
 8     Account account2 = new Account("411856198009092332", "6220202", "关羽", "18513668891", "guanyu@qq.com");
 9     Account account3 = new Account("411856198009092333", "6220203", "张飞", "18513668892", "zhangfei@qq.com");
10     Account account4 = new Account("411856198009092334", "6220204", "赵云", "18513668893", "zhaoyun@qq.com");
11     accountList.addAll(Arrays.asList(account1, account2, account3, account4));
12     System.out.println(accountList.size());
13     System.out.println(accountList);
14     for (Account account : accountList) {
15         System.out.println(account);
16     }
17     Map<String, String> streamMap = accountList.stream().collect(Collectors.toMap(Account::getId, Account::getUsername));
18     System.out.println("streamMap.size():" + streamMap.size());
19     System.out.println(JSON.toJSONString(streamMap));
20 }

 

 

2、搜集list集合中的身份证号和账户实体，将其转换成Map<String,Account>

 1 /**
 2  * 搜集list集合中的身份证号和账户实体，将其转换成Map<String,Account>
 3  * account -> account是一个返回本身lambda的表达式，其实还可以使用Function接口中的一个默认方法Function.identity()，这个方法返回自身对象，更加简洁
 4  */
 5 @Test
 6 public void convertList2Map2() {
 7     List<Account> accountList = new ArrayList<>();
 8     Account account1 = new Account("411856198009092331", "6220201", "刘备", "18513668898", "liubei@qq.com");
 9     Account account2 = new Account("411856198009092332", "6220202", "关羽", "18513668891", "guanyu@qq.com");
10     Account account3 = new Account("411856198009092333", "6220203", "张飞", "18513668892", "zhangfei@qq.com");
11     Account account4 = new Account("411856198009092334", "6220204", "赵云", "18513668893", "zhaoyun@qq.com");
12     accountList.addAll(Arrays.asList(account1, account2, account3, account4));
13     Map<String, Account> streamMap1 = accountList.stream().collect(Collectors.toMap(Account::getId, account -> account));
14     Map<String, Account> streamMap2 = accountList.stream().collect(Collectors.toMap(Account::getId, Function.identity()));
15     System.out.println("streamMap1.size():" + streamMap1.size());
16     System.out.println("streamMap1 json:" + JSON.toJSONString(streamMap1));
17     System.out.println("streamMap2.size():" + streamMap2.size());
18     System.out.println("streamMap2 json:" + JSON.toJSONString(streamMap2));
19 }

 

 

 

 

3、搜集list集合中的身份证号和账户实体，将其转换成Map<String,Account>,重复key情况

 1 /**
 2  * 搜集list集合中的身份证号和账户实体，将其转换成Map<String,Account>,重复key情况
 3  * 在list转为map时，作为key的值有可能重复，这时候流的处理会抛出个异常：Java.lang.IllegalStateException:Duplicate key,
 4  * 这时候就要在toMap方法中指定当key冲突时key的选择。(这里是选择第二个key2覆盖第一个key1)
 5  */
 6 @Test
 7 public void convertList2Map3() {
 8     List<Account> accountList = new ArrayList<>();
 9     Account account1 = new Account("411856198009092331", "6220201", "刘备", "18513668898", "liubei@qq.com");
10     Account account2 = new Account("411856198009092331", "6220202", "刘备", "18513668898", "liubei@qq.com");
11     Account account3 = new Account("411856198009092333", "6220203", "张飞", "18513668892", "zhangfei@qq.com");
12     Account account4 = new Account("411856198009092334", "6220204", "赵云", "18513668893", "zhaoyun@qq.com");
13     accountList.addAll(Arrays.asList(account1, account2, account3, account4));
14     //Map<String, Account> streamMap = accountList.stream().collect(Collectors.toMap(Account::getId, Function.identity()));
15     Map<String, Account> streamMap = accountList.stream().collect(Collectors.toMap(Account::getId, Function.identity(),(key1,key2) -> key2));
16     System.out.println(streamMap.size());
17     System.out.println(JSON.toJSONString(streamMap));
18 }

 

 

 

 参考：https://blog.csdn.net/zlj1217/article/details/81611834

 

posted @ 2022-01-05 13:33  谁怕？一蓑烟雨任平生  阅读(49)  评论(0)    收藏  举报