Java-Stream Collectors.toMap() list转map

分手后的思念是犯贱 2022-09-06 12:29 174阅读 0赞

## **一、list转map** ##

List<UserInfoDetailVo> userList = new ArrayList<>();
    //userInfo字段 id主键; nickname名称; className班级名称; age年龄； sex性别; money零花钱
    userList.add(new UserInfoDetailVo(1L,"李明","一年级一班", 7,"男",new BigDecimal("45")));
    userList.add(new UserInfoDetailVo(2L,"李明","一年级二班", 8,"男",new BigDecimal("34")));
    userList.add(new UserInfoDetailVo(3L,"张三","一年级一班", 6,"男",new BigDecimal("14")));
    userList.add(new UserInfoDetailVo(4L,"李四","一年级二班", 8,"男",new BigDecimal("11")));
    
    //以用户id值作为map的key, UserInfo对象作为map的value
    Map<Long,UserInfoDetailVo> map1 = userList.stream().collect(Collectors.toMap(UserInfoDetailVo::getId, m -> m));
    //或着：
    Map<Long,UserInfoDetailVo> map2 = userList.stream().collect(Collectors.toMap(it -> it.getId(), m -> m));
    //或着：
    Map<Long,UserInfoDetailVo> map3 = userList.stream().collect(Collectors.toMap(UserInfoDetailVo::getId, Function.identity()));
    //或着：
    Map<Long,UserInfoDetailVo> map4 = userList.stream().collect(Collectors.toMap(it -> it.getId(), Function.identity()));

***注意：Collectors.toMap必须保证key的唯一性，getId值是具有唯一性,否则会抛出异常***

## **二、list转map过程中，添加不同业务** ##

#### （1）对map的key添加具体业务 ####

Map<Long,UserInfoDetailVo> map5 = userList.stream().collect(Collectors.toMap(it -> { 
                                                        //可添加具体业务，需要return map的key
                                                        return it.getId() + 1;
                                                        }, Function.identity()));

#### （2）对map的value添加具体业务 ####

Map<Long,UserInfoDetailVo> map6 = userList.stream().collect(Collectors.toMap(it -> it.getId(), m -> { 
                                                        //可添加具体业务，需要return map的value
                                                        m.setNickname("你好：" + m.getNickname());
                                                        return m;
                                                    }));

#### （3）对map的key和value添加具体业务 ####

Map<Long,UserInfoDetailVo> map7 = userList.stream().collect(Collectors.toMap(it -> { 
    				                                        //可添加具体业务，需要return map的key
    				                                        return it.getId() + 1;
    				                                    }, m -> { 
    				                                        //可添加具体业务，需要return map的value
    				                                        m.setNickname("你好：" + m.getNickname());
    				                                        return m;
    				                                    }));

## **三、指定字段返回给map的value** ##

Map<Long,Integer> map8 = userList.stream().collect(Collectors.toMap(it -> it.getId(), it -> it.getAge());
    //或着：
    Map<Long,Integer> map9 = userList.stream().collect(Collectors.toMap(UserInfoDetailVo::getId, UserInfoDetailVo::getAge));
    
    //对map的key添加具体业务，对map的value添加具体业务
    Map<Long,String> map10 = userList.stream().collect(Collectors.toMap(it -> { 
    			                                            //可添加具体业务，需要return map的key
    			                                            return it.getId() + 1;
    			                                        }, m -> { 
    			                                            //可添加具体业务，需要return map的value
    			                                            return "你好：".concat(m.getNickname());
    			                                        }));

## **四、Collectors.toMap()有分组的功能** ##

***key重复时，可以使用(oldVal, currVal)进行特定业务处理***

//下面会抛出异常,因为key重复了
    Map<String,String> map11 = userList.stream().collect(Collectors.toMap(it -> it.getClassName(), it -> it.getNickname()));
    
    //以学生班级为key, 学生名称为value
    Map<String,String> map12 = userList.stream().collect(Collectors.toMap(it -> it.getClassName(), it -> it.getNickname(),
                                                            (oldVal, currVal) -> oldVal.concat(",").concat(currVal)));
    //"一年级二班" -> "李明,李四"
    //"一年级一班" -> "李明,张三"