stream流中toMap()api和Duplicate key问题

1、指定key-value，value是对象中的某个属性值。

 Map<Integer,String> userMap = userList.stream().collect(Collectors.toMap(User::getId,User::getName));

2、指定key-value，value是对象本身

User->User 是一个返回本身的lambda表达式

Map<Integer,User> userMap = userList.stream().collect(Collectors.toMap(User::getId,User->User));

Function.identity()是简洁写法，也是返回对象本身

Map<Integer,User> userMap = userList.stream().collect(Collectors.toMap(User::getId, Function.identity()));

3、返回其他Map

解决了Key冲突并转为了TreeMap

userList.stream().collect(Collectors.toMap(User::getName, Function.identity(), (oldVal, newVal) -> newVal, TreeMap::new));

4、解决重复key问题

如果生成Map时有重复key(通过key类型的equals方法来判断)就会报错：java.lang.IllegalStateException: Duplicate key。

当发生重复时这里选择第二个key覆盖第一个key的value值,当然如果需要第一个key的value值那就选择oldKey(可提前先进行指定规则的排序)

Map<Integer,User> userMap4 = userList.stream().collect(Collectors.toMap(User::getId, Function.identity(),(oldKey,newKey)->newKey));

PS:当然也可以提前将集合进行去重处理(stream根据指定字段去重)再进行简单的toMap操作。

5、其他示例

• 重复时将前面的value 和后面的value拼接起来

Map<String, String> map = list.stream().collect(Collectors.toMap(Person::getId, Person::getName,(key1 , key2)-> key1+","+key2 ));

//或者将其值相加
Map<String, BigDecimal> map = list.stream().collect(Collectors.toMap(Person::getNo, Person::amount,BigDecimal::add));

• value转为其他DTO实体并解决key重复问题

Map<Integer, ProductCategoryResultDTO> resultDTOMap = byCategoryFidAndEnvironmentTag.stream()
                    .collect(Collectors.toMap(ProductCategory::getCategoryId, v -> {
                        ProductCategoryResultDTO productCategoryResultDTO = new ProductCategoryResultDTO();
productCategoryResultDTO.setStatus(v.getUplowState());
                        return productCategoryResultDTO;
                    }, (key1, key2) -> key2));

• 代码实战：

public static void main(String[] args) throws JsonProcessingException {
        UserInfo u1 = new UserInfo(1, "张三");
        UserInfo u2 = new UserInfo(2, "李四");
        UserInfo u3 = new UserInfo(3, "王五");
        UserInfo u4 = new UserInfo(4, "赵六1");
        UserInfo u5 = new UserInfo(4, "赵六2");
        List<UserInfo> list = Arrays.asList(u1,u2,u3,u4,u5);

        Map<Integer, String> map = list.stream()
                .collect(Collectors.toMap(UserInfo::getId, UserInfo::getUserName, (key1, key2) -> key1 + "," + key2));
        String mapStr = new ObjectMapper().writeValueAsString(map);
        System.out.println(mapStr);

        Set<Object> set = new HashSet<>();
        Map<Integer, String> map1 = list.stream()
                .collect(Collectors.toMap(UserInfo::getId, UserInfo::getUserName, (key1, key2) -> {
                    set.add(key1);
                    return key1;
                }));
        System.out.println(new ObjectMapper().writeValueAsString(map1));
        System.out.println("set:" + set);
    }

执行结果：