通过流将地图列表转换为单个地图

Question

我在DB中查询两列,其中第一列是第二列的键.如何将结果列表转换为单个地图？它甚至可能吗？我刚看过bean的例子.

List<Map<String, Object>> steps = jdbcTemplate.queryForList("SELECT key, value FROM table");

// well this doesn't work
Map<String, String> result = steps.stream().collect(Collectors.toMap(s -> s.get("key"), s -> s.get("value")));

Answer 1

我们可以使用java流的reduce()将一系列map转换为java中的单个map。

请检查以下代码以了解其使用方式。

例如：

@Data
@AllArgsConstructor
public class Employee {

    private String employeeId;
    private String employeeName;
    private Map<String,Object> employeeMap;
}


public class Test{
 public static void main(String[] args) {
 
        Map<String, Object> map1 = new HashMap<>();
        Map<String, Object> map2 = new HashMap<>();
        Map<String, Object> map3 = new HashMap<>();
        
        
        map1.put("salary", 1000);
        Employee e1 = new Employee("e1", "employee1", map1);

        map2.put("department", "HR");
        Employee e2 = new Employee("e2", "employee2", map2);

        map3.put("leave balance", 14);
        Employee e3 = new Employee("e3", "employee3", map3);
        
        //now we create a employees list and add the employees e1,e2 and e3.
        List<Employee> employeeList = Arrays.asList(e1,e2,e3);
         
        //now we retreive employeeMap from all employee objects and therefore have a List of employee maps.
        List<Map<String, Object>> employeeMaps = employeeList
        .stream()
        .map(Employee::getEmployeeMap)
        .collect(Collectors.toList());

        System.out.println("List of employee maps: " + employeeMaps);
        
        // to reduce a list of maps to a single map, we use the reduce function of stream.
        
        Map<String, Object> finalMap = employeeMaps
        .stream()
        .reduce((firstMap, secondMap) -> {
                firstMap.putAll(secondMap);
                 return firstMap;
              }).orElse(null);
               
        System.out.println("final Map: "+ finalMap);             
        
}
}

输出：员工地图列表：[{salary=1000}, {department=HR}, {leave Balance=14}]。

最终地图：{工资=1000，部门=HR，休假余额=14}

PS：对扩展答案表示歉意，这是我第一次使用 stackoverflow。谢谢 ：-）

Answer 2

您忘记转换键和值映射以生成String:

final Map<String, String> result = steps
                .stream()
                .collect(Collectors.toMap(s -> (String) s.get("key"), s -> (String) s.get("value")));

完整的例子

public static void main(String[] args) {
    final List<Map<String, Object>> steps = queryForList("SELECT key, value FROM table");
    final Map<String, String> result = steps
            .stream()
            .collect(Collectors.toMap(s -> (String) s.get("key"), s -> (String) s.get("value")));
    result.entrySet().forEach(e -> System.out.println(e.getKey() + " -> " + e.getValue()));
}

private static List<Map<String, Object>> queryForList(String s) {
    final List<Map<String, Object>> result = new ArrayList<>();

    for (int i = 0; i < 10; i++) {
        final Map<String, Object> map = new HashMap<>();
        map.put("key", "key" + i);
        map.put("value", "value" + i);
        result.add(map);
    }

    return result;
}

哪个打印

key1 -> value1
key2 -> value2
key0 -> value0
key5 -> value5
key6 -> value6
key3 -> value3
key4 -> value4
key9 -> value9
key7 -> value7
key8 -> value8