Convert List of Maps to single Map via streams

Question

I query the DB for two columns where the first one is the key to the second one. How can I convert the resulting list to a single map? Is it even possible? I have just seen examples with beans.

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

We can use the reduce() of java streams to convert a list of maps to a single map in java.

Please check the following code on how it is used.

For example:

@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);             
        
}
}

Output: List of employee maps: [{salary=1000}, {department=HR}, {leave balance=14}].

final Map: {salary=1000, department=HR, leave balance=14}

PS: Apologies for the extended answer, this is my first time in stackoverflow. Thank you :-)