通过流将地图列表转换为单个地图

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

You forgot to convert your key and value mappings to produce String :

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

Full example

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;
}

Which prints

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

Answer 2

The problem is you have a list of maps. The code below should work:

Map<String, String> result = new HashMap<>();
steps.stream().forEach(map -> {
    result.putAll(map.entrySet().stream()
        .collect(Collectors.toMap(entry -> entry.getKey(), entry -> (String) entry.getValue())));
});

If we try to run this example

Map<String, Object> steps1 = new HashMap<>();
steps1.put("key11", "value11");
steps1.put("key12", "value12");

Map<String, Object> steps2 = new HashMap<>();
steps2.put("key21", "value21");
steps2.put("key22", "value22");

List<Map<String, Object>> steps = new ArrayList<>();
steps.add(steps1);
steps.add(steps2);

Map<String, String> result = new HashMap<>();
steps.stream().forEach(map -> {
    result.putAll(map.entrySet().stream()
        .collect(Collectors.toMap(entry -> entry.getKey(), entry -> (String) entry.getValue())));
});
System.out.println(result);

It happily gives us the output like that:

{key12=value12, key11=value11, key22=value22, key21=value21}

Answer 3

Adding on to what esin88 said above, if there is a possibility that we get duplicate keys in the list of maps, we would like to get a map with key and list of values rather than key value pair which can be done like below.

    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")));*/
    final Map<String, List<String>> result1 = steps.stream()
            .collect(Collectors.groupingBy(k -> String.valueOf(k.get("key")),
                    Collectors.mapping(l -> String.valueOf(l.get("value")), Collectors.toList())));
    result1.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<>();
    Map<String, Object> map = new HashMap<>();
    for (int i = 0; i < 10; i++) {
        map = new HashMap<>();
        map.put("key", "key" + i);
        map.put("value", "value" + i);
        result.add(map);
    }
    map = new HashMap<>();
    map.put("key", "key1");
    map.put("value", "value20");
    result.add(map);

    return result;
}

The result would look like

key1 -> [value1, value20]
key2 -> [value2]
key0 -> [value0]
key5 -> [value5]
key6 -> [value6]
key3 -> [value3]
key4 -> [value4]
key9 -> [value9]
key7 -> [value7]
key8 -> [value8]

Answer 4

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 :-)