How to create list of Maps from List of Object in java without having getKey method?

praxnet :

How to create a list of maps, where each key name is inferred from name of the class attribute, and value is to be put by getter method

I am having following class in java

class DTA {
  private String id;
  private String age;

  @Override
  public String toString() {
    return "DTA{" +
            "id='" + id + '\'' +
            ", age='" + age + '\'' +
            '}';
  }

  public DTA(String id, String age) {
    this.id = id;
    this.age = age;
  }

  public String getId() {
    return id;
  }

  public void setId(String id) {
    this.id = id;
  }

  public String getAge() {
    return age;
  }

  public void setAge(String age) {
    this.age = age;
  }
}

I am having a list of objects of type DTA

List<DTA> listA = new ArrayList<>();
listA.add(new DTA("A", "15"));
listA.add(new DTA("B", "25"));

I want to create an ordered list of maps (somewhat like scala) which has following content.

List<? extends Map<String, String>>

List(Map("id"->"A", "age"->"15"), Map("id"->"B", "age"->"25"))

GhostCat salutes Monica C. :

Without "dynamics", the straight forward thing might look like:

List<Map<String, String>> x = listA
            .stream()
            .map(this::toMap)
            .collect(Collectors.toList());

with a local helper, such as:

private Map<String, String> toMap(DTA dta) {
    Map<String, String> rv = new HashMap<>();
    rv.put("id", dta.getId());
    rv.put("age", dta.getAge());
    return rv;
}

In order to be fully dynamic here, you would have to use reflection to query the field names. You can find examples how to do that here.

But I strongly suggest to not do that: reflection should always be your last resort. The notion of DTA suggests that you have that object data coming from some "service" anyway. If so, why first serialize into a specific DTA class, to then "flatten" that information into some generic Map structure?!

Meaning: when that service gives you objects that are serialized as, say JSON, or XML ... then it would be much better to simply use a library like gson or jackson to directly deserialize that data into such generic "flat" Map-based objects. Jackson for example has a JsonNode class. When you deserialize into such objects, you get that mapping of field names for free! See here more example code.

The point is: identifying fields using reflection is possible. But reflection code is always tedious, and error prone. If possible, stay away from doing that yourself.