Java : How to create map using the class attributes?

Question

I am trying to create a map from all the attributes that a class have.My class looks like :

public class MyInventory
{
   private int tiers = 80;
   private int stearing =135;
   private int battery = 46;

 }

Now I have collected all the methods that the class has as :

Field[] fields = this.getClass().getDeclaredFields();

Now , I am trying to create a Map out of it where keys are the values of the fields and the values are the name of the fields. Example :

Map<46,battery> ...etc

Is there a way to do it? The attribute values for the above mentioned class were generated by mapping to properties file and by using spring annotation @ConfigurationProperties. Now I need to create the Map but keys the values of the attributes. I tried to use reflect. However did not find a way to get the value of the fields.

Thanks

So you want to create a Map<int,String> - what's the issue? — Balwinder Singh
– Balwinder Singh, Commented Oct 12, 2018 at 3:30
It sound strange, we often create key by field name not vice versa. Anyway, it's possible. Just use reflect api docs.oracle.com/javase/7/docs/api/java/lang/reflect/Field.html — Huy Nguyen
– Huy Nguyen, Commented Oct 12, 2018 at 3:36
Why in the world would you need a map if you already have your class done? Wouldn't it be simpler if you work with a list of MyInventory? — nicolasl
– nicolasl, Commented Oct 12, 2018 at 4:25

Firman Wijaya · Accepted Answer · 2018-10-12 04:39:15Z

2

You can use Introspector class.

public Map<Object, String> populateMap(final Object o) throws Exception {
  Map<Object, String> result = new HashMap<>();
  for (PropertyDescriptor pd : Introspector.getBeanInfo(o.getClass()).getPropertyDescriptors()) {
    String fieldName = pd.getName();
    if("class".equals(fieldName) continue;
    Object value = pd.getReadMethod().invoke(o);
    result.put(value, fieldName);
  }
  return result;
}

You can call the above method, passing your class as argument.

MyInventory mi = new MyInventory();
// Sets the properties of mi
mi.setXXX...
// Populates map
populateMap(mi);

edited Oct 12, 2018 at 4:39

answered Oct 12, 2018 at 3:47

Firman Wijaya

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1 Comment

VictorGram Over a year ago

I am getting :Exception in thread "main" java.lang.ClassCastException: java.lang.Class cannot be cast to java.lang.Integer

c0der · Accepted Answer · 2018-10-12 03:52:59Z

2

  Map<Integer, String> map() throws IllegalArgumentException, IllegalAccessException {

       Field[] fields = getClass().getDeclaredFields();
       Map<Integer,String> map = new HashMap<>();
       for (Field field : fields) {
           map.put(field.getInt(this), field.getName());
       }
       return map;
   }

Of course it will not map properly if different fields have the same value.

answered Oct 12, 2018 at 3:52

c0der

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Comments

Ravi · Accepted Answer · 2018-10-12 03:35:34Z

1

I think, you can have getter method in your class

public class MyInventory
{
   private int tiers = 80;
   private int stearing =135;
   private int battery = 46;

   public int getBattery()
   {
       return battery;
   }
  //and other getter 
 }

and then you can populate your map as

map.put(inventory.getBattery(),"battery");

Because, when you have value, which means you know what is the type for which you are populating map.

answered Oct 12, 2018 at 3:35

Ravi

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Comments

Uros Velickovic · Accepted Answer · 2021-01-23 21:38:19Z

0

You can use json parser. For example jackson:

  import com.fasterxml.jackson.databind.ObjectMapper;
  ... 
  ObjectMapper mapper = new ObjectMapper();
  return mapper.readValue(mapper.writeValueAsString(fooOject), HashMap.class);

answered Jan 23, 2021 at 21:38

Uros Velickovic

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