Java, Convert instance of Class to HashMap

Question

If I have class like this:

class MyObject {
    public int myInt;
    public String myString;
}

Is it possible to convert instance of this class to HashMap without implementing converting code?

MyObject obj = new MyObject();
obj.myInt = 1; obj.myString = "string";
HashMap<String, Object> hs = convert(obj);

hs.getInt("myInt"); // returns 1
hs.getString("myString"); // returns "string"

Does Java provide that kind of solution, or I need to implement convert by myself?

My Class has more than 50 fields and writing converter for each field is not so good idea.

Hashmaps have keys and values (2 things), rather than 50 fields. Can you clarify what you are trying to achieve here? — Tim Biegeleisen
– Tim Biegeleisen, Commented May 4, 2016 at 9:59
you´d also have to provide a custom Map implementation, because the valid method to get the value for a key from the Map is the get(Object) method. There is nothing like getInt or getString in order to return different types from the Map.In the end it wouldn´t make any sense to have them, because the return type is defined by the second generic type. — SomeJavaGuy
– SomeJavaGuy, Commented May 4, 2016 at 9:59
@TimBiegeleisen I want to convert all fields of my object to keys and values of hashmap. — Mr.D
– Mr.D, Commented May 4, 2016 at 10:02
@Mr.D could you eleborate a usecase for this, because i can´t see any. Calling a simple getter method would be more easy to me, especially since changing a single value in any of this fields would make the formerly generated Map an invalid one. — SomeJavaGuy
– SomeJavaGuy, Commented May 4, 2016 at 10:04

Ömer Erden · Accepted Answer · 2019-12-06 07:07:07Z

28

With jackson library this is also possible

MyObject obj = new MyObject();
obj.myInt = 1;
obj.myString = "1";
ObjectMapper mapObject = new ObjectMapper();
Map < String, Object > mapObj = mapObject.convertValue(obj, Map.class);

edited Dec 6, 2019 at 7:07

Ömer Erden

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answered Aug 14, 2018 at 18:19

profcalculus

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2 Comments

hmartos Over a year ago

To me, this is the quickest and less prone to error solution. Here is a reference with a more complete example using this solution mkyong.com/java/java-convert-object-to-map-example

somebody Over a year ago

One caveat is that this solution will not keep the enum values type, it will transform them to String name /or index.

PKuhn · Accepted Answer · 2016-05-04 10:01:54Z

10

You can use reflection for implementing this behavior. You can get all fields of the class you want to convert to map iterate over this fields and take the name of each field as key of the map. This will result in a map from String to object.

Map<String, Object> myObjectAsDict = new HashMap<>();    
Field[] allFields = SomeClass.class.getDeclaredFields();
    for (Field field : allFields) {
        Class<?> targetType = field.getType();
        Object objectValue = targetType.newInstance();
        Object value = field.get(objectValue);
        myObjectAsDict.put(field.getName(), value);
    }
}

answered May 4, 2016 at 10:01

PKuhn

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2 Comments

Mr.D Over a year ago

If some fields are Lists of objects? How to "reflect" them?

PKuhn Over a year ago

If the object relations form a tree and not a graph you can use the method described here stackoverflow.com/questions/6133660/…

Nicolas Filotto · Accepted Answer · 2016-05-04 10:07:36Z

5

Something like that will do the trick:

MyObject obj = new MyObject();
obj.myInt = 1; obj.myString = "string";
Map<String, Object> map = new HashMap<>();
// Use MyObject.class.getFields() instead of getDeclaredFields()
// If you are interested in public fields only
for (Field field : MyObject.class.getDeclaredFields()) {
    // Skip this if you intend to access to public fields only
    if (!field.isAccessible()) {
        field.setAccessible(true);
    }
    map.put(field.getName(), field.get(obj));
}
System.out.println(map);

Output:

{myString=string, myInt=1}

answered May 4, 2016 at 10:07

Nicolas Filotto

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Comments

Mike Murphy · Accepted Answer · 2016-05-04 10:05:20Z

0

You might consider using a map instead of a class.

Or have your class extend a map such as

public class MyObject extends HashMap<String, Object> {

}

answered May 4, 2016 at 10:05

Mike Murphy

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Comments

Abdul Mohsin · Accepted Answer · 2018-12-19 10:12:24Z

If you don't want to use Reflection then you can use my trick. hope this may help for someone.

Suppose your class looks like this.

public class MyClass {
         private int id;
         private String name; 
}

Now Override toString() method in this class. in Eclipse there is a shortcut for generating this method also.

public class MyClass {
    private int id;
    private String name; 

    @Override
    public String toString() {
        StringBuilder builder = new StringBuilder();
        builder.append("MyClass [id=").append(id).append(", name=").append(name).append("]");
        return builder.toString();
    }

}

Now write a method inside this class that will convert your object into Map<String,String>

public Map<String, String> asMap() {
        Map<String, String> map = new HashMap<String, String>();
        String stringRepresentation = this.toString();
        if (stringRepresentation == null || stringRepresentation.trim().equals("")) {
            return map;
        }
        if (stringRepresentation.contains("[")) {
            int index = stringRepresentation.indexOf("[");
            stringRepresentation = stringRepresentation.substring(index + 1, stringRepresentation.length());
        }
        if (stringRepresentation.endsWith("]")) {
            stringRepresentation = stringRepresentation.substring(0, stringRepresentation.length() - 1);
        }
        String[] commaSeprated = stringRepresentation.split(",");
        for (int i = 0; i < commaSeprated.length; i++) {
            String keyEqualsValue = commaSeprated[i];
            keyEqualsValue = keyEqualsValue.trim();
            if (keyEqualsValue.equals("") || !keyEqualsValue.contains("=")) {
                continue;
            }
            String[] keyValue = keyEqualsValue.split("=", 2);
            if (keyValue.length > 1) {
                map.put(keyValue[0].trim(), keyValue[1].trim());
            }

        }
        return map;
    }

Now from any where in your application you can simply call this method to get your HashMap from the Object. Cheers

A.Casanova · Accepted Answer · 2023-01-31 12:37:08Z

0

Updated approach using reflection:

  public static <T> Map<String, String> parseInnerClass(T classInstance) {
    LinkedHashMap<String, String> ret = new LinkedHashMap<>();

    for (Field attr : classInstance.getClass().getDeclaredFields()) {
      String attrValue = "";
      attr.setAccessible(true);

      try {
        attrValue = attr.get(classInstance).toString();
      } catch (IllegalAccessException | NullPointerException e) {
        // Do not add nothing
      }

      ret.put(attr.getName(), attrValue);
    }
    return ret;
  }

answered Jan 31, 2023 at 12:37

A.Casanova

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