how to init fields of a java class using annotations

init null fields

@Retention(value=RUNTIME)
@Target({ElementType=FIELD})
public @interface DefaultIfNUll {
  String stringValue();
  int intValue();
  boolean booleanValue();
  byte byteValue();
  short shortValue();
  long longValue();
  float floatValue();
  double doubleValue();
}

//...

public class MyClass {
  @DefaultIfNull(stringValue="some value") private String field1;
  @DefaultIfNull(intValue=123) private int field2;

  public void setField1(Object field1) {
    this.field1 = field1;
  }
}

//...

public class Defaulter {
  public static void initNullFields(Object obj) {
    for(Field field in obj.getClass().getFields()) {
      if(field.get(obj) != null) {
        DefaultIfNull dflt = field.getAnnotation(DefaultIfNull.class);
        if(anno != null) {
          
          // handle fields type
          
          field.set(obj, dflt.stringValue())
        }
      }
    }
  }
}

//...

MyClass obj = new MyClass();
obj.setField1(null);

Defaulter.initNullFields(obj);