java - Jackson serialise map with extra fields -

if class extends map , includes fields. e.g last modified date, jackson seems ignore thing not contained property of map.

i have class looks thing following:

import java.io.ioexception; import java.time.offsetdatetime; import java.util.hashmap; import java.util.objects;  import com.fasterxml.jackson.databind.objectmapper;  public class foo extends hashmap<string, string> {     private offsetdatetime lastmodifiedtime;     public foo() {       super();       lastmodifiedtime = offsetdatetime.now();    }     public void setlastmodifiedtime(offsetdatetime newtime) {       this.lastmodifiedtime = newtime;    }     public offsetdatetime getlastmodifiedtime() {       return this.lastmodifiedtime;    }     public static void main(string[] args) throws ioexception {       foo f = new foo();       f.put("hello", "world");        objectmapper om = new objectmapper();       om.findandregistermodules();        string result = om.writevalueasstring(f);       if(f.equals(om.readvalue(result, foo.class))) {          system.out.println("wooo");       } else {          system.out.println("booo: " + result);       }     }     @override    public boolean equals(object obj) {       if (this == obj) {          return true;       }       if (!(obj instanceof foo)) {          return false;       }       if (!super.equals(obj)) {          return false;       }       foo foo = (foo) obj;       return objects.equals(getlastmodifiedtime(), foo.getlastmodifiedtime());    }     @override    public int hashcode() {       return objects.hash(super.hashcode(), getlastmodifiedtime());    }  } 

when runs outputs booo: {"hello":"world"} not include last modified date.

i've tried adding @jsoninclude annotations property , getters doesn't cause include field.

what correct way make jackson include field?

it's considered bad practice extend hashmap. instead, should create class has map, , additional field.

this solve problem, have additional advantages:

• you able encapsulate rules (like preventing keys/values added, etc.)
• you able change map implementation if needed (linkedhashmap, synchronized map, concurrent map)