GSON: Illegal Argument exception when converting a json string to a Java object -

i'm working youtube api. can convert java object json string. when try reverse (json string java object) throws exception: can not set com.google.api.services.youtube.model.commentthreadsnippet field com.google.api.services.youtube.model.commentthread.snippet java.util.linkedhashmap. why error , how fix it?

this works:

  commentthread commentthread = getitsomehow()   gson gson = new gson();   string jsonstr = gson.tojson( commentthread ); 

reverse throws exception:

 commentthread fromjson = gson.fromjson( jsonstr, commentthread.class ); 

here commentthread class:

@suppresswarnings("javadoc")     public final class commentthread extends com.google.api.client.json.genericjson {    /**    * etag of resource.    * value may {@code null}.    */   @com.google.api.client.util.key   private java.lang.string etag;    /**    * id youtube uses uniquely identify comment thread.    * value may {@code null}.    */   @com.google.api.client.util.key   private java.lang.string id;    /**    * identifies kind of resource is. value: fixed string "youtube#commentthread".    * value may {@code null}.    */   @com.google.api.client.util.key   private java.lang.string kind;    /**    * replies object contains limited number of replies (if any) top level comment found    * in snippet.    * value may {@code null}.    */   @com.google.api.client.util.key   private commentthreadreplies replies;    /**    * snippet object contains basic details comment thread , top level    * comment.    * value may {@code null}.    */   @com.google.api.client.util.key   private commentthreadsnippet snippet;    /**    * etag of resource.    * @return value or {@code null} none    */   public java.lang.string getetag() {     return etag;   }    /**    * etag of resource.    * @param etag etag or {@code null} none    */   public commentthread setetag(java.lang.string etag) {     this.etag = etag;     return this;   }    /**    * id youtube uses uniquely identify comment thread.    * @return value or {@code null} none    */   public java.lang.string getid() {     return id;   }    /**    * id youtube uses uniquely identify comment thread.    * @param id id or {@code null} none    */   public commentthread setid(java.lang.string id) {     this.id = id;     return this;   }    /**    * identifies kind of resource is. value: fixed string "youtube#commentthread".    * @return value or {@code null} none    */   public java.lang.string getkind() {     return kind;   }    /**    * identifies kind of resource is. value: fixed string "youtube#commentthread".    * @param kind kind or {@code null} none    */   public commentthread setkind(java.lang.string kind) {     this.kind = kind;     return this;   }    /**    * replies object contains limited number of replies (if any) top level comment found    * in snippet.    * @return value or {@code null} none    */   public commentthreadreplies getreplies() {     return replies;   }    /**    * replies object contains limited number of replies (if any) top level comment found    * in snippet.    * @param replies replies or {@code null} none    */   public commentthread setreplies(commentthreadreplies replies) {     this.replies = replies;     return this;   }    /**    * snippet object contains basic details comment thread , top level    * comment.    * @return value or {@code null} none    */   public commentthreadsnippet getsnippet() {     return snippet;   }    /**    * snippet object contains basic details comment thread , top level    * comment.    * @param snippet snippet or {@code null} none    */   public commentthread setsnippet(commentthreadsnippet snippet) {     this.snippet = snippet;     return this;   }    @override   public commentthread set(string fieldname, object value) {     return (commentthread) super.set(fieldname, value);   }    @override   public commentthread clone() {     return (commentthread) super.clone();   }  } 

below details of exception.

throwable: can not set com.google.api.services.youtube.model.commentthreadsnippet field com.google.api.services.youtube.model.commentthread.snippet java.util.linkedhashmap java.lang.illegalargumentexception: can not set com.google.api.services.youtube.model.commentthreadsnippet field com.google.api.services.youtube.model.commentthread.snippet java.util.linkedhashmap     @ sun.reflect.unsafefieldaccessorimpl.throwsetillegalargumentexception(unsafefieldaccessorimpl.java:167)     @ sun.reflect.unsafefieldaccessorimpl.throwsetillegalargumentexception(unsafefieldaccessorimpl.java:171)     @ sun.reflect.unsafeobjectfieldaccessorimpl.set(unsafeobjectfieldaccessorimpl.java:81)     @ java.lang.reflect.field.set(field.java:764)     @ com.google.api.client.util.fieldinfo.setfieldvalue(fieldinfo.java:245)     @ com.google.api.client.util.fieldinfo.setvalue(fieldinfo.java:206)     @ com.google.api.client.util.genericdata.put(genericdata.java:103)     @ com.google.api.client.util.genericdata.put(genericdata.java:47)     @ com.google.gson.internal.bind.maptypeadapterfactory$adapter.read(maptypeadapterfactory.java:189)     @ com.google.gson.internal.bind.maptypeadapterfactory$adapter.read(maptypeadapterfactory.java:146)     @ com.google.gson.gson.fromjson(gson.java:755)     @ com.google.gson.gson.fromjson(gson.java:721)     @ com.google.gson.gson.fromjson(gson.java:670)     @ com.google.gson.gson.fromjson(gson.java:642)     @ sam.ut.commentthreads.handlecommentsthreads(commentthreads.java:157)     @ sam.ut.commentthreads.downloadcommentsforthisvideo(commentthreads.java:84)     @ sam.ut.commentthreads.main(commentthreads.java:66) 

edit: , here json:

{   "etag": "\"0fu6li6vpldrmlqu3wwncowdaus/f7byskj8z3darwhusu5zinnzkqw\"",   "id": "z12wxpxozzy1tdxop04cjd1rjzq4cp2jgfw",   "kind": "youtube#commentthread",   "snippet": {     "canreply": true,     "ispublic": true,     "toplevelcomment": {       "etag": "\"0fu6li6vpldrmlqu3wwncowdaus/pzvv_vfrn7czovsiilpnvseiwe0\"",       "id": "z12wxpxozzy1tdxop04cjd1rjzq4cp2jgfw",       "kind": "youtube#comment",       "snippet": {         "authorchannelid": {           "value": "ucojtszsvhfakfqprwn3etlq"         },         "authorchannelurl": "http://www.youtube.com/channel/ucojtszsvhfakfqprwn3etlq",         "authordisplayname": "spruce moose arts",         "authorgoogleplusprofileurl": "https://plus.google.com/102869292172467105853",         "authorprofileimageurl": "https://lh6.googleusercontent.com/-ejgw19enrkq/aaaaaaaaaai/aaaaaaaaayg/khd3hglrox4/photo.jpg?sz=50",         "canrate": true,         "likecount": 0,         "publishedat": "2016-06-13t22:49:52.000z",         "textdisplay": "the man behind camera recorded this, must shaky, \ncondsidering amount of warp stabilization on video :p also, \nthink matter of personal taste. think car \npol filter looked better.",         "updatedat": "2016-06-13t22:49:52.000z",         "videoid": "yczkcnps45s",         "viewerrating": "none"       }     },     "totalreplycount": 0,     "videoid": "yczkcnps45s"   } }