Cant write json output to a XML file

Below is my code to specifically get values at 

result._source.city

node.

import net.sf.*;
import net.sf.json.*;
import net.sf.json.groovy.*;
import net.sf.json.groovy.JsonSlurper;

def ResFile ="C:/driver/Response1.xml"
def Res = context.expand( '${GetAllSchoolsForNZ#Response}' )

def slurper = new JsonSlurper()

def json = slurper.parseText Res

def countryList = json.result._source.city
log.info countryList
def j = new File(ResFile)
j.write(countryList, "UTF-8",true)

But when i execute it im gettint an error like this.

No signature of method: java.io.File.write() is applicable for argument types: (java.util.ArrayList, java.lang.String, java.lang.Boolean) values: [[Rotorua, Christchurch, Auckland, Auckland, ...], ...] Possible solutions: write(java.lang.String, java.lang.String), write(java.lang.String), wait(), size(), canWrite(), delete()