Java Mapping error

Hi Experts,

I am practicing the java mapping in PI,  My requirement is simple I just want to concatenate the two input strings. And my code is like this

I have few doubts :-

1) execute method is mandatory 

CODE :-

import java.io.*;
import java.util.*;
import com.sap.aii.mapping.api.AbstractTransformation;
import com.sap.aii.mapping.api.StreamTransformationException;
import com.sap.aii.mapping.api.TransformationInput;
import com.sap.aii.mapping.api.TransformationOutput;
import com.sap.aii.mapping.api.AbstractTrace;

import javax.xml.parsers.DocumentBuilder;
import javax.xml.parsers.DocumentBuilderFactory;

import org.w3c.dom.Document;
import org.w3c.dom.NodeList;
import org.w3c.dom.Node;

public class Java_Test extends AbstractTransformation {
public String var1;
public String var2;

public void transform(TransformationInput input, TransformationOutput output)throws StreamTransformationException
{
 
  InputStream ins=input.getInputPayload().getInputStream();
  OutputStream outs=output.getOutputPayload().getOutputStream();
  int n,m;
  String result=null;
   
  try{
   DocumentBuilderFactory dfactory=DocumentBuilderFactory.newInstance();
   DocumentBuilder dbuilder=dfactory.newDocumentBuilder();
   Document doc=dbuilder.parse(ins);
   NodeList nl=doc.getElementsByTagName("PMI_SOURCE_PROD");
   NodeList nl1=doc.getElementsByTagName("LOGISTA_CODE");
  
   String PMI_SOURCE_PROD=nl.item(0).getTextContent();
   String LOGISTA_CODE=nl1.item(0).getTextContent();
  
   n=nl.getLength();
   m=nl.getLength();
   if(n==m)
   {
   for(int i=0;i<=n;i++)
   {
    result=PMI_SOURCE_PROD+LOGISTA_CODE;
   }
  
   outs.getOutputPayload().getOutputStream().Writes(result.getBytes());  // I am getting the error in this line Kindly check and help me in this.
    }
  }
  catch(Exception e)
  {
   System.out.println(e);
  }
 
}
 
  }

Thanks ,

Vijay Kumar K.V.N