//To following code creates a metadata connection
import com.sforce.soap.metadata.*;
import com.sforce.soap.partner.LoginResult;
import com.sforce.soap.partner.PartnerConnection;
import com.sforce.ws.ConnectionException;
import com.sforce.ws.ConnectorConfig;
/**
* Login utility.
*/
public class MetadataLoginUtil {
public static MetadataConnection login() throws ConnectionException {
  final String USERNAME = ""username@gmail.com"";
  // This is only a sample. Hard coding passwords in source files is a bad practice.
  final String PASSWORD = ""Password+SecurityToken"";
  final String URL = ""https://login.salesforce.com/services/Soap/u/31.0"";

  //Quick Start Step 3: Walk through the Java Sample Code
  final LoginResult loginResult = loginToSalesforce(USERNAME, PASSWORD, URL);
  //System.out.println(loginResult);
  return createMetadataConnection(loginResult);
}
private static MetadataConnection createMetadataConnection(
   final LoginResult loginResult) throws ConnectionException {
  final ConnectorConfig config = new ConnectorConfig();
  config.setServiceEndpoint(loginResult.getMetadataServerUrl());
  config.setSessionId(loginResult.getSessionId());
  return new MetadataConnection(config);
}
private static LoginResult loginToSalesforce(
   final String username,
   final String password,
   final String loginUrl) throws ConnectionException {
  final ConnectorConfig config = new ConnectorConfig();
  config.setAuthEndpoint(loginUrl);
  config.setServiceEndpoint(loginUrl);
  config.setManualLogin(true);
  return (new PartnerConnection(config)).login(username, password);
}

}

//The following code creates a custom field using  metadata conncetion and createMetadata() method.

import java.util.ArrayList;
import java.util.concurrent.ForkJoinPool.ManagedBlocker;
import com.sforce.async.Error;
import com.sforce.soap.metadata.*;
import com.sforce.soap.metadata.FieldType;
import com.sforce.soap.metadata.Package;
import com.sforce.soap.metadata.SaveResult;
import com.sforce.soap.partner.*;
import com.sforce.ws.ConnectionException;
import com.sforce.ws.ConnectorConfig;
import com.sforce.ws.wsdl.SfdcApiType;
/**
* Login utility.
*/
public class PartnerLoginUtil {

public static void main(String[] args) throws Exception {
  // TODO Auto-generated method stub
  try {
   PartnerLoginUtil p= new PartnerLoginUtil( );
   p.createCustomExtField(""CustomObjectName"");

  } catch (ConnectionException e) {
   // TODO Auto-generated catch block
   e.printStackTrace();
  }
}

private void createCustomExtField(String name) {
  // TODO Auto-generated method stub
String country[] = {"US","UK"};
CustomField c = new CustomField();
 c.setType(FieldType.Picklist);
 c.setPicklist(country);
  cs.setFullName(name+"".CustExtField__c"");
  cs.setLabel(""CustExtField"");
    try {
   MetadataConnection metadataConnection = MetadataLoginUtil.login();
   SaveResult[] results = metadataConnection
     .updateMetadata(new Metadata[] {cs});

   for (SaveResult r : results) {
    if (r.isSuccess()) {
     System.out.println(""Created component: "" + r.getFullName());
    } else {
     System.out
     .println(""Errors were encountered while creating ""
       + r.getFullName());
     for (com.sforce.soap.metadata.Error e : r.getErrors()) {
      System.out.println(""Error message: "" + e.getMessage());
      System.out.println(""Status code: "" + e.getStatusCode());
     }
    }
   }
  } catch (ConnectionException e) {
   // TODO Auto-generated catch block
   e.printStackTrace();
  }

}

}