one.java

package somepackage;

import java.util.Optional;
import org.stellar.sdk.Account;
import org.stellar.sdk.Operation;
import org.stellar.sdk.Server;
import org.stellar.sdk.SubmissionResponse;
import org.stellar.sdk.SubmissionSuccessResponse;

public class Main {

  //  A simple example that illustrates refunding the last payment sent to
  //  an address
  public static void main(String[] args) {
    String id = "GCEJJI5DOVSLDVXROAU6J66T67YONNWUU2BTDQZDXXQM4NJ4JMNEASVA";
    Server s  = Server.testNet();
    Account a = s.getAccount(id);

    // reading data from horizon is simple (assuming java 8 here and us
    // implementing Stream<T> for Page<T>).
    Optional<Operation> lastReceived = a.payments()
      .order("DESC")
      .execute()
      .stream()
      .filter(p -> p.getTo() == id)
      .findFirst()

    if (!lastReceived.isPresent()) {
      return;
    }

    // the account object provides a method into the authoring portions of the
    // library, returning a builder that we can call methods on to construct a
    // new payments.
    SubmissionResponse resp = a.newPayment()
      .destination(lastReceived.getFrom())
      .asset(lastReceived.getAsset())
      .amount(lastReceived.getAmount())
      .memoReturn(lastReceived.getTxHash())
      .sign("SCZJN2SXIIVJ5TCN72ZPNMSGMOMXB3XP3UC66OPQ3YP6GZNGFL2HVFZA")
      .submit();

    if (resp.isFailure()) {
      // TODO
      return
    } 

    SubmissionSuccessResponse success = resp.getSuccess();

    System.out.println("tx:" + success.getTxHash() + " paging_token: " + success.getTransaction().getPagingToken());
  }
}