My code(below) displays only one "message" at a time and then prompts the user to delete the message in order to create a new one.

How can I make it so that the user can constantly add new messages without having to delete the previous message?

import java.security.SecureRandom;
import java.util.Iterator;
import java.util.LinkedList;
import java.util.List;

import com.google.appengine.api.datastore.DatastoreService;
import com.google.appengine.api.datastore.DatastoreServiceFactory;
import com.google.appengine.api.datastore.Entity;
import com.google.appengine.api.datastore.Key;
import com.google.appengine.api.datastore.Query;
import com.google.appengine.api.datastore.Query.FilterOperator;
import com.google.appengine.api.datastore.Query.SortDirection;
import com.google.appengine.api.datastore.Text;

// The application design is that authenticated users have zero or one messages.
// This class is not thread safe because it is possible to create 2 messages for
// a single user under the right conditions.  Also, the static initialization
// code is not thread safe either.  These problems can be solved using transactions. 
// All application messages are placed under a single parent entity.
public class Message {
	private static final String parentKind = "messageParent";
	private static final String entityKind = "message";
	private static final String userIdPropertyName = "userId";
	private static final String nicknamePropertyName = "nickname";
	private static final String textPropertyName = "text";
	private static final String csrfTokenPropertyName = "csrfToken";
	private static Key parentKey = null;
	private static SecureRandom secureRandom = new SecureRandom();
	// Check if message parent exists; if not, then create it.
	static {
		Query query = new Query(parentKind);
		DatastoreService datastore = DatastoreServiceFactory.getDatastoreService();
		Entity parent = datastore.prepare(query).asSingleEntity();
		if (parent == null) {
			parent = new Entity(parentKind);
		parentKey = parent.getKey();
	private Entity entity = null;

	private Message(Entity entity) {
		this.entity = entity;
	public Long getID() {
		return (Long) entity.getKey().getId();
	public String getText() {
		return ((Text) entity.getProperty(textPropertyName)).getValue();
	public String getNickname() {
		return (String) entity.getProperty(nicknamePropertyName);
	private void setNickname(String nickname) {
		entity.setProperty(nicknamePropertyName, nickname);
	private void setText(String text) {
		entity.setProperty(textPropertyName, new Text(text));
	public String getCsrfToken() {
		return (String) entity.getProperty(csrfTokenPropertyName);
	private void save() {
		DatastoreService datastore = DatastoreServiceFactory.getDatastoreService();
	public void delete() {
		DatastoreService datastore = DatastoreServiceFactory.getDatastoreService();

	public static Message createOrUpdate(String userId, String nickname, String text) {
		Message message = getByUserId(userId);
		if (message != null) {
		} else {
			Entity entity = new Entity(entityKind, parentKey);
			entity.setProperty(userIdPropertyName, userId);
			entity.setProperty(nicknamePropertyName, nickname);
			entity.setProperty(textPropertyName, new Text(text));
			entity.setProperty(csrfTokenPropertyName, "" + secureRandom.nextLong());
			message = new Message(entity);
		return message;

	public static List<Message> getAll() {
		Query query = new Query(entityKind, parentKey).addSort(nicknamePropertyName, SortDirection.ASCENDING);
		DatastoreService datastore = DatastoreServiceFactory.getDatastoreService();
		Iterator<Entity> entityIter = datastore.prepare(query).asIterator();
		List<Message> messageList = new LinkedList<Message>();
		while (entityIter.hasNext()) {
			messageList.add(new Message(entityIter.next()));
		return messageList;
	public static Message getByUserId(String userId) {
		Query query = new Query(entityKind);
		query.addFilter(userIdPropertyName, FilterOperator.EQUAL, userId);
		DatastoreService datastore = DatastoreServiceFactory.getDatastoreService();
		Entity entity = datastore.prepare(query).asSingleEntity();
		if (entity == null) {
			return null;
		} else {
			return new Message(entity);
5 Years
Discussion Span
Last Post by stultuske

well ... not sure what you're trying to do, but by your description: instead of having one single instance of your message (is that the Entity variable?) create either an Array of the type, or an ArrayList which takes elements of that type, and add to it, instead of overwriting.

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