Total Noob to gwt / gson in eclipse, error: "Cannot invoke the no-args constructor for the class"

I play with this example of a Google Web / GSON tool that receives data from Twitter.

Everything compiles just fine, but when I debug the web application, everything loads just fine, but when I click the Search button, I get the following error:

SEVERE: javax.servlet.ServletContext log: exception when sending an incoming RPC call com.google.gwt.user.server.rpc.UnexpectedException: service method 'public abstract java.util.List com.google.gwt.twittersearch.client.TwitterService .searchTweets (java.lang.String) throws java.io.IOException, java.lang.IllegalArgumentException 'threw an unexpected exception: java.lang.RuntimeException: Failed to call the no-args constructor for class com.google.gwt.twittersearch.server .TwitterServiceImpl $ SearchResponse. Registering InstanceCreator with Gson for this type can solve this problem. at com.google.gwt.user.server.rpc.RPC.encodeResponseForFailure (RPC.javahaps85) at com.google.gwt.user.server.rpc.RPC.invokeAndEncodeResponse (RPC.java∗88) on com.google .gwt.user.server.rpc.RemoteServiceServlet.processCall (RemoteServiceServlet.java:208) on com.google.gwt.user.server.rpc.RemoteServiceServlet.processPost (RemoteServiceServlet.java:248) on com.google.gwt.user.server.rpc.AbstractRemoteServiceServlet.doPost (AbstractRemoteServiceServletjavajava.ava .servlet.http.HttpServlet.service (HttpServlet.java:637) in javax.servlet.http.HttpServlet.service (HttpServlet.java:717) at org.mortbay.jetty.servlet.ServletHolder.handle (ServletHolder. ) at org.mortbay.jetty.servlet.ServletHandler $ CachedChain.doFilter (ServletHandler.java:1166) at com.google.appengine.tools.development.HeaderVerificationFilter.doFilter (HeaderVerificationFilter.java:35) at org.mortbay.jet servlet.ServletHandler $ CachedChain.doFilter (ServletHandler.java:1157) at com.google.appengine.api.blobstore.dev.ServeBlobFilter.doFilter (ServeBlobFilter.java:60) at org.mortbay.jetty.servlet.ServletHandler $ CachedChain.doFilter (ServletHandler.java:1157) at com.google.apphosting.utils.servlet.TransactionCleanupFilter.doFilter (TransactionCleanupFilter.java:43) at org.mortbay.jetty.servlet.ServletHanddo $ ServletHandler.java:1157) at com.google.appengine.tools.development.StaticFileFilter.doFilter (StaticFileFilter.java:125) at org.mortbay.jetty.servlet.ServletHandler $ CachedChain.doFilter (ServletHandler) comava11 .google.appengine.tools.development.BackendServersFilter.doFilter (BackendServersFilter.java:97) at org.mortbay.jetty.servlet.ServletHandler $ CachedChain.doFilter (ServletHandler.java:1157) at org.mortbay.jlettyervletandjervtyervlet.jlettyervlet .handle (ServletHandler.javahaps88) at org.mortbay.jetty.security.SecurityHandler.handle (SecurityHandler.java:216) at org.mortbay.jetty.servlet.SessionHandler.handle (SessionHandler.java:182) at org.mortbay.jetty.handler.ContextHandler.handle (ContextHandler.java:765) in org.mortbay.jetty.webapp.WebAppContext.handle (WebAppContext.java:418) at com.google.appengine.tools.development.DevAppEngineWebAppContext.handle (DevAppEngineWebAppContext.java:94) at org.mortbay.jetty.handler.HandlerWrapper.handle (HandlerWrapper.java:152) on com.google.dement .JettyContainerService $ ApiProxyHandler.handle (JettyContainerService.javahaps70) at org.mortbay.jetty.handler.HandlerWrapper.handle (HandlerWrapper.java:152) at org.mortbay.jetty.Server.handle (Server.java:6 org.mortbay.jetty.HttpConnection.handleRequest (HttpConnection.java∗42) at org.mortbay.jetty.HttpConnection $ RequestHandler.content (HttpConnection.java:938) at org.mortbay.jetty.HttpParser.parseNext (HttpParser.java:755) at org.mortbay.jetty.HttpParser.parseAvailable (HttpParser.java:218) at org.mortbay.jetty.HttpConnection.handle (HttpConnection.java404 .nio.SelectChannelEndPoint.run (SelectChannelEndPoint.java:409) at org.mortbay.thread.QueuedThreadPool $ PoolThread.run (QueuedThreadPool.java∗82) Raised: java.lang.RuntimeException: Cannot call the no-args constructor for class com. google.gwt.twittersearch.server.TwitterServiceImpl $ SearchResponse. Registering InstanceCreator with Gson for this type can solve this problem. at com.google.gson.internal.ConstructorConstructor $ 8.construct (ConstructorConstructor.java:167) at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory $ Adapter.read (ReflectiveTypeAdapterFactory.java:162) in com.google.gson. Gson.fromJson (Gson.java: 795) on com.google.gson.Gson.fromJson (Gson.java:734) on com.google.gwt.twittersearch.server.TwitterServiceImpl.parseSearchResponse (TwitterServiceImpl.java:80) on com.google.gwt.twittersearch .server.TwitterServiceImpl.searchTweets (TwitterServiceImpl.java:35) at sun.reflect.NativeMethodAccessorImpl.invoke0 (native method) at sun.reflect.NativeMethodAccessorImpl.invoke (NativeMethodAccessorImpl.javalectMethodativePloteMethreativeMethreleccessPlote.LetMeodreLessfReLetf.AccessMelodfreatLaccessMeTod .java: 25) in java.lang.reflect.Method.invoke (Method.java∗97) in com.google.appengine.tools.development.agent.runtime.Runtime.invoke (Runtime.java:115) on com. google.gwt.user.server.rpc.RPC.invokeAndEncodeResponse (RPC.java//69) ... 34 more. Called: java.lang.UnsupportedOperationException: class com.google.gwt cannot be allocated.twittersearch.server.TwitterServiceImpl $ SearchResponse at com.google.gson.internal.UnsafeAllocator $ 4.newInstance (UnsafeAllocator.java:100) at com.google.gson.internal.ConstructorConstructor $ 8.construct (ConstructorConstructor.java:164) 45 more

TwitterServiceImpl:

package com.google.gwt.twittersearch.server;

import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.io.Reader;
import java.net.HttpURLConnection;
import java.net.URL;
import java.net.URLEncoder;
import java.util.List;

import com.google.gwt.twittersearch.client.Tweet;
import com.google.gwt.twittersearch.client.TwitterService;
import com.google.gson.Gson;
import com.google.gwt.user.server.rpc.RemoteServiceServlet;

@SuppressWarnings("serial") 
public class TwitterServiceImpl extends RemoteServiceServlet implements
        TwitterService {

    @Override
    public List<Tweet> searchTweets(String query) throws IllegalArgumentException, IOException {
          query = query.trim();
          if (query.isEmpty()) {
            throw new IllegalArgumentException("No search query specified.");
          }

          // see: https://dev.twitter.com/docs/api/1/get/search
          String q = URLEncoder.encode(query, "UTF-8");
          URL url = new URL("http://search.twitter.com/search.json?q=" + q);
          HttpURLConnection connection = (HttpURLConnection) url.openConnection();
          InputStream response = null;
          try {
            response = connection.getInputStream();
            return parseSearchResponse(response);
          } finally {
            if (response != null) {
              response.close();
            }
          }
    }

    @Override
    public String getPrivacyPolicy() throws IOException {
        // see: https://dev.twitter.com/docs/api/1/get/legal/privacy
        URL url = new URL("https://api.twitter.com/1/legal/privacy.json");
        HttpURLConnection connection = (HttpURLConnection) url.openConnection();
        InputStream response = null;
        try {
            response = connection.getInputStream();
            return parsePolicyResponse(response);
        } finally {
            if (response != null) {
                response.close();
            }
        }
    }

    /**
     * Parses the privacy policy response returned from the Twitter API.
     * @param response the response
     * @return the privacy policy
     * @throws IOException if there was a problem reading the response
     */
    private String parsePolicyResponse(InputStream response) throws IOException {
        Reader reader = new InputStreamReader(response);
        PrivacyPolicyResponse privacyPolicyResponse = new Gson().fromJson(reader, PrivacyPolicyResponse.class);
        return privacyPolicyResponse.privacy;
    }

    /**
     * Parses the search response returned from the Twitter API.
     * @param response the response
     * @return the search results
     * @throws IOException if there was a problem reading the response
     */

    private List<Tweet> parseSearchResponse(InputStream response) throws IOException {
          Reader reader = new InputStreamReader(response);
          SearchResponse searchResponse = new Gson().fromJson(reader, SearchResponse.class);
          return searchResponse.results;
        }

    private class PrivacyPolicyResponse {
    public String privacy;
    }


    private class SearchResponse {
      public List<Tweet> results;
    }

}

:

    package com.google.gwt.twittersearch.client;

import java.util.List;
import com.google.gwt.safehtml.shared.SafeHtmlBuilder;
import com.google.gwt.core.client.EntryPoint;
import com.google.gwt.core.client.GWT;
import com.google.gwt.event.dom.client.ClickEvent;
import com.google.gwt.event.dom.client.ClickHandler;
import com.google.gwt.user.client.rpc.AsyncCallback;
import com.google.gwt.user.client.ui.Button;
import com.google.gwt.user.client.ui.HTML;
import com.google.gwt.user.client.ui.HorizontalPanel;
import com.google.gwt.user.client.ui.Image;
import com.google.gwt.user.client.ui.Label;
import com.google.gwt.user.client.ui.Panel;
import com.google.gwt.user.client.ui.RootPanel;
import com.google.gwt.user.client.ui.TextBox;
import com.google.gwt.user.client.ui.VerticalPanel;

/**
 * Entry point classes define <code>onModuleLoad()</code>.
 */
public class TwitterSearch implements EntryPoint {
  private Button privacyPolicyButton;
  private Button searchButton;
  private TextBox searchQueryTextBox;
  private Panel resultsPanel;
  private Label errorLabel;
  private Image loadingImage;
  private final TwitterServiceAsync service = GWT.create(TwitterService.class);

/**
 * This is the entry point method.
 */
public void onModuleLoad() {
    createWidgets();
    layoutWidgets();

}

private void createWidgets() {
    searchQueryTextBox = new TextBox();

    searchButton = new Button("Search");
    searchButton.addClickHandler(new ClickHandler() {
        @Override
        public void onClick(ClickEvent event) {
            setLoading(true);
            String query = searchQueryTextBox.getText();
            service.searchTweets(query, new AsyncCallback<List<Tweet>>() {
                @Override
                public void onFailure(Throwable caught) {
                    errorLabel.setText(caught.getMessage());
                    errorLabel.setVisible(true);
                    setLoading(false);
                }

                @Override
                public void onSuccess(List<Tweet> result) {
                    resultsPanel.clear();
                    for (Tweet tweet : result) {
                        SafeHtmlBuilder builder = new SafeHtmlBuilder();
                        builder.appendHtmlConstant("<b>User: </b>");
                        builder.appendEscaped(tweet.getFrom_user());
                        builder.appendHtmlConstant("<br /><b>Created: </b>");
                        builder.appendEscaped(tweet.getCreated_at());
                        builder.appendHtmlConstant("<br /><b>Tweet: </b>");
                        builder.appendEscaped(tweet.getText());
                        builder.appendHtmlConstant("<br /><br />");
                        resultsPanel.add(new HTML(builder.toSafeHtml()));
                    }
                    setLoading(false);
                }
            });
        }
    });

    privacyPolicyButton = new Button("Privacy Policy");
    privacyPolicyButton.addClickHandler(new ClickHandler() {
        @Override
        public void onClick(ClickEvent event) {
            setLoading(true);
            service.getPrivacyPolicy(new AsyncCallback<String>() {
                @Override
                public void onFailure(Throwable caught) {
                    errorLabel.setText(caught.getMessage());
                    errorLabel.setVisible(true);
                    setLoading(false);
                }

                @Override
                public void onSuccess(String result) {
                    resultsPanel.clear();

                    // convert newlines to <br />
                    SafeHtmlBuilder builder = new SafeHtmlBuilder();
                    builder.appendEscapedLines(result);

                    resultsPanel.add(new HTML(builder.toSafeHtml()));

                    setLoading(false);
                }
            });
        }
    });

    resultsPanel = new VerticalPanel();

    errorLabel = new Label();
    errorLabel.addStyleName("errorLabel");
    errorLabel.setVisible(false);

    //image from http://loadinfo.net/
    loadingImage = new Image("loading.gif");
    loadingImage.setVisible(false);
}

private void layoutWidgets() {
    Panel panel = new VerticalPanel();

    panel.add(errorLabel);

    Panel horizPanel = new HorizontalPanel();
    horizPanel.add(searchQueryTextBox);
    horizPanel.add(searchButton);
    horizPanel.add(privacyPolicyButton);
    horizPanel.add(loadingImage);
    panel.add(horizPanel);

    panel.add(resultsPanel);

    RootPanel.get().add(panel);
}

/**
 * Updates the UI for when a RPC call is made.
 * @param loading true if an RPC call is being sent, false if not
 */
private void setLoading(boolean loading) {
    if (loading) {
        errorLabel.setVisible(false);
    }
    searchQueryTextBox.setEnabled(!loading);
    searchButton.setEnabled(!loading);
    privacyPolicyButton.setEnabled(!loading);
    loadingImage.setVisible(loading);
}
}

:

    package com.google.gwt.twittersearch.client;

import java.io.Serializable;

@SuppressWarnings("serial")
public class Tweet implements Serializable{
private String id;
  private String from_user;
  private String created_at;
  private String text;

  public String getId() {
        return id;
    }

    public void setId(String id) {
        this.id = id;
    }

    public String getFrom_user() {
        return from_user;
    }

    public void setFrom_user(String from_user) {
        this.from_user = from_user;
    }

    public String getCreated_at() {
        return created_at;
    }

    public void setCreated_at(String created_at) {
        this.created_at = created_at;
    }

    public String getText() {
        return text;
    }

    public void setText(String text) {
        this.text = text;
    }


}

, , !

, brl