Adds the following feature.

src/main/java/shef/data/User.java

@@ -28,14 +28,27 @@ public class User {
     private String bio;
     private String profilePageUrl;
     private boolean isCurrentUser;
+    private boolean isFollowedByCurrentUser;
+    private long followingCount;
+    private long followersCount;
 
-    public User(String key, String email, String username, String location, String profilePicKey, String bio, boolean isCurrentUser) {
+
+    public User(String key, String email, String username, String location, String profilePicKey, String bio, boolean isCurrentUser, boolean isFollowedByCurrentUser, long followingCount, long followersCount) {
       this.key = key;
       this.email = email;
       this.username = username;
       this.location = location;
       this.profilePicKey = profilePicKey;
       this.bio = bio;
       this.isCurrentUser = isCurrentUser;
+      this.isFollowedByCurrentUser = isFollowedByCurrentUser;
+      this.followingCount = followingCount;
+      this.followersCount = followersCount;
+    }
+
+    public User(String key, String username, String profilePicKey) {
+      this.key = key;
+      this.username = username;
+      this.profilePicKey = profilePicKey;
     }
 }

src/main/java/shef/servlets/FollowersServlet.java

@@ -0,0 +1,92 @@
+// Copyright 2019 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package shef.servlets;
+
+import shef.data.User;
+import java.io.IOException;
+import java.util.ArrayList;
+import java.util.List;
+import java.util.LinkedList;
+import javax.servlet.annotation.WebServlet;
+import javax.servlet.http.HttpServlet;
+import javax.servlet.http.HttpServletRequest;
+import javax.servlet.http.HttpServletResponse;
+import com.google.gson.Gson;
+import com.google.appengine.api.users.UserService;
+import com.google.appengine.api.users.UserServiceFactory;
+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.PreparedQuery;
+import com.google.appengine.api.datastore.Query;
+import com.google.appengine.api.datastore.Key;
+import com.google.appengine.api.datastore.KeyFactory;
+import com.google.appengine.api.datastore.EntityNotFoundException;
+
+/** Servlet that retrieves follower information. */
+@WebServlet("/followers")
+public class FollowersServlet extends HttpServlet {
+
+  @Override
+  public void doGet(HttpServletRequest request, HttpServletResponse response) throws IOException {
+    UserService userService = UserServiceFactory.getUserService();
+    DatastoreService datastore = DatastoreServiceFactory.getDatastoreService();
+
+    // Get the key from the query string.
+    String userKeyString = request.getParameter("key");
+    Key userKey;
+    if(userKeyString == null && userService.isUserLoggedIn()) {
+      userKey = KeyFactory.createKey("User", userService.getCurrentUser().getUserId());
+    } else {
+      userKey = KeyFactory.stringToKey(userKeyString);
+    }
+
+    List<String> followers;
+    try {
+      Entity userEntity = datastore.get(userKey);
+      followers = (List<String>) userEntity.getProperty("followers");
+    } catch(EntityNotFoundException e) {
+      // User doesn't exist.
+      e.printStackTrace();
+      return;      
+    }
+
+
+    LinkedList<User> followerUsers = new LinkedList<User>();
+
+    if(followers != null) {
+      for(String keyString : followers) {
+        try {
+          Key key = KeyFactory.stringToKey(keyString);
+          Entity userEntity = datastore.get(key);
+          String name = (String) userEntity.getProperty("username");
+          String profilePicKey = (String) userEntity.getProperty("profile-pic");
+
+          User user = new User(keyString, name, profilePicKey);
+          followerUsers.add(user);
+        } catch(EntityNotFoundException e) {
+          e.printStackTrace();
+          return;
+        }
+      }
+    }
+
+    // Convert to JSON and send it as the response.
+    Gson gson = new Gson();
+
+    response.setContentType("application/json");
+    response.getWriter().println(gson.toJson(followerUsers));   
+  }
+}

src/main/java/shef/servlets/FollowingServlet.java

@@ -0,0 +1,150 @@
+// Copyright 2019 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package shef.servlets;
+
+import shef.data.User;
+import java.io.IOException;
+import java.util.ArrayList;
+import java.util.List;
+import java.util.LinkedList;
+import javax.servlet.annotation.WebServlet;
+import javax.servlet.http.HttpServlet;
+import javax.servlet.http.HttpServletRequest;
+import javax.servlet.http.HttpServletResponse;
+import com.google.gson.Gson;
+import com.google.appengine.api.users.UserService;
+import com.google.appengine.api.users.UserServiceFactory;
+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.PreparedQuery;
+import com.google.appengine.api.datastore.Query;
+import com.google.appengine.api.datastore.Key;
+import com.google.appengine.api.datastore.KeyFactory;
+import com.google.appengine.api.datastore.EntityNotFoundException;
+
+/** Servlet that updates and retrieves following information. */
+@WebServlet("/following")
+public class FollowingServlet extends HttpServlet {
+
+  @Override
+  public void doGet(HttpServletRequest request, HttpServletResponse response) throws IOException {
+    UserService userService = UserServiceFactory.getUserService();
+    DatastoreService datastore = DatastoreServiceFactory.getDatastoreService();
+
+    // Get the key from the query string.
+    String userKeyString = request.getParameter("key");
+    Key userKey;
+    if(userKeyString == null && userService.isUserLoggedIn()) {
+      userKey = KeyFactory.createKey("User", userService.getCurrentUser().getUserId());
+    } else {
+      userKey = KeyFactory.stringToKey(userKeyString);
+    }
+
+    List<String> following;
+    try {
+      Entity userEntity = datastore.get(userKey);
+      following = (List<String>) userEntity.getProperty("following");
+    } catch(EntityNotFoundException e) {
+      // User doesn't exist.
+      e.printStackTrace();
+      return;      
+    }
+
+    LinkedList<User> usersFollowed = new LinkedList<User>();
+
+    if(following != null) {
+      for(String keyString : following) {
+        try {
+          Key key = KeyFactory.stringToKey(keyString);
+          Entity userEntity = datastore.get(key);
+          String name = (String) userEntity.getProperty("username");
+          String profilePicKey = (String) userEntity.getProperty("profile-pic");
+
+          User user = new User(keyString, name, profilePicKey);
+          usersFollowed.add(user);
+        } catch(EntityNotFoundException e) {
+          e.printStackTrace();
+          return;
+        }
+      }
+    }
+
+    // Convert to JSON and send it as the response.
+    Gson gson = new Gson();
+
+    response.setContentType("application/json");
+    response.getWriter().println(gson.toJson(usersFollowed));   
+  }
+
+  @Override
+  public void doPost(HttpServletRequest request, HttpServletResponse response) throws IOException {
+    UserService userService = UserServiceFactory.getUserService();
+    DatastoreService datastore = DatastoreServiceFactory.getDatastoreService();
+
+    // Retrieve the key of the user to follow.
+    String userToFollowKeyString = request.getParameter("user");
+    Key userToFollowKey = KeyFactory.stringToKey(userToFollowKeyString);
+    Entity userToFollow;
+
+    // Get the current user's key.
+    Key currentUserKey = KeyFactory.createKey("User", userService.getCurrentUser().getUserId());
+    String currentUserKeyString = KeyFactory.keyToString(currentUserKey);
+    Entity currentUser;
+
+    boolean unfollow = Boolean.parseBoolean(request.getParameter("unfollow"));
+
+    try {
+      // Retrieve the current user from Datastore and add to their following list.
+      currentUser = datastore.get(currentUserKey);
+      ArrayList<String> following = (ArrayList<String>) currentUser.getProperty("following");
+      // Retrieve the user to follow, and add the current user as a follow
+      userToFollow = datastore.get(userToFollowKey);
+      ArrayList<String> followers = (ArrayList<String>) userToFollow.getProperty("followers");
+
+      if(unfollow) {
+        if(following != null) {
+          following.remove(userToFollowKeyString);
+        }
+        if(followers != null) {
+          followers.remove(currentUserKeyString);
+        }
+      } else {
+        if(following == null) {
+          following = new ArrayList<String>();
+        }
+        if(followers == null) {
+          followers = new ArrayList<String>();
+        }
+        following.add(userToFollowKeyString);
+        followers.add(currentUserKeyString);
+        currentUser.setProperty("following", following);
+        userToFollow.setProperty("followers", followers);
+      }
+
+    } catch(EntityNotFoundException e) {
+      /** This means the current user, for whom we're trying to set following for, doesn't exist.
+        * This should never happen. If it does, multiple things have gone seriously wrong.
+        */
+      e.printStackTrace();
+      return;
+    }
+
+    // Store the user in Datastore.
+    datastore.put(currentUser);
+    datastore.put(userToFollow);
+    response.sendRedirect("/profile-page.html?key=" + userToFollowKeyString);
+  }
+}

src/main/java/shef/servlets/UserServlet.java

@@ -66,12 +66,27 @@ public void doGet(HttpServletRequest request, HttpServletResponse response) thro
       String location = (String) userEntity.getProperty("location");
       String profilePicKey = (String) userEntity.getProperty("profile-pic");
       String bio = (String) userEntity.getProperty("bio");
+      List<String> following = (List<String>) userEntity.getProperty("following");
+      List<String> followers = (List<String>) userEntity.getProperty("followers");
+
+      long followingCount = 0;
+      long followersCount = 0;
+      boolean isFollowedByCurrentUser = false;
+
+      if(following != null) {
+        followingCount = following.size();
+      }
+      if(followers != null) {
+        followersCount = followers.size();
+        String currentUserKeyString = KeyFactory.createKeyString("User", userService.getCurrentUser().getUserId());
+        isFollowedByCurrentUser = followers.contains(currentUserKeyString);
+      }
 
       // Since we chose to store the id as a string in Datastore, it is referred to as "name".
       String id = (String) userKey.getName();
       boolean isCurrentUser = id.equals(userService.getCurrentUser().getUserId());
 
-      User user = new User(keyString, email, username, location, profilePicKey, bio, isCurrentUser);
+      User user = new User(keyString, email, username, location, profilePicKey, bio, isCurrentUser, isFollowedByCurrentUser, followingCount, followersCount);
 
       // Convert to JSON and send it as the response.
       Gson gson = new Gson();