diff --git a/src/main/java/io/socket/client/Socket.java b/src/main/java/io/socket/client/Socket.java
index 05feff39..9e844d94 100644
--- a/src/main/java/io/socket/client/Socket.java
+++ b/src/main/java/io/socket/client/Socket.java
@@ -376,7 +376,10 @@ private void onconnect(String id) {
     private void emitBuffered() {
         List<Object> data;
         while ((data = this.receiveBuffer.poll()) != null) {
-            String event = (String)data.get(0);
+            if (data.isEmpty()) {
+                continue;
+            }
+            String event = data.remove(0).toString();
             super.emit(event, data.toArray());
         }
         this.receiveBuffer.clear();
diff --git a/src/site/markdown/emitting_events.md b/src/site/markdown/emitting_events.md
index fce31092..5f298ffb 100644
--- a/src/site/markdown/emitting_events.md
+++ b/src/site/markdown/emitting_events.md
@@ -1,6 +1,6 @@
 # Emitting events
 
-See also: https://socket.io/docs/v3/emitting-events/
+See also: https://socket.io/docs/v4/emitting-events/
 
 **Table of content**
 
@@ -77,6 +77,27 @@ Events are great, but in some cases you may want a more classic request-response
 
 You can add a callback as the last argument of the `emit()`, and this callback will be called once the other side acknowledges the event:
 
+### From client to server
+
+*Client*
+
+```java
+// Java 7
+socket.emit("update item", 1, new JSONObject(singletonMap("name", "updated")), new Ack() {
+    @Override
+    public void call(Object... args) {
+        JSONObject response = (JSONObject) args[0];
+        System.out.println(response.getString("status")); // "ok"
+    }
+});
+
+// Java 8 and above
+socket.emit("update item", 1, new JSONObject(singletonMap("name", "updated")), (Ack) args -> {
+    JSONObject response = (JSONObject) args[0];
+    System.out.println(response.getString("status")); // "ok"
+});
+```
+
 *Server*
 
 ```js
@@ -91,15 +112,37 @@ io.on("connection", (socket) => {
 });
 ```
 
+### From server to client
+
+*Server*
+
+```js
+io.on("connection", (socket) => {
+  socket.emit("hello", "please acknowledge", (response) => {
+    console.log(response); // prints "hi!"
+  });
+});
+```
+
 *Client*
 
 ```java
-socket.emit("update item", 1, new JSONObject(singletonMap("name", "updated")), new Ack() {
+// Java 7
+socket.on("hello", new Emitter.Listener() {
     @Override
     public void call(Object... args) {
-        JSONObject response = (JSONObject) args[0];
-        System.out.println(response.getString("status")); // "ok"
+        System.out.println(args[0]); // "please acknowledge"
+        if (args.length > 1 && args[1] instanceof Ack) {
+            ((Ack) args[1]).call("hi!");
+        }
     }
 });
-```
 
+// Java 8 and above
+socket.on("hello", args -> {
+    System.out.println(args[0]); // "please acknowledge"
+    if (args.length > 1 && args[1] instanceof Ack) {
+        ((Ack) args[1]).call("hi!");
+    }
+});
+```
diff --git a/src/site/markdown/initialization.md b/src/site/markdown/initialization.md
index efef638f..72cca0d0 100644
--- a/src/site/markdown/initialization.md
+++ b/src/site/markdown/initialization.md
@@ -25,7 +25,7 @@ Socket socket = IO.socket("wss://example.com"); // OK, similar to the example ab
 Socket socket = IO.socket("192.168.0.1:1234"); // NOT OK, missing the scheme part
 ```
 
-The path represents the [Namespace](https://socket.io/docs/v3/namespaces/), and not the actual path (see [below](#path)) of the HTTP requests:
+The path represents the [Namespace](https://socket.io/docs/v4/namespaces/), and not the actual path (see [below](#path)) of the HTTP requests:
 
 ```java
 Socket socket = IO.socket(URI.create("https://example.com")); // the main namespace
@@ -76,7 +76,7 @@ Whether to create a new Manager instance.
 
 A Manager instance is in charge of the low-level connection to the server (established with HTTP long-polling or WebSocket). It handles the reconnection logic.
 
-A Socket instance is the interface which is used to sends events to — and receive events from — the server. It belongs to a given [namespace](https://socket.io/docs/v3/namespaces).
+A Socket instance is the interface which is used to sends events to — and receive events from — the server. It belongs to a given [namespace](https://socket.io/docs/v4/namespaces).
 
 A single Manager can be attached to several Socket instances.
 
@@ -131,7 +131,7 @@ IO.Options options = IO.Options.builder()
 Socket socket = IO.socket(URI.create("https://example.com"), options);
 ```
 
-Note: in that case, sticky sessions are not required on the server side (more information [here](https://socket.io/docs/v3/using-multiple-nodes/)).
+Note: in that case, sticky sessions are not required on the server side (more information [here](https://socket.io/docs/v4/using-multiple-nodes/)).
 
 #### `upgrade`
 
@@ -177,7 +177,7 @@ IO.Options options = IO.Options.builder()
 Socket socket = IO.socket(URI.create("https://example.com"), options);
 ```
 
-Please note that this is different from the path in the URI, which represents the [Namespace](https://socket.io/docs/v3/namespaces/).
+Please note that this is different from the path in the URI, which represents the [Namespace](https://socket.io/docs/v4/namespaces/).
 
 Example:
 
@@ -274,7 +274,7 @@ These settings are specific to the given Socket instance.
 
 Default value: -
 
-Credentials that are sent when accessing a namespace (see also [here](https://socket.io/docs/v3/middlewares/#Sending-credentials)).
+Credentials that are sent when accessing a namespace (see also [here](https://socket.io/docs/v4/middlewares/#sending-credentials)).
 
 Example:
 
diff --git a/src/site/markdown/installation.md b/src/site/markdown/installation.md
index 1eb05d80..532dc4d3 100644
--- a/src/site/markdown/installation.md
+++ b/src/site/markdown/installation.md
@@ -3,7 +3,7 @@
 | Client version | Socket.IO server |
 | -------------- | ---------------- |
 | 0.9.x  | 1.x |
-| 1.x    | 2.x (or 3.1.x / 4.x with [`allowEIO3: true`](https://socket.io/docs/v4/server-initialization/#allowEIO3)) |
+| 1.x    | 2.x (or 3.1.x / 4.x with [`allowEIO3: true`](https://socket.io/docs/v4/server-options/#alloweio3)) |
 | 2.x    | 3.x / 4.x |
 
 ## Installation
@@ -26,7 +26,7 @@ Add the following dependency to your `pom.xml`.
 Add it as a gradle dependency for Android Studio, in `build.gradle`:
 
 ```groovy
-compile ('io.socket:socket.io-client:2.0.1') {
+implementation ('io.socket:socket.io-client:2.0.1') {
   // excluding org.json which is provided by Android
   exclude group: 'org.json', module: 'json'
 }
diff --git a/src/site/markdown/listening_to_events.md b/src/site/markdown/listening_to_events.md
index 6caf0a61..6a740658 100644
--- a/src/site/markdown/listening_to_events.md
+++ b/src/site/markdown/listening_to_events.md
@@ -1,6 +1,6 @@
 # Listening to events
 
-See also: https://socket.io/docs/v3/listening-to-events/
+See also: https://socket.io/docs/v4/listening-to-events/
 
 **Table of content**
 
diff --git a/src/site/markdown/migrating_from_1_x.md b/src/site/markdown/migrating_from_1_x.md
index f13bebf3..37c56550 100644
--- a/src/site/markdown/migrating_from_1_x.md
+++ b/src/site/markdown/migrating_from_1_x.md
@@ -7,12 +7,12 @@ Here is the compatibility table:
 | Java client version | Socket.IO server |
 | -------------- | ---------------- |
 | 0.9.x  | 1.x |
-| 1.x    | 2.x (or 3.1.x / 4.x with [`allowEIO3: true`](https://socket.io/docs/v4/server-initialization/#allowEIO3)) |
+| 1.x    | 2.x (or 3.1.x / 4.x with [`allowEIO3: true`](https://socket.io/docs/v4/server-options/#alloweio3)) |
 | 2.x    | 3.x / 4.x |
 
 **Important note:** due to the backward incompatible changes to the Socket.IO protocol, a 2.x Java client will not be able to reach a 2.x server, and vice-versa
 
-Since the Java client matches the Javascript client quite closely, most of the changes listed in the migration guide [here](https://socket.io/docs/v3/migrating-from-2-x-to-3-0) also apply to the Java client:
+Since the Java client matches the Javascript client quite closely, most of the changes listed in the migration guide [here](https://socket.io/docs/v4/migrating-from-2-x-to-3-0) also apply to the Java client:
 
 - [A middleware error will now emit an Error object](#A_middleware_error_will_now_emit_an_Error_object)
 - [The Socket `query` option is renamed to `auth`](#The_Socket_query_option_is_renamed_to_auth)
diff --git a/src/site/markdown/socket_instance.md b/src/site/markdown/socket_instance.md
index 26457164..5a40be45 100644
--- a/src/site/markdown/socket_instance.md
+++ b/src/site/markdown/socket_instance.md
@@ -113,7 +113,7 @@ socket.on("data", new Emitter.Listener() {
 
 ### `Socket.EVENT_CONNECT_ERROR`
 
-This event is fired when the server does not accept the connection (in a [middleware function](https://socket.io/docs/v3/middlewares/#Sending-credentials)).
+This event is fired when the server does not accept the connection (in a [middleware function](https://socket.io/docs/v4/middlewares/#sending-credentials)).
 
 You need to manually reconnect. You might need to update the credentials:
 
@@ -144,7 +144,7 @@ Here is the list of possible reasons:
 
 Reason | Description
 ------ | -----------
-`io server disconnect` | The server has forcefully disconnected the socket with [socket.disconnect()](https://socket.io/docs/v3/server-api/#socket-disconnect-close)
+`io server disconnect` | The server has forcefully disconnected the socket with [socket.disconnect()](https://socket.io/docs/v4/server-api/#socketdisconnectclose)
 `io client disconnect` | The socket was manually disconnected using `socket.disconnect()`
 `ping timeout` | The server did not respond in the `pingTimeout` range
 `transport close` | The connection was closed (example: the user has lost connection, or the network was changed from WiFi to 4G)