refactor: Reactor pattern - Polish Javadoc and Using 'Map.computeIfAbsent' instead of 'if' statement (#2805)

* refactor: using computeIfAbsent instead of if check.

* docs: Polish Javadoc and comments

Author: yuhangbin

reactor/src/main/java/com/iluwatar/reactor/framework/AbstractNioChannel.java

@@ -162,11 +162,7 @@ public void write(Object data, SelectionKey key) {
     var pendingWrites = this.channelToPendingWrites.get(key.channel());
     if (pendingWrites == null) {
       synchronized (this.channelToPendingWrites) {
-        pendingWrites = this.channelToPendingWrites.get(key.channel());
-        if (pendingWrites == null) {
-          pendingWrites = new ConcurrentLinkedQueue<>();
-          this.channelToPendingWrites.put(key.channel(), pendingWrites);
-        }
+        pendingWrites = this.channelToPendingWrites.computeIfAbsent(key.channel(), k -> new ConcurrentLinkedQueue<>());
       }
     }
     pendingWrites.add(data);

reactor/src/main/java/com/iluwatar/reactor/framework/NioReactor.java

@@ -37,7 +37,7 @@
 
 /**
  * This class acts as Synchronous Event De-multiplexer and Initiation Dispatcher of Reactor pattern.
- * Multiple handles i.e. {@link AbstractNioChannel}s can be registered to the reactor and it blocks
+ * Multiple handles i.e. {@link AbstractNioChannel}s can be registered to the reactor, and it blocks
  * for events from all these handles. Whenever an event occurs on any of the registered handles, it
  * synchronously de-multiplexes the event which can be any of read, write or accept, and dispatches
  * the event to the appropriate {@link ChannelHandler} using the {@link Dispatcher}.
@@ -46,7 +46,7 @@
  * #start()} method. {@link NioReactor} uses {@link Selector} for realizing Synchronous Event
  * De-multiplexing.
  *
- * <p>NOTE: This is one of the ways to implement NIO reactor and it does not take care of all
+ * <p>NOTE: This is one of the ways to implement NIO reactor, and it does not take care of all
  * possible edge cases which are required in a real application. This implementation is meant to
  * demonstrate the fundamental concepts that lie behind Reactor pattern.
  */
@@ -212,7 +212,7 @@ private void onChannelAcceptable(SelectionKey key) throws IOException {
 
   /**
    * Queues the change of operations request of a channel, which will change the interested
-   * operations of the channel sometime in future.
+   * operations of the channel sometime in the future.
    *
    * <p>This is a non-blocking method and does not guarantee that the operations have changed when
    * this method returns.
@@ -228,7 +228,7 @@ public void changeOps(SelectionKey key, int interestedOps) {
   /**
    * A command that changes the interested operations of the key provided.
    */
-  class ChangeKeyOpsCommand implements Runnable {
+  static class ChangeKeyOpsCommand implements Runnable {
     private final SelectionKey key;
     private final int interestedOps;