diff --git a/src/main/java/com/github/nkzawa/engineio/client/Socket.java b/src/main/java/com/github/nkzawa/engineio/client/Socket.java
index 241a82d..79507d1 100644
--- a/src/main/java/com/github/nkzawa/engineio/client/Socket.java
+++ b/src/main/java/com/github/nkzawa/engineio/client/Socket.java
@@ -27,16 +27,14 @@ public abstract class Socket extends Emitter {
 
     private static final Gson gson = new Gson();
 
-    private static final int OPENING = 0;
-    private static final int OPEN = 1;
-    private static final int CLOSING = 2;
-    private static final int CLOSED = 3;
-    private static final Map<Integer, String> STATE_MAP = new HashMap<Integer, String>() {{
-        put(OPENING, "opening");
-        put(OPEN, "open");
-        put(CLOSING, "closing");
-        put(CLOSED, "closed");
-    }};
+    private enum ReadyState {
+        OPENING, OPEN, CLOSING, CLOSED;
+
+        @Override
+        public String toString() {
+            return super.toString().toLowerCase();
+        }
+    }
 
     /**
      * Called on successful connection.
@@ -98,7 +96,6 @@ public abstract class Socket extends Emitter {
     private int port;
     private int policyPort;
     private int prevBufferLen;
-    private int readyState = -1;
     private long pingInterval;
     private long pingTimeout;
     private String id;
@@ -114,6 +111,7 @@ public abstract class Socket extends Emitter {
     private Future pingTimeoutTimer;
     private Future pingIntervalTimer;
 
+    private ReadyState readyState;
     private ScheduledExecutorService heartbeatScheduler = Executors.newSingleThreadScheduledExecutor();
 
 
@@ -179,7 +177,7 @@ public abstract class Socket extends Emitter {
         EventThread.exec(new Runnable() {
             @Override
             public void run() {
-                Socket.this.readyState = OPENING;
+                Socket.this.readyState = ReadyState.OPENING;
                 Transport transport = Socket.this.createTransport(Socket.this.transports.get(0));
                 Socket.this.setTransport(transport);
                 transport.open();
@@ -297,7 +295,7 @@ public abstract class Socket extends Emitter {
                                 @Override
                                 public void run() {
                                     if (failed[0]) return;
-                                    if (self.readyState == CLOSED || self.readyState == CLOSING) {
+                                    if (self.readyState == ReadyState.CLOSED || self.readyState == ReadyState.CLOSING) {
                                         return;
                                     }
 
@@ -354,12 +352,12 @@ public abstract class Socket extends Emitter {
 
     private void onOpen() {
         logger.fine("socket open");
-        this.readyState = OPEN;
+        this.readyState = ReadyState.OPEN;
         this.emit(EVENT_OPEN);
         this.onopen();
         this.flush();
 
-        if (this.readyState == OPEN && this.upgrade && this.transport instanceof Polling) {
+        if (this.readyState == ReadyState.OPEN && this.upgrade && this.transport instanceof Polling) {
             logger.fine("starting upgrade probes");
             for (String upgrade: this.upgrades) {
                 this.probe(upgrade);
@@ -368,7 +366,7 @@ public abstract class Socket extends Emitter {
     }
 
     private void onPacket(Packet packet) {
-        if (this.readyState == OPENING || this.readyState == OPEN) {
+        if (this.readyState == ReadyState.OPENING || this.readyState == ReadyState.OPEN) {
             logger.fine(String.format("socket received: type '%s', data '%s'", packet.type, packet.data));
 
             this.emit(EVENT_PACKET, packet);
@@ -389,7 +387,7 @@ public abstract class Socket extends Emitter {
                 this.onmessage(packet.data);
             }
         } else {
-            logger.fine(String.format("packet received with socket readyState '%s'", STATE_MAP.get(this.readyState)));
+            logger.fine(String.format("packet received with socket readyState '%s'", this.readyState));
         }
     }
 
@@ -430,7 +428,7 @@ public abstract class Socket extends Emitter {
                 EventThread.exec(new Runnable() {
                     @Override
                     public void run() {
-                        if (self.readyState == CLOSED) return;
+                        if (self.readyState == ReadyState.CLOSED) return;
                         self.onClose("ping timeout");
                     }
                 });
@@ -485,7 +483,7 @@ public abstract class Socket extends Emitter {
     }
 
     private void flush() {
-        if (this.readyState != CLOSED && this.transport.writable &&
+        if (this.readyState != ReadyState.CLOSED && this.transport.writable &&
                 !this.upgrading && this.writeBuffer.size() != 0) {
             logger.fine(String.format("flushing %d packets in socket", this.writeBuffer.size()));
             this.prevBufferLen = this.writeBuffer.size();
@@ -552,7 +550,7 @@ public abstract class Socket extends Emitter {
         EventThread.exec(new Runnable() {
             @Override
             public void run() {
-                if (Socket.this.readyState == OPENING || Socket.this.readyState == OPEN) {
+                if (Socket.this.readyState == ReadyState.OPENING || Socket.this.readyState == ReadyState.OPEN) {
                     Socket.this.onClose("forced close");
                     logger.fine("socket closing - telling transport to close");
                     Socket.this.transport.close();
@@ -575,7 +573,7 @@ public abstract class Socket extends Emitter {
     }
 
     private void onClose(String reason, Exception desc) {
-        if (this.readyState == OPENING || this.readyState == OPEN) {
+        if (this.readyState == ReadyState.OPENING || this.readyState == ReadyState.OPEN) {
             logger.fine(String.format("socket close with reason: %s", reason));
             if (this.pingIntervalTimer != null) {
                 this.pingIntervalTimer.cancel(true);
@@ -590,7 +588,7 @@ public abstract class Socket extends Emitter {
                     Socket.this.callbackBuffer.clear();
                 }
             });
-            this.readyState = CLOSED;
+            this.readyState = ReadyState.CLOSED;
             this.emit(EVENT_CLOSE, reason, desc);
             this.onclose();
             this.id = null;
diff --git a/src/main/java/com/github/nkzawa/engineio/client/Transport.java b/src/main/java/com/github/nkzawa/engineio/client/Transport.java
index bb241f6..b72cecb 100644
--- a/src/main/java/com/github/nkzawa/engineio/client/Transport.java
+++ b/src/main/java/com/github/nkzawa/engineio/client/Transport.java
@@ -5,21 +5,18 @@ import com.github.nkzawa.emitter.Emitter;
 import com.github.nkzawa.engineio.parser.Packet;
 import com.github.nkzawa.engineio.parser.Parser;
 
-import java.util.HashMap;
 import java.util.Map;
 
 public abstract class Transport extends Emitter {
 
-    protected static final int OPENING = 0;
-    protected static final int OPEN = 1;
-    protected static final int CLOSED = 2;
-    protected static final int PAUSED = 3;
-    protected static final Map<Integer, String> STATE_MAP = new HashMap<Integer, String>() {{
-        put(OPENING, "opening");
-        put(OPEN, "open");
-        put(CLOSED, "closed");
-        put(PAUSED, "paused");
-    }};
+    protected enum ReadyState {
+        OPENING, OPEN, CLOSED, PAUSED;
+
+        @Override
+        public String toString() {
+            return super.toString().toLowerCase();
+        }
+    }
 
     public static final String EVENT_OPEN = "open";
     public static final String EVENT_CLOSE = "close";
@@ -34,11 +31,11 @@ public abstract class Transport extends Emitter {
     protected boolean secure;
     protected boolean timestampRequests;
     protected int port;
-    protected int readyState = -1;
     protected String path;
     protected String hostname;
     protected String timestampParam;
 
+    protected ReadyState readyState;
 
     public Transport(Options opts) {
         this.path = opts.path;
@@ -61,8 +58,8 @@ public abstract class Transport extends Emitter {
         exec(new Runnable() {
             @Override
             public void run() {
-                if (Transport.this.readyState == CLOSED || Transport.this.readyState < 0) {
-                    Transport.this.readyState = OPENING;
+                if (Transport.this.readyState == ReadyState.CLOSED || Transport.this.readyState == null) {
+                    Transport.this.readyState = ReadyState.OPENING;
                     Transport.this.doOpen();
                 }
             }
@@ -74,7 +71,7 @@ public abstract class Transport extends Emitter {
         exec(new Runnable() {
             @Override
             public void run() {
-                if (Transport.this.readyState == OPENING || Transport.this.readyState == OPEN) {
+                if (Transport.this.readyState == ReadyState.OPENING || Transport.this.readyState == ReadyState.OPEN) {
                     Transport.this.doClose();
                     Transport.this.onClose();
                 }
@@ -87,7 +84,7 @@ public abstract class Transport extends Emitter {
         exec(new Runnable() {
             @Override
             public void run() {
-                if (Transport.this.readyState == OPEN) {
+                if (Transport.this.readyState == ReadyState.OPEN) {
                     Transport.this.write(packets);
                 } else {
                     throw new RuntimeException("Transport not open");
@@ -97,7 +94,7 @@ public abstract class Transport extends Emitter {
     }
 
     protected void onOpen() {
-        this.readyState = OPEN;
+        this.readyState = ReadyState.OPEN;
         this.writable = true;
         this.emit(EVENT_OPEN);
     }
@@ -111,7 +108,7 @@ public abstract class Transport extends Emitter {
     }
 
     protected void onClose() {
-        this.readyState = CLOSED;
+        this.readyState = ReadyState.CLOSED;
         this.emit(EVENT_CLOSE);
     }
 
diff --git a/src/main/java/com/github/nkzawa/engineio/client/transports/Polling.java b/src/main/java/com/github/nkzawa/engineio/client/transports/Polling.java
index 344c5bf..33f8dd7 100644
--- a/src/main/java/com/github/nkzawa/engineio/client/transports/Polling.java
+++ b/src/main/java/com/github/nkzawa/engineio/client/transports/Polling.java
@@ -38,13 +38,13 @@ abstract public class Polling extends Transport {
             public void run() {
                 final Polling self = Polling.this;
 
-                Polling.this.readyState = PAUSED;
+                Polling.this.readyState = ReadyState.PAUSED;
 
                 final Runnable pause = new Runnable() {
                     @Override
                     public void run() {
                         logger.fine("paused");
-                        self.readyState = PAUSED;
+                        self.readyState = ReadyState.PAUSED;
                         onPause.run();
                     }
                 };
@@ -100,7 +100,7 @@ abstract public class Polling extends Transport {
         Parser.decodePayload(data, new Parser.DecodePayloadCallback() {
             @Override
             public boolean call(Packet packet, int index, int total) {
-                if (self.readyState == OPENING) {
+                if (self.readyState == ReadyState.OPENING) {
                     self.onOpen();
                 }
 
@@ -114,14 +114,14 @@ abstract public class Polling extends Transport {
             }
         });
 
-        if (this.readyState != CLOSED) {
+        if (this.readyState != ReadyState.CLOSED) {
             this.polling = false;
             this.emit(EVENT_POLL_COMPLETE);
 
-            if (this.readyState == OPEN) {
+            if (this.readyState == ReadyState.OPEN) {
                 this.poll();
             } else {
-                logger.fine(String.format("ignoring poll - transport state '%s'", STATE_MAP.get(this.readyState)));
+                logger.fine(String.format("ignoring poll - transport state '%s'", this.readyState));
             }
         }
     }