diff --git a/src/Cron/Protocol/Frame.php b/src/Cron/Protocol/Frame.php
index 9d054de..b6117d9 100644
--- a/src/Cron/Protocol/Frame.php
+++ b/src/Cron/Protocol/Frame.php
@@ -13,12 +13,8 @@ class Frame
 {
     /**
      * Check the integrity of the package.
-     *
-     * @param string $buffer
-     *
-     * @return int
      */
-    public static function input($buffer, TcpConnection $connection)
+    public static function input(string $buffer, TcpConnection $connection): int
     {
         if (\strlen($buffer) < 4)
         {
@@ -31,24 +27,16 @@ public static function input($buffer, TcpConnection $connection)
 
     /**
      * Decode.
-     *
-     * @param string $buffer
-     *
-     * @return string
      */
-    public static function decode($buffer)
+    public static function decode(string $buffer): string
     {
         return substr($buffer, 4);
     }
 
     /**
      * Encode.
-     *
-     * @param string $buffer
-     *
-     * @return string
      */
-    public static function encode($buffer)
+    public static function encode(string $buffer): string
     {
         return pack('N', \strlen($buffer)) . $buffer;
     }
diff --git a/src/Server/Protocol/FrameWithLength.php b/src/Server/Protocol/FrameWithLength.php
index 44bd4cf..907af0d 100644
--- a/src/Server/Protocol/FrameWithLength.php
+++ b/src/Server/Protocol/FrameWithLength.php
@@ -13,12 +13,8 @@ class FrameWithLength
 {
     /**
      * Check the integrity of the package.
-     *
-     * @param string $buffer
-     *
-     * @return int
      */
-    public static function input($buffer, TcpConnection $connection)
+    public static function input(string $buffer, TcpConnection $connection): int
     {
         if (\strlen($buffer) < 4)
         {
@@ -31,24 +27,16 @@ public static function input($buffer, TcpConnection $connection)
 
     /**
      * Decode.
-     *
-     * @param string $buffer
-     *
-     * @return string
      */
-    public static function decode($buffer)
+    public static function decode(string $buffer): string
     {
         return $buffer;
     }
 
     /**
      * Encode.
-     *
-     * @param string $buffer
-     *
-     * @return string
      */
-    public static function encode($buffer)
+    public static function encode(string $buffer): string
     {
         $total_length = \strlen($buffer);
 
diff --git a/src/Server/Protocol/TextCRLF.php b/src/Server/Protocol/TextCRLF.php
index 0025cc4..0ebc21f 100644
--- a/src/Server/Protocol/TextCRLF.php
+++ b/src/Server/Protocol/TextCRLF.php
@@ -13,12 +13,8 @@ class TextCRLF
 {
     /**
      * Check the integrity of the package.
-     *
-     * @param string $buffer
-     *
-     * @return int
      */
-    public static function input($buffer, ConnectionInterface $connection)
+    public static function input(string $buffer, ConnectionInterface $connection): int
     {
         // Judge whether the package length exceeds the limit.
         if (isset($connection->maxPackageSize) && \strlen($buffer) >= $connection->maxPackageSize)
@@ -40,12 +36,8 @@ public static function input($buffer, ConnectionInterface $connection)
 
     /**
      * Encode.
-     *
-     * @param string $buffer
-     *
-     * @return string
      */
-    public static function encode($buffer)
+    public static function encode(string $buffer): string
     {
         // Add "\r\n"
         return $buffer . "\r\n";
@@ -53,12 +45,8 @@ public static function encode($buffer)
 
     /**
      * Decode.
-     *
-     * @param string $buffer
-     *
-     * @return string
      */
-    public static function decode($buffer)
+    public static function decode(string $buffer): string
     {
         // Remove "\r\n"
         return rtrim($buffer, "\r\n");