diff --git a/composer.lock b/composer.lock
index 64c24da..8e682e4 100644
--- a/composer.lock
+++ b/composer.lock
@@ -884,16 +884,16 @@
         },
         {
             "name": "phpunit/phpunit",
-            "version": "9.5.1",
+            "version": "9.5.2",
             "source": {
                 "type": "git",
                 "url": "https://github.com/sebastianbergmann/phpunit.git",
-                "reference": "e7bdf4085de85a825f4424eae52c99a1cec2f360"
+                "reference": "f661659747f2f87f9e72095bb207bceb0f151cb4"
             },
             "dist": {
                 "type": "zip",
-                "url": "https://api.github.com/repos/sebastianbergmann/phpunit/zipball/e7bdf4085de85a825f4424eae52c99a1cec2f360",
-                "reference": "e7bdf4085de85a825f4424eae52c99a1cec2f360",
+                "url": "https://api.github.com/repos/sebastianbergmann/phpunit/zipball/f661659747f2f87f9e72095bb207bceb0f151cb4",
+                "reference": "f661659747f2f87f9e72095bb207bceb0f151cb4",
                 "shasum": ""
             },
             "require": {
@@ -979,7 +979,7 @@
                     "type": "github"
                 }
             ],
-            "time": "2021-01-17T07:42:25+00:00"
+            "time": "2021-02-02T14:45:58+00:00"
         },
         {
             "name": "sebastian/cli-parser",
diff --git a/src/Application.php b/src/Application.php
index b5895c8..2e32fd6 100644
--- a/src/Application.php
+++ b/src/Application.php
@@ -2,23 +2,21 @@
 namespace Gt\Cli;
 
 use Gt\Cli\Argument\ArgumentValueList;
-use Gt\Cli\Argument\NotEnoughArgumentsException;
 use Gt\Cli\Command\Command;
-use Gt\Cli\Command\CommandException;
 use Gt\Cli\Command\HelpCommand;
 use Gt\Cli\Command\InvalidCommandException;
 use Gt\Cli\Command\VersionCommand;
 use Gt\Cli\Parameter\MissingRequiredParameterException;
-use Gt\Cli\Parameter\MissingRequiredParameterValueException;
 use Gt\Cli\Argument\ArgumentList;
 
 class Application {
-	protected $description;
-	protected $arguments;
-	protected $commands;
-	protected $stream;
-	protected $helpCommand;
-	protected $versionCommand;
+	protected string $description;
+	protected ?ArgumentList $arguments;
+	/** @var Command[]  */
+	protected array $commands;
+	protected Stream $stream;
+	protected HelpCommand $helpCommand;
+	protected VersionCommand $versionCommand;
 
 	public function __construct(
 		string $description,
@@ -53,7 +51,7 @@ public function __construct(
 		$this->versionCommand->setOutput($this->stream);
 	}
 
-	public function setStream($in, $out, $error) {
+	public function setStream(string $in, string $out, string $error):void {
 		$this->stream->setStream($in, $out, $error);
 	}
 
@@ -137,4 +135,4 @@ protected function findCommandByName(string $name):Command {
 
 		throw new InvalidCommandException($name);
 	}
-}
\ No newline at end of file
+}
diff --git a/src/Argument/Argument.php b/src/Argument/Argument.php
index 6ec88b7..62feda4 100644
--- a/src/Argument/Argument.php
+++ b/src/Argument/Argument.php
@@ -3,8 +3,8 @@
 
 abstract class Argument {
 	const USER_DATA = "__user-data__";
-	protected $key;
-	protected $value;
+	protected string $key;
+	protected ?string $value;
 
 	public function __construct(string $rawKey, string $value = null) {
 		$this->key = $this->processRawKey($rawKey);
@@ -29,4 +29,4 @@ public function getKey():string {
 	public function getValue():?string {
 		return $this->value;
 	}
-}
\ No newline at end of file
+}
diff --git a/src/Argument/ArgumentList.php b/src/Argument/ArgumentList.php
index 0001834..8722475 100644
--- a/src/Argument/ArgumentList.php
+++ b/src/Argument/ArgumentList.php
@@ -4,13 +4,14 @@
 use Gt\Cli\Parameter\Parameter;
 use Iterator;
 
+/** @implements Iterator<int, Argument> */
 class ArgumentList implements Iterator {
 	const DEFAULT_COMMAND = "help";
 
-	protected $script;
+	protected string $script;
 	/** @var Argument[] */
-	protected $argumentList = [];
-	protected $iteratorIndex;
+	protected array $argumentList = [];
+	protected int $iteratorIndex;
 
 	public function __construct(string $script, string...$arguments) {
 		$this->script = $script;
@@ -25,18 +26,20 @@ public function getCommandName():string {
 		return $this->argumentList[0]->getValue();
 	}
 
+	/** @param string[] $arguments */
 	protected function buildArgumentList(array $arguments):void {
 		$commandArgument = array_shift($arguments);
 		if($commandArgument) {
-			$this->argumentList []= new CommandArgument(
-				$commandArgument
+			array_push(
+				$this->argumentList,
+				new CommandArgument($commandArgument)
 			);
 		}
 		else {
 			$defaultCommandArgument = new CommandArgument(
 				self::DEFAULT_COMMAND
 			);
-			$this->argumentList []= $defaultCommandArgument;
+			array_push($this->argumentList, $defaultCommandArgument);
 		}
 
 		$skipNextArgument = false;
@@ -82,19 +85,27 @@ protected function buildArgumentList(array $arguments):void {
 				}
 
 				if ($arg[1] === "-") {
-					$this->argumentList []= new LongOptionArgument(
-						$name,
-						$value
+					array_push(
+						$this->argumentList,
+						new LongOptionArgument(
+							$name,
+							$value
+						)
 					);
 				}
 				else {
-					$this->argumentList []= new ShortOptionArgument(
-						$arg,
-						$value
+					array_push($this->argumentList,
+						new ShortOptionArgument(
+							$arg,
+							$value
+						)
 					);
 				}
 			} else {
-				$this->argumentList []= new NamedArgument($arg);
+				array_push(
+					$this->argumentList,
+					new NamedArgument($arg)
+				);
 			}
 		}
 	}
@@ -177,4 +188,4 @@ public function getValueForParameter(Parameter $parameter):?string {
 
 		return null;
 	}
-}
\ No newline at end of file
+}
diff --git a/src/Argument/ArgumentValue.php b/src/Argument/ArgumentValue.php
index 1b4973a..8641722 100644
--- a/src/Argument/ArgumentValue.php
+++ b/src/Argument/ArgumentValue.php
@@ -2,9 +2,10 @@
 namespace Gt\Cli\Argument;
 
 class ArgumentValue {
-	protected $key;
-	protected $valueList;
-	protected $queueIndex;
+	protected string $key;
+	/** @var string[] */
+	protected array $valueList;
+	protected int $queueIndex;
 
 	public function __construct(string $key) {
 		$this->key = $key;
@@ -17,7 +18,7 @@ public function __toString():string {
 	}
 
 	public function push(string $value = null):void {
-		$this->valueList []= $value;
+		array_push($this->valueList, $value);
 	}
 
 	public function getKey():string {
@@ -30,7 +31,8 @@ public function get():?string {
 		return $value;
 	}
 
+	/** @return string[] */
 	public function getAll():array {
 		return $this->valueList;
 	}
-}
\ No newline at end of file
+}
diff --git a/src/Argument/ArgumentValueList.php b/src/Argument/ArgumentValueList.php
index 2636572..520864b 100644
--- a/src/Argument/ArgumentValueList.php
+++ b/src/Argument/ArgumentValueList.php
@@ -3,13 +3,15 @@
 
 use Iterator;
 
+/** @implements Iterator<int, ArgumentValue> */
 class ArgumentValueList implements Iterator {
 	const DEFAULT_ARGUMENT_VALUE = "//\\DEFAULT ARGUMENT VALUE\\//";
 
 	/** @var ArgumentValue[] */
-	protected $valueList = [];
-	protected $argumentValueMap = [];
-	protected $iteratorIndex;
+	protected array $valueList = [];
+	/** @var array<string, ArgumentValue> */
+	protected array $argumentValueMap = [];
+	protected int $iteratorIndex;
 
 	public function set(string $key, string $value = null):void {
 		if($this->contains($key)) {
@@ -28,7 +30,7 @@ public function set(string $key, string $value = null):void {
 
 	public function get(
 		string $key,
-		$default = self::DEFAULT_ARGUMENT_VALUE
+		string $default = self::DEFAULT_ARGUMENT_VALUE
 	):ArgumentValue {
 // $default is handled in this manner because we want to allow null to be a
 // valid value for the default.
@@ -48,12 +50,12 @@ public function contains(string $key):bool {
 	}
 
 	/** @link https://php.net/manual/en/iterator.rewind.php */
-	public function rewind() {
+	public function rewind():void {
 		$this->iteratorIndex = 0;
 	}
 
 	/** @link https://php.net/manual/en/iterator.key.php */
-	public function key() {
+	public function key():int {
 		return $this->iteratorIndex;
 	}
 
@@ -65,7 +67,7 @@ public function valid():bool {
 	}
 
 	/** @link https://php.net/manual/en/iterator.current.php */
-	public function current() {
+	public function current():?ArgumentValue {
 		if(!$this->iteratorIndex) {
 			$this->rewind();
 		}
@@ -73,11 +75,11 @@ public function current() {
 	}
 
 	/** @link https://php.net/manual/en/iterator.next.php */
-	public function next() {
+	public function next():void {
 		$this->iteratorIndex++;
 	}
 
-	public function first() {
+	public function first():?ArgumentValue {
 		return $this->valueList[0] ?? null;
 	}
-}
\ No newline at end of file
+}
diff --git a/src/Argument/NamedArgument.php b/src/Argument/NamedArgument.php
index 6e9d9ce..c97ba54 100644
--- a/src/Argument/NamedArgument.php
+++ b/src/Argument/NamedArgument.php
@@ -2,11 +2,11 @@
 namespace Gt\Cli\Argument;
 
 class NamedArgument extends Argument {
-	public function __construct($value) {
+	public function __construct(string $value) {
 		parent::__construct("", $value);
 	}
 
 	protected function processRawKey(string $rawKey): string {
 		return "";
 	}
-}
\ No newline at end of file
+}
diff --git a/src/Command/Command.php b/src/Command/Command.php
index 9aef5e4..10c3a59 100644
--- a/src/Command/Command.php
+++ b/src/Command/Command.php
@@ -15,10 +15,9 @@
 use Gt\Cli\Stream;
 
 abstract class Command {
-	/** @var Stream */
-	protected $output;
+	protected ?Stream $output;
 
-	public function setOutput(Stream $output = null) {
+	public function setOutput(Stream $output = null):void {
 		$this->output = $output;
 	}
 
@@ -231,7 +230,7 @@ public function getArgumentValueList(
 				continue;
 			}
 			elseif($argument instanceof NamedArgument) {
-				/** @var NamedParameter $parameter */
+				/** @var NamedParameter|null $parameter */
 				$parameter = $namedParameterList[
 					$namedParameterIndex
 				] ?? null;
@@ -285,7 +284,7 @@ protected function write(
 		string $message,
 		string $streamName = Stream::OUT
 	):void {
-		if(is_null($this->output)) {
+		if(!isset($this->output) || is_null($this->output)) {
 			return;
 		}
 
@@ -298,4 +297,4 @@ protected function writeLine(
 	):void {
 		$this->write($message . PHP_EOL, $streamName);
 	}
-}
\ No newline at end of file
+}
diff --git a/src/Command/HelpCommand.php b/src/Command/HelpCommand.php
index 366cadd..940da96 100644
--- a/src/Command/HelpCommand.php
+++ b/src/Command/HelpCommand.php
@@ -8,10 +8,10 @@
 class HelpCommand extends Command {
 	const ALL_COMMANDS = "*";
 
-	protected $applicationDescription;
+	protected string $applicationDescription;
 	/** @var Command[] */
-	protected $applicationCommandList;
-	protected $scriptName;
+	protected array $applicationCommandList;
+	protected ?string $scriptName;
 
 	/**
 	 * @param Command[] $applicationCommandList
@@ -140,4 +140,4 @@ protected function getHelpForCommand(string $commandName = null):string {
 
 		return $output;
 	}
-}
\ No newline at end of file
+}
diff --git a/src/Command/VersionCommand.php b/src/Command/VersionCommand.php
index b649ba5..d8c4785 100644
--- a/src/Command/VersionCommand.php
+++ b/src/Command/VersionCommand.php
@@ -6,17 +6,18 @@
 use Gt\Cli\Parameter\Parameter;
 
 class VersionCommand extends Command {
-	protected $script;
+	protected string $script;
 
 	public function __construct(string $script) {
 		$this->script = $script;
 	}
 
 	public function run(ArgumentValueList $arguments = null):void {
-		$this->writeLine($this->getVersion($arguments->get(
-			"command",
-			null
-		)));
+		$this->writeLine(
+			$this->getVersion($arguments->get(
+				"command"
+			)
+		));
 	}
 
 	public function getName():string {
@@ -88,4 +89,4 @@ protected function getVersion(string $command = null):string {
 
 		return $version;
 	}
-}
\ No newline at end of file
+}
diff --git a/src/ErrorCode.php b/src/ErrorCode.php
index 292fc26..339818f 100644
--- a/src/ErrorCode.php
+++ b/src/ErrorCode.php
@@ -11,7 +11,8 @@
 class ErrorCode {
 	const DEFAULT_CODE = 1000;
 
-	protected static $classList = [
+	/** @var string[] */
+	protected static array $classList = [
 		NotEnoughArgumentsException::class,
 		CommandException::class,
 		InvalidCommandException::class,
@@ -19,6 +20,7 @@ class ErrorCode {
 		MissingRequiredParameterException::class,
 	];
 
+	/** @param string|Exception $exception */
 	public static function get($exception):int {
 		if($exception instanceof Exception) {
 			$exception = get_class($exception);
@@ -29,4 +31,4 @@ public static function get($exception):int {
 			self::$classList
 		) ?: self::DEFAULT_CODE;
 	}
-}
\ No newline at end of file
+}
diff --git a/src/Parameter/Parameter.php b/src/Parameter/Parameter.php
index ada36e8..9b23465 100644
--- a/src/Parameter/Parameter.php
+++ b/src/Parameter/Parameter.php
@@ -2,11 +2,11 @@
 namespace Gt\Cli\Parameter;
 
 class Parameter {
-	protected $takesValue;
-	protected $longOption;
-	protected $shortOption;
-	protected $documentation;
-	protected $exampleValue;
+	protected bool $takesValue;
+	protected string $longOption;
+	protected ?string $shortOption;
+	protected ?string $documentation;
+	protected ?string $exampleValue;
 
 	public function __construct(
 		bool $takesValue,
@@ -54,4 +54,4 @@ public function getExampleValue():string {
 		return $this->exampleValue
 			?? strtoupper($this->longOption);
 	}
-}
\ No newline at end of file
+}
diff --git a/src/Stream.php b/src/Stream.php
index f0a7754..f3dedda 100644
--- a/src/Stream.php
+++ b/src/Stream.php
@@ -35,7 +35,7 @@ public function __construct(
 		$this->setStream($in, $out, $error);
 	}
 
-	public function setStream(string $in, string $out, string $error) {
+	public function setStream(string $in, string $out, string $error):void {
 		$this->in = new SplFileObject(
 			$in,
 			"r"
@@ -93,4 +93,4 @@ protected function getNamedStream(string $streamName):SplFileObject {
 
 		throw new InvalidStreamNameException($streamName);
 	}
-}
\ No newline at end of file
+}