diff --git a/.travis.yml b/.travis.yml
new file mode 100644
index 0000000..16c2c0a
--- /dev/null
+++ b/.travis.yml
@@ -0,0 +1,22 @@
+anguage: php
+
+os:
+  - linux
+
+php:
+  - 7.0
+  - 7.1
+
+matrix:
+  include:
+    - php: 7.0
+      env: PHPSTAN=1
+    - php: 7.1
+      env: PHPSTAN=1
+
+install:
+    - composer self-update
+    - composer install
+
+script:
+  - if ["$PHPSTAN" = "1"]; then vendor/bin/phpstan analyse src --level=7 --ansi --no-progress; fi
diff --git a/README.md b/README.md
index ba0fb86..41af695 100755
--- a/README.md
+++ b/README.md
@@ -1,11 +1,21 @@
+[![Build Status](https://travis-ci.org/WebChemistry/invoice.svg?branch=master)](https://travis-ci.org/WebChemistry/invoice)
+
 # PHP Invoice
 
 ## Installation
 
+php 7.0
+
 ```
 composer require webchemistry/invoice
 ```
 
+php 5.5
+
+```
+composer require webchemistry/invoice:^1.0
+```
+
 ## Usage
 
 ### Company
diff --git a/composer.json b/composer.json
index 2a50ce6..ad4de83 100755
--- a/composer.json
+++ b/composer.json
@@ -7,12 +7,13 @@
 		"invoice"
 	],
 	"require": {
-		"php": ">=5.5",
+		"php": ">=7.0",
 		"intervention/image": "^2.3",
 		"nette/utils": "^2.3"
 	},
 	"require-dev": {
-		"nette/di": "^2.3"
+		"nette/di": "^2.3",
+		"phpstan/phpstan": "^0.7.0"
 	},
 	"autoload": {
 		"psr-4": {
diff --git a/src/Components/IPaginator.php b/src/Components/IPaginator.php
index e55a216..3a3d65e 100755
--- a/src/Components/IPaginator.php
+++ b/src/Components/IPaginator.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace WebChemistry\Invoice\Components;
 
 use WebChemistry\Invoice\Data\Item;
@@ -9,26 +11,26 @@ interface IPaginator {
 	/**
 	 * @return int
 	 */
-	public function getTotalPages();
+	public function getTotalPages(): int;
 
 	/**
 	 * @return Item[]
 	 */
-	public function getItems();
+	public function getItems(): array;
 
 	/**
 	 * @return bool
 	 */
-	public function isLastPage();
+	public function isLastPage(): bool;
 
 	/**
 	 * @return int
 	 */
-	public function getCurrentPage();
+	public function getCurrentPage(): int;
 
 	/**
 	 * @return bool
 	 */
-	public function hasNextPage();
+	public function hasNextPage(): bool;
 
 }
diff --git a/src/Components/IPaginatorFactory.php b/src/Components/IPaginatorFactory.php
index c40716d..226f473 100755
--- a/src/Components/IPaginatorFactory.php
+++ b/src/Components/IPaginatorFactory.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace WebChemistry\Invoice\Components;
 
 use WebChemistry\Invoice\Data\Item;
@@ -10,6 +12,6 @@ interface IPaginatorFactory {
 	 * @param Item[] $items
 	 * @return IPaginator
 	 */
-	public function createPaginator(array $items);
+	public function createPaginator(array $items): IPaginator;
 
 }
diff --git a/src/Components/Paginator.php b/src/Components/Paginator.php
index 08139ae..a388d09 100755
--- a/src/Components/Paginator.php
+++ b/src/Components/Paginator.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace WebChemistry\Invoice\Components;
 
 use WebChemistry\Invoice\Data\Item;
@@ -28,14 +30,14 @@ public function __construct(array $items) {
 	/**
 	 * @return int
 	 */
-	public function getTotalPages() {
+	public function getTotalPages(): int {
 		return $this->totalPages;
 	}
 
 	/**
 	 * @return Item[]
 	 */
-	public function getItems() {
+	public function getItems(): array {
 		$page = $this->currentPage - 1;
 
 		return array_slice($this->items, $page * self::ITEMS_PER_PAGE, $page * self::ITEMS_PER_PAGE + self::ITEMS_PER_PAGE);
@@ -44,21 +46,21 @@ public function getItems() {
 	/**
 	 * @return bool
 	 */
-	public function isLastPage() {
+	public function isLastPage(): bool {
 		return $this->currentPage >= $this->getTotalPages();
 	}
 
 	/**
 	 * @return int
 	 */
-	public function getCurrentPage() {
+	public function getCurrentPage(): int {
 		return $this->currentPage;
 	}
 
 	/**
 	 * @return bool
 	 */
-	public function hasNextPage() {
+	public function hasNextPage(): bool {
 		if ($this->isLastPage()) {
 			return FALSE;
 		}
diff --git a/src/Components/PaginatorFactory.php b/src/Components/PaginatorFactory.php
index 31c22ca..c5a92ab 100755
--- a/src/Components/PaginatorFactory.php
+++ b/src/Components/PaginatorFactory.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace WebChemistry\Invoice\Components;
 
 use WebChemistry\Invoice\Data\Item;
@@ -8,9 +10,9 @@ class PaginatorFactory implements IPaginatorFactory {
 
 	/**
 	 * @param Item[] $items
-	 * @return Paginator
+	 * @return IPaginator
 	 */
-	public function createPaginator(array $items) {
+	public function createPaginator(array $items): IPaginator {
 		return new Paginator($items);
 	}
 
diff --git a/src/DI/InvoiceExtension.php b/src/DI/InvoiceExtension.php
index 01d3e64..078966d 100755
--- a/src/DI/InvoiceExtension.php
+++ b/src/DI/InvoiceExtension.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace WebChemistry\Invoice\DI;
 
 use Nette\DI\CompilerExtension;
diff --git a/src/Data/Account.php b/src/Data/Account.php
index f9cdbfe..057e61b 100755
--- a/src/Data/Account.php
+++ b/src/Data/Account.php
@@ -1,8 +1,8 @@
 <?php
 
-namespace WebChemistry\Invoice\Data;
+declare(strict_types=1);
 
-use WebChemistry\Invoice\InvoiceException;
+namespace WebChemistry\Invoice\Data;
 
 class Account {
 
@@ -20,29 +20,10 @@ class Account {
 	 * @param string|null $iBan
 	 * @param string|null $swift
 	 */
-	public function __construct($accountNumber, $iBan = NULL, $swift = NULL) {
+	public function __construct(string $accountNumber, ?string $iBan = NULL, ?string $swift = NULL) {
 		$this->accountNumber = $accountNumber;
 		$this->iBan = $iBan;
 		$this->swift = $swift;
-
-		$this->validate();
-	}
-
-	/**
-	 * Validates properties
-	 *
-	 * @throws InvoiceException
-	 */
-	private function validate() {
-		if (!$this->accountNumber || !is_string($this->accountNumber)) {
-			throw InvoiceException::wrongType('non-empty string', $this->accountNumber);
-		}
-		if ($this->iBan !== NULL && !$this->iBan || !is_string($this->iBan)) {
-			throw InvoiceException::wrongType('non-empty string or null', $this->iBan);
-		}
-		if ($this->swift !== NULL && !$this->swift || !is_string($this->swift)) {
-			throw InvoiceException::wrongType('non-empty string or null', $this->iBan);
-		}
 	}
 
 	/////////////////////////////////////////////////////////////////
@@ -50,21 +31,21 @@ private function validate() {
 	/**
 	 * @return string
 	 */
-	public function getAccountNumber() {
+	public function getAccountNumber(): string {
 		return $this->accountNumber;
 	}
 
 	/**
 	 * @return null|string
 	 */
-	public function getIBan() {
+	public function getIBan(): ?string {
 		return $this->iBan;
 	}
 
 	/**
 	 * @return null|string
 	 */
-	public function getSwift() {
+	public function getSwift(): ?string {
 		return $this->swift;
 	}
 
diff --git a/src/Data/Company.php b/src/Data/Company.php
index a9d9140..f688004 100755
--- a/src/Data/Company.php
+++ b/src/Data/Company.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace WebChemistry\Invoice\Data;
 
 class Company extends Subject {
@@ -17,16 +19,16 @@ class Company extends Subject {
 	 * @param string|null $vaTin
 	 * @param bool $hasTax
 	 */
-	public function __construct($name, $town, $address, $zip, $country, $tin = NULL, $vaTin = NULL,
-								$hasTax = FALSE) {
+	public function __construct(string $name, string $town, string $address, string $zip, string $country, ?string $tin = NULL, ?string $vaTin = NULL,
+								bool $hasTax = FALSE) {
 		parent::__construct($name, $town, $address, $zip, $country, $tin, $vaTin);
-		$this->hasTax = (bool) $hasTax;
+		$this->hasTax = $hasTax;
 	}
 
 	/**
 	 * @return bool
 	 */
-	public function hasTax() {
+	public function hasTax(): bool {
 		return $this->hasTax;
 	}
 
diff --git a/src/Data/Customer.php b/src/Data/Customer.php
index 995561f..fdfe50a 100755
--- a/src/Data/Customer.php
+++ b/src/Data/Customer.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace WebChemistry\Invoice\Data;
 
 class Customer extends Subject {
diff --git a/src/Data/Item.php b/src/Data/Item.php
index 7f38386..4d7fbf0 100755
--- a/src/Data/Item.php
+++ b/src/Data/Item.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace WebChemistry\Invoice\Data;
 
 use WebChemistry\Invoice\InvoiceException;
@@ -9,19 +11,19 @@ class Item {
 	/** @var string */
 	protected $name;
 
-	/** @var int */
+	/** @var int|float */
 	protected $count;
 
-	/** @var int */
+	/** @var int|float */
 	protected $price;
 
 	/**
 	 * @param string $name
-	 * @param int $count
-	 * @param int $price
+	 * @param int|float $count
+	 * @param int|float $price
 	 * @throws InvoiceException
 	 */
-	public function __construct($name, $count, $price) {
+	public function __construct(string $name, $count, $price) {
 		$this->name = $name;
 		$this->count = $count;
 		$this->price = $price;
@@ -35,9 +37,6 @@ public function __construct($name, $count, $price) {
 	 * @throws InvoiceException
 	 */
 	private function validate() {
-		if (!$this->name || !is_string($this->name)) {
-			throw InvoiceException::wrongType('non-empty string', $this->name);
-		}
 		if (!is_numeric($this->count)) {
 			throw InvoiceException::wrongType('numeric', $this->count);
 		}
@@ -51,19 +50,19 @@ private function validate() {
 	/**
 	 * @return string
 	 */
-	public function getName() {
+	public function getName(): string {
 		return $this->name;
 	}
 
 	/**
-	 * @return int
+	 * @return int|float
 	 */
 	public function getCount() {
 		return $this->count;
 	}
 
 	/**
-	 * @return int
+	 * @return int|float
 	 */
 	public function getPrice() {
 		return $this->price;
diff --git a/src/Data/Order.php b/src/Data/Order.php
index 3c20be8..523c505 100755
--- a/src/Data/Order.php
+++ b/src/Data/Order.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace WebChemistry\Invoice\Data;
 
 use WebChemistry\Invoice\InvoiceException;
@@ -48,7 +50,7 @@ public function __construct($number, \DateTime $dueDate, Account $account, Payme
 	 * @param int|float $count
 	 * @return Item
 	 */
-	public function addItem($name, $price, $count = 1) {
+	public function addItem(string $name, $price, $count = 1) {
 		return $this->items[] = new Item($name, $price, $count);
 	}
 
@@ -75,35 +77,35 @@ public function getNumber() {
 	/**
 	 * @return \DateTime
 	 */
-	public function getDueDate() {
+	public function getDueDate(): \DateTime {
 		return $this->dueDate;
 	}
 
 	/**
 	 * @return Account
 	 */
-	public function getAccount() {
+	public function getAccount(): Account {
 		return $this->account;
 	}
 
 	/**
 	 * @return PaymentInformation
 	 */
-	public function getPayment() {
+	public function getPayment(): PaymentInformation {
 		return $this->payment;
 	}
 
 	/**
 	 * @return \DateTime
 	 */
-	public function getCreated() {
+	public function getCreated(): \DateTime {
 		return $this->created;
 	}
 
 	/**
 	 * @return Item[]
 	 */
-	public function getItems() {
+	public function getItems(): array {
 		return $this->items;
 	}
 
diff --git a/src/Data/PaymentInformation.php b/src/Data/PaymentInformation.php
index 3c74b48..beb6c38 100755
--- a/src/Data/PaymentInformation.php
+++ b/src/Data/PaymentInformation.php
@@ -1,18 +1,18 @@
 <?php
 
-namespace WebChemistry\Invoice\Data;
+declare(strict_types=1);
 
-use WebChemistry\Invoice\InvoiceException;
+namespace WebChemistry\Invoice\Data;
 
 class PaymentInformation {
 
 	/** @var string */
 	private $currency;
 
-	/** @var string */
+	/** @var string|null */
 	private $variableSymbol;
 
-	/** @var string */
+	/** @var string|null */
 	private $constantSymbol;
 
 	/** @var float|null */
@@ -24,54 +24,38 @@ class PaymentInformation {
 	 * @param string|null $constantSymbol
 	 * @param float|null $tax
 	 */
-	public function __construct($currency, $variableSymbol = NULL, $constantSymbol = NULL, $tax = NULL) {
+	public function __construct(string $currency, ?string $variableSymbol = NULL, ?string $constantSymbol = NULL, ?float $tax = NULL) {
 		$this->currency = $currency;
 		$this->variableSymbol = $variableSymbol;
 		$this->constantSymbol = $constantSymbol;
 		$this->tax = $tax;
-
-		$this->validate();
-	}
-
-	/**
-	 * @throws InvoiceException
-	 */
-	private function validate() {
-		if (!$this->currency || !is_string($this->currency)) {
-			throw InvoiceException::wrongType('non-empty string', $this->currency);
-		}
-		if ($this->tax && !is_float($this->tax)) {
-			throw InvoiceException::wrongType('float', $this->tax);
-		}
 	}
 
-	/////////////////////////////////////////////////////////////////
-
 	/**
 	 * @return string
 	 */
-	public function getCurrency() {
+	public function getCurrency(): string {
 		return $this->currency;
 	}
 
 	/**
-	 * @return string
+	 * @return string|null
 	 */
-	public function getVariableSymbol() {
+	public function getVariableSymbol(): ?string {
 		return $this->variableSymbol;
 	}
 
 	/**
-	 * @return string
+	 * @return string|null
 	 */
-	public function getConstantSymbol() {
+	public function getConstantSymbol(): ?string {
 		return $this->constantSymbol;
 	}
 
 	/**
 	 * @return float|null
 	 */
-	public function getTax() {
+	public function getTax(): ?float {
 		return $this->tax;
 	}
 
diff --git a/src/Data/Subject.php b/src/Data/Subject.php
index 141f30d..501cfda 100755
--- a/src/Data/Subject.php
+++ b/src/Data/Subject.php
@@ -1,8 +1,8 @@
 <?php
 
-namespace WebChemistry\Invoice\Data;
+declare(strict_types=1);
 
-use WebChemistry\Invoice\InvoiceException;
+namespace WebChemistry\Invoice\Data;
 
 abstract class Subject {
 
@@ -21,10 +21,10 @@ abstract class Subject {
 	/** @var string */
 	protected $country;
 
-	/** @var string */
+	/** @var string|null */
 	protected $tin;
 
-	/** @var string */
+	/** @var string|null */
 	protected $vaTin;
 
 	/**
@@ -35,9 +35,8 @@ abstract class Subject {
 	 * @param string $country
 	 * @param string|null $tin
 	 * @param string|null $vaTin
-	 * @throws InvoiceException
 	 */
-	public function __construct($name, $town, $address, $zip, $country, $tin = NULL, $vaTin = NULL) {
+	public function __construct(string $name, string $town, string $address, string $zip, string $country, ?string $tin = NULL, ?string $vaTin = NULL) {
 		$this->name = $name;
 		$this->town = $town;
 		$this->address = $address;
@@ -45,71 +44,54 @@ public function __construct($name, $town, $address, $zip, $country, $tin = NULL,
 		$this->country = $country;
 		$this->tin = $tin;
 		$this->vaTin = $vaTin;
-
-		$this->validate();
-	}
-
-	/**
-	 * Validates properties
-	 *
-	 * @throws InvoiceException
-	 */
-	private function validate() {
-		foreach (['name', 'town', 'address', 'zip', 'country'] as $val) {
-			if (!$this->$val || !is_string($this->$val)) {
-				throw InvoiceException::wrongType('non-empty string', $this->$val);
-			}
-		}
 	}
 
-	/////////////////////////////////////////////////////////////////
-
 	/**
 	 * @return string
 	 */
-	public function getName() {
+	public function getName(): string {
 		return $this->name;
 	}
 
 	/**
 	 * @return string
 	 */
-	public function getTown() {
+	public function getTown(): string {
 		return $this->town;
 	}
 
 	/**
 	 * @return string
 	 */
-	public function getAddress() {
+	public function getAddress(): string {
 		return $this->address;
 	}
 
 	/**
 	 * @return string
 	 */
-	public function getZip() {
+	public function getZip(): string {
 		return $this->zip;
 	}
 
 	/**
 	 * @return string
 	 */
-	public function getCountry() {
+	public function getCountry(): string {
 		return $this->country;
 	}
 
 	/**
-	 * @return string
+	 * @return string|null
 	 */
-	public function getTin() {
+	public function getTin(): ?string {
 		return $this->tin;
 	}
 
 	/**
-	 * @return string
+	 * @return string|null
 	 */
-	public function getVaTin() {
+	public function getVaTin(): ?string {
 		return $this->vaTin;
 	}
 
diff --git a/src/Data/Template.php b/src/Data/Template.php
index 6690f7e..d6b3822 100755
--- a/src/Data/Template.php
+++ b/src/Data/Template.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace WebChemistry\Invoice\Data;
 
 use WebChemistry\Invoice\InvoiceException;
@@ -30,10 +32,10 @@ class Template {
 	/** @var array */
 	protected $colorEven = [241, 240, 240];
 
-	/** @var string */
+	/** @var string|null */
 	protected $footer;
 
-	/** @var string */
+	/** @var string|null */
 	protected $logo;
 
 	public function __construct() {
@@ -45,7 +47,7 @@ public function __construct() {
 	/**
 	 * @return string
 	 */
-	public function getFont() {
+	public function getFont(): string {
 		return $this->font;
 	}
 
@@ -54,7 +56,7 @@ public function getFont() {
 	 * @throws InvoiceException
 	 * @return self
 	 */
-	public function setFont($font) {
+	public function setFont(string $font): self {
 		if (!file_exists($font) || !is_file($font)) {
 			throw new InvoiceException("File '$font' not exists.");
 		}
@@ -66,7 +68,7 @@ public function setFont($font) {
 	/**
 	 * @return string
 	 */
-	public function getFontBold() {
+	public function getFontBold(): string {
 		return $this->fontBold;
 	}
 
@@ -75,7 +77,7 @@ public function getFontBold() {
 	 * @throws InvoiceException
 	 * @return self
 	 */
-	public function setFontBold($fontBold) {
+	public function setFontBold(string $fontBold): self {
 		if (!file_exists($fontBold) || !is_file($fontBold)) {
 			throw new InvoiceException("File '$fontBold' not exists.");
 		}
@@ -87,7 +89,7 @@ public function setFontBold($fontBold) {
 	/**
 	 * @return string
 	 */
-	public function getIconFont() {
+	public function getIconFont(): string {
 		return $this->iconFont;
 	}
 
@@ -96,7 +98,7 @@ public function getIconFont() {
 	 * @throws InvoiceException
 	 * @return self
 	 */
-	public function setIconFont($iconFont) {
+	public function setIconFont(string $iconFont): self {
 		if (!file_exists($iconFont) || !is_file($iconFont)) {
 			throw new InvoiceException("File '$iconFont' not exists.");
 		}
@@ -108,16 +110,16 @@ public function setIconFont($iconFont) {
 	/**
 	 * @return array
 	 */
-	public function getBaseColor() {
+	public function getBaseColor(): array {
 		return $this->baseColor;
 	}
 
 	/**
 	 * @param array $baseColor
-	 * @throws
+	 * @throws InvoiceException
 	 * @return self
 	 */
-	public function setBaseColor(array $baseColor) {
+	public function setBaseColor(array $baseColor): self {
 		if (count($baseColor) !== 3) {
 			throw new InvoiceException('Color must have 3 items.');
 		}
@@ -129,7 +131,7 @@ public function setBaseColor(array $baseColor) {
 	/**
 	 * @return array
 	 */
-	public function getPrimaryColor() {
+	public function getPrimaryColor(): array {
 		return $this->primaryColor;
 	}
 
@@ -138,7 +140,7 @@ public function getPrimaryColor() {
 	 * @throws InvoiceException
 	 * @return self
 	 */
-	public function setPrimaryColor($primaryColor) {
+	public function setPrimaryColor(array $primaryColor): self {
 		if (count($primaryColor) !== 3) {
 			throw new InvoiceException('Color must have 3 items.');
 		}
@@ -150,7 +152,7 @@ public function setPrimaryColor($primaryColor) {
 	/**
 	 * @return array
 	 */
-	public function getFontColor() {
+	public function getFontColor(): array {
 		return $this->fontColor;
 	}
 
@@ -159,7 +161,7 @@ public function getFontColor() {
 	 * @throws InvoiceException
 	 * @return self
 	 */
-	public function setFontColor($fontColor) {
+	public function setFontColor(array $fontColor): self {
 		if (count($fontColor) !== 3) {
 			throw new InvoiceException('Color must have 3 items.');
 		}
@@ -171,7 +173,7 @@ public function setFontColor($fontColor) {
 	/**
 	 * @return array
 	 */
-	public function getColorOdd() {
+	public function getColorOdd(): array {
 		return $this->colorOdd;
 	}
 
@@ -180,7 +182,7 @@ public function getColorOdd() {
 	 * @throws InvoiceException
 	 * @return self
 	 */
-	public function setColorOdd($colorOdd) {
+	public function setColorOdd(array $colorOdd): self {
 		if (count($colorOdd) !== 3) {
 			throw new InvoiceException('Color must have 3 items.');
 		}
@@ -192,7 +194,7 @@ public function setColorOdd($colorOdd) {
 	/**
 	 * @return array
 	 */
-	public function getColorEven() {
+	public function getColorEven(): array {
 		return $this->colorEven;
 	}
 
@@ -201,7 +203,7 @@ public function getColorEven() {
 	 * @throws InvoiceException
 	 * @return self
 	 */
-	public function setColorEven($colorEven) {
+	public function setColorEven(array $colorEven): self {
 		if (count($colorEven) !== 3) {
 			throw new InvoiceException('Color must have 3 items.');
 		}
@@ -211,30 +213,30 @@ public function setColorEven($colorEven) {
 	}
 
 	/**
-	 * @param string $footer
+	 * @param string|null $footer
 	 */
-	public function setFooter($footer) {
+	public function setFooter(?string $footer = NULL) {
 		$this->footer = $footer;
 	}
 
 	/**
-	 * @return string
+	 * @return string|null
 	 */
-	public function getFooter() {
+	public function getFooter(): ?string {
 		return $this->footer;
 	}
 
 	/**
-	 * @param string $logo
+	 * @param string|null $logo
 	 */
-	public function setLogo($logo) {
+	public function setLogo(?string $logo = NULL) {
 		$this->logo = $logo;
 	}
 
 	/**
 	 * @return string
 	 */
-	public function getLogo() {
+	public function getLogo(): ?string {
 		return $this->logo;
 	}
 
diff --git a/src/Formatter.php b/src/Formatter.php
index 9e63d8c..f8c53d5 100644
--- a/src/Formatter.php
+++ b/src/Formatter.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace WebChemistry\Invoice;
 
 class Formatter implements IFormatter {
@@ -30,19 +32,19 @@ class Formatter implements IFormatter {
 	/** @var string */
 	private $lang;
 
-	public function __construct($lang = self::ENGLISH) {
+	public function __construct(string $lang = self::ENGLISH) {
 		$this->lang = $lang;
 	}
 
-	public function formatNumber($float) {
+	public function formatNumber($float): string {
 		return number_format($float, 2, self::$options[$this->lang]['number']['dec'], self::$options[$this->lang]['number']['sep']);
 	}
 
-	public function formatMoney($float, $currency) {
+	public function formatMoney($float, string $currency): string {
 		return strtr(self::$options[$this->lang]['money'], ['%money' => $this->formatNumber($float), '%currency' => $currency]);
 	}
 
-	public function formatDate(\DateTime $date) {
+	public function formatDate(\DateTime $date): string {
 		return $date->format(self::$options[$this->lang]['date']);
 	}
 
diff --git a/src/IFormatter.php b/src/IFormatter.php
index 031dc4b..5874736 100644
--- a/src/IFormatter.php
+++ b/src/IFormatter.php
@@ -10,19 +10,19 @@ interface IFormatter {
 	 * @param float $float
 	 * @return string
 	 */
-	public function formatNumber($float);
+	public function formatNumber($float): string;
 
 	/**
 	 * @param float $float
 	 * @param string $currency
 	 * @return string
 	 */
-	public function formatMoney($float, $currency);
+	public function formatMoney($float, string $currency): string;
 
 	/**
 	 * @param \DateTime $date
 	 * @return string
 	 */
-	public function formatDate(\DateTime $date);
+	public function formatDate(\DateTime $date): string;
 
 }
diff --git a/src/ITranslator.php b/src/ITranslator.php
index 4993fe0..31dfd7a 100755
--- a/src/ITranslator.php
+++ b/src/ITranslator.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace WebChemistry\Invoice;
 
 interface ITranslator {
@@ -8,6 +10,6 @@ interface ITranslator {
 	 * @param string $message
 	 * @return string
 	 */
-	public function translate($message);
+	public function translate(string $message): string;
 
 }
diff --git a/src/Invoice.php b/src/Invoice.php
index f5b65e8..3b30fd9 100755
--- a/src/Invoice.php
+++ b/src/Invoice.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace WebChemistry\Invoice;
 
 use Intervention\Image\AbstractShape;
@@ -33,7 +35,7 @@ class Invoice {
 	/** @var Image */
 	private $image;
 	
-	/** @var Translator */
+	/** @var ITranslator */
 	private $translator;
 
 	/** @var IPaginatorFactory */
@@ -62,7 +64,7 @@ public function __construct(Company $company, Template $template = NULL, ITransl
 	 * @param string $message
 	 * @return string
 	 */
-	protected function translate($message) {
+	protected function translate(string $message): string {
 		return $this->translator->translate($message);
 	}
 
@@ -71,7 +73,7 @@ protected function translate($message) {
 	/**
 	 * @return string Encoded invoice
 	 */
-	public function generatePreview() {
+	public function generatePreview(): string {
 		$factory = new InvoiceFactory();
 
 		$tax = $this->company->hasTax() ? 0.21 : NULL;
@@ -89,7 +91,7 @@ public function generatePreview() {
 
 		$images = $this->create($customer, $order);
 
-		return $images[0]->encode();
+		return (string) $images[0]->encode();
 	}
 
 	/**
@@ -98,7 +100,7 @@ public function generatePreview() {
 	 * @throws InvoiceException
 	 * @return Image[]
 	 */
-	public function create(Customer $customer, Order $order) {
+	public function create(Customer $customer, Order $order): array {
 		$this->customer = $customer;
 		$this->order = $order;
 		$paginator = $this->paginatorFactory->createPaginator($order->getItems());
@@ -133,7 +135,6 @@ public function create(Customer $customer, Order $order) {
 
 			$pages[] = $this->image;
 		}
-		$this->image = NULL;
 
 		return $pages;
 	}
@@ -543,7 +544,7 @@ protected function item($multiplier, Item $item) {
 			$font->color($this->template->getFontColor());
 		});
 
-		$this->image->text($this->formatter->formatNumber($item->getCount(), 0), 1255, 1350 + $plus, function (Font $font) {
+		$this->image->text($this->formatter->formatNumber($item->getCount()), 1255, 1350 + $plus, function (Font $font) {
 			$font->size(27);
 			$font->file($this->template->getFont());
 			$font->valign('center');
diff --git a/src/InvoiceException.php b/src/InvoiceException.php
index 43642e5..adec629 100755
--- a/src/InvoiceException.php
+++ b/src/InvoiceException.php
@@ -4,7 +4,7 @@
 
 class InvoiceException extends \Exception {
 
-	public static function wrongType($need, $given) {
+	public static function wrongType($need, $given): self {
 		$given = is_object($given) ? get_class($given) : gettype($given);
 
 		return new self(sprintf('%s expected, %s given.', $need, $given));
diff --git a/src/InvoiceFactory.php b/src/InvoiceFactory.php
index 9d2ffde..703405f 100755
--- a/src/InvoiceFactory.php
+++ b/src/InvoiceFactory.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace WebChemistry\Invoice;
 
 use WebChemistry\Invoice\Data\Account;
@@ -17,7 +19,7 @@ class InvoiceFactory {
 	 * @param string|null $swift
 	 * @return Account
 	 */
-	public function createAccount($accountNumber, $iBan = NULL, $swift = NULL) {
+	public function createAccount($accountNumber, $iBan = NULL, $swift = NULL): Account {
 		return new Account($accountNumber, $iBan, $swift);
 	}
 
@@ -32,7 +34,7 @@ public function createAccount($accountNumber, $iBan = NULL, $swift = NULL) {
 	 * @param bool $hasTax
 	 * @return Company
 	 */
-	public function createCompany($name, $town, $address, $zip, $country, $tin = NULL, $vaTin = NULL, $hasTax = FALSE) {
+	public function createCompany($name, $town, $address, $zip, $country, $tin = NULL, $vaTin = NULL, $hasTax = FALSE): Company {
 		return new Company($name, $town, $address, $zip, $country, $tin, $vaTin, $hasTax);
 	}
 
@@ -46,7 +48,7 @@ public function createCompany($name, $town, $address, $zip, $country, $tin = NUL
 	 * @param string|null $vaTin
 	 * @return Customer
 	 */
-	public function createCustomer($name, $town, $address, $zip, $country, $tin = NULL, $vaTin = NULL) {
+	public function createCustomer($name, $town, $address, $zip, $country, $tin = NULL, $vaTin = NULL): Customer {
 		return new Customer($name, $town, $address, $zip, $country, $tin, $vaTin);
 	}
 
@@ -58,7 +60,7 @@ public function createCustomer($name, $town, $address, $zip, $country, $tin = NU
 	 * @param \DateTime|NULL $created
 	 * @return Order
 	 */
-	public function createOrder($number, \DateTime $dueDate, Account $account, PaymentInformation $payment, \DateTime $created = NULL) {
+	public function createOrder($number, \DateTime $dueDate, Account $account, PaymentInformation $payment, \DateTime $created = NULL): Order {
 		return new Order($number, $dueDate, $account, $payment, $created);
 	}
 
@@ -69,14 +71,14 @@ public function createOrder($number, \DateTime $dueDate, Account $account, Payme
 	 * @param float|null $tax
 	 * @return PaymentInformation
 	 */
-	public function createPaymentInformation($currency, $variableSymbol = NULL, $constantSymbol = NULL, $tax = NULL) {
+	public function createPaymentInformation($currency, $variableSymbol = NULL, $constantSymbol = NULL, $tax = NULL): PaymentInformation {
 		return new PaymentInformation($currency, $variableSymbol, $constantSymbol, $tax);
 	}
 
 	/**
 	 * @return Template
 	 */
-	public function createTemplate() {
+	public function createTemplate(): Template {
 		return new Template();
 	}
 
diff --git a/src/Translator.php b/src/Translator.php
index a55c50a..86fef4a 100755
--- a/src/Translator.php
+++ b/src/Translator.php
@@ -70,7 +70,7 @@ class Translator implements ITranslator {
 	 * @param string $lang
 	 * @throws InvoiceException
 	 */
-	public function __construct($lang = self::ENGLISH) {
+	public function __construct(string $lang = self::ENGLISH) {
 		$this->lang = $lang;
 		if (!isset(self::$translations[$this->lang])) {
 			throw new InvoiceException("Language $lang not exists.");
@@ -81,7 +81,7 @@ public function __construct($lang = self::ENGLISH) {
 	 * @param string $message
 	 * @return string
 	 */
-	public function translate($message) {
+	public function translate(string $message): string {
 		return self::$translations[$this->lang][$message];
 	}