diff --git a/composer.json b/composer.json
index c3a3767..8088333 100644
--- a/composer.json
+++ b/composer.json
@@ -16,7 +16,7 @@
         }
     },
     "require": {
-        "php": "^7.2",
-        "phpunit/phpunit": "^8.5"
+        "php": "^7.3",
+        "phpunit/phpunit": "^8.5|^9.0"
     }
 }
diff --git a/src/EasyMock.php b/src/EasyMock.php
index 27c5b52..8e02dfe 100644
--- a/src/EasyMock.php
+++ b/src/EasyMock.php
@@ -22,10 +22,8 @@ trait EasyMock
      *
      * @param string|MockObject $classname The class to mock. Can also be an existing mock to mock new methods.
      * @param array             $methods   Array of values to return, indexed by the method name.
-     *
-     * @return MockObject
      */
-    protected function easyMock($classname, array $methods = array()): MockObject
+    protected function easyMock($classname, array $methods = []): MockObject
     {
         $mock = $classname instanceof MockObject ? $classname : $this->createMock($classname);
 
@@ -45,10 +43,8 @@ protected function easyMock($classname, array $methods = array()): MockObject
      *
      * @param string|MockObject $classname The class to mock. Can also be an existing mock to mock new methods.
      * @param array             $methods   Array of values to return, indexed by the method name.
-     *
-     * @return MockObject
      */
-    protected function easySpy($classname, array $methods = array()): MockObject
+    protected function easySpy($classname, array $methods = []): MockObject
     {
         $mock = $classname instanceof MockObject ? $classname : $this->createMock($classname);
 
@@ -59,9 +55,7 @@ protected function easySpy($classname, array $methods = array()): MockObject
         return $mock;
     }
 
-    abstract protected function createMock($originalClassName): MockObject;
-
-    private function mockMethod(MockObject $mock, $method, InvocationOrder $invocation, $return): void
+    private function mockMethod(MockObject $mock, string $method, InvocationOrder $invocation, $return): void
     {
         $methodAssertion = $mock->expects($invocation)->method($method);
 
diff --git a/tests/EasyMockTest.php b/tests/EasyMockTest.php
index b0ed11e..3c9fc7d 100644
--- a/tests/EasyMockTest.php
+++ b/tests/EasyMockTest.php
@@ -25,7 +25,7 @@ public function should_mock_objects(): void
         /** @var ClassFixture $mock */
         $mock = $this->easyMock(ClassFixture::class);
 
-        $this->assertNull($mock->foo());
+        self::assertNull($mock->foo());
     }
 
     /**
@@ -36,7 +36,7 @@ public function should_skip_the_constructor(): void
         /** @var ClassWithConstructor $mock */
         $mock = $this->easyMock(ClassWithConstructor::class);
 
-        $this->assertFalse($mock->constructorCalled);
+        self::assertFalse($mock->constructorCalled);
     }
 
     /**
@@ -47,7 +47,7 @@ public function should_mock_interfaces(): void
         /** @var InterfaceFixture $mock */
         $mock = $this->easyMock(InterfaceFixture::class);
 
-        $this->assertNull($mock->foo());
+        self::assertNull($mock->foo());
     }
 
     /**
@@ -58,7 +58,7 @@ public function not_mocked_methods_should_return_null(): void
         /** @var ClassFixture $mock */
         $mock = $this->easyMock(ClassFixture::class);
 
-        $this->assertNull($mock->foo());
+        self::assertNull($mock->foo());
     }
 
     /**
@@ -71,7 +71,7 @@ public function should_mock_existing_method_with_a_value(): void
             'foo' => 'bar',
         ));
 
-        $this->assertSame('bar', $mock->foo());
+        self::assertSame('bar', $mock->foo());
     }
 
     /**
@@ -86,7 +86,7 @@ public function should_mock_existing_method_with_a_callback(): void
             },
         ));
 
-        $this->assertSame('bar', $mock->foo());
+        self::assertSame('bar', $mock->foo());
     }
 
     /**
@@ -116,7 +116,7 @@ public function should_mock_new_methods_on_existing_mock(): void
             'foo' => 'bar',
         ));
 
-        $this->assertSame('bar', $mock->foo());
+        self::assertSame('bar', $mock->foo());
     }
 
     /**
@@ -134,8 +134,8 @@ public function should_allow_to_spy_method_calls(): void
         $property->setAccessible(true);
         $mockObjects = $property->getValue($this);
 
-        $this->assertCount(1, $mockObjects);
-        $this->assertSame($mock, $mockObjects[0]);
+        self::assertCount(1, $mockObjects);
+        self::assertSame($mock, $mockObjects[0]);
 
         // Cannot use @expectedException because PHPUnit has specific behavior for this
         try {