public function testContainsKey()
{
$elements = [1, 'A' => 'a', 2, 'null' => null, 3, 'A2' => 'a', 'B2' => 'b'];
$collection = new ArrayCollection($elements);
$this->assertTrue($collection->containsKey(0), 'Contains index 0');
$this->assertTrue($collection->containsKey('A'), 'Contains key "A"');
$this->assertTrue($collection->containsKey('null'), 'Contains key "null", with value null');
$this->assertFalse($collection->containsKey('non-existent'), "Doesn't contain key");
}
