Why are static methods untestable?

Question

Why are static methods untestable? Kindly exemplify (in PHP if possible).

Answer 1

Static methods themselves aren't untestable, but if the object being tested calls a static method then a test cannot "get in between" and make it call a stub method instead. If the object being tested instead calls a regular method, the test can give it an alternative object with a stub implementation of that method.

In general, rigid dependencies are less testable while dependency injection (google it) makes code more testable.

For instance, let's say we have a static method getCurrentUser() that is used by the class we are testing, as follows

class PostModel {
    //...
    public function getRecentPosts() {
        return $this->database->from('posts')
                ->where(array('user' => UserModel::getCurrentUser()))
                ->limit(10);
    }
}

Now UserModel::getCurrentUser() cannot be replaced with a stub method. If we make it a regular method that we call through an object reference instead, we can pass in an alternative stub object in our test.

class PostModel {
    private $userModel;
    public function __construct($userModel) { 
        $this->userModel = $userModel;
    }
    //...
    public function getRecentPosts() {
        return $this->database->from('posts')
                ->where(array('user' => $this->userModel->getCurrentUser()))
                ->limit(10);
    }
}