Testing Private Methods

Testing Private Methods

// Generally: test behavior, not implementation
// If private methods are complex, they may need extracting

class Calculator {
    private function validateInput(int $n): void {
        if ($n < 0) throw new InvalidArgumentException("Must be positive");
    }

    public function factorial(int $n): int {
        $this->validateInput($n);
        return $n <= 1 ? 1 : $n * $this->factorial($n - 1);
    }
}

// Test THROUGH the public method
public function testFactorialValidatesInput(): void {
    $this->expectException(InvalidArgumentException::class);
    (new Calculator)->factorial(-1);
}

// Access private via Reflection (when necessary)
public function testPrivateMethod(): void {
    $calc   = new Calculator();
    $method = new ReflectionMethod($calc, "validateInput");
    // PHP 8.1+ — no need for setAccessible(true)
    $method->setAccessible(true);

    $this->expectException(InvalidArgumentException::class);
    $method->invoke($calc, -5);
}