Test Stub

A Test Stub is a controllable replacement for an existing dependency (or collaborator) in the system. The stub has the same API as the dependency, but the implementation is simplified. It's designed to return specific, hard-coded values or responses to the system under test (SUT).

Stubs are a type of Test Double, a general term for any object that stands in for a real object during a test. The primary purpose of a stub is to provide the SUT with the necessary data or state to execute a specific test case, thereby isolating the SUT from its actual dependencies.

Key Concepts

• State Verification: Stubs are primarily used for state verification. This means you use a stub to provide input to the SUT, and then you check the final state of the SUT to see if it behaved correctly. The stub itself is not verified; it's just a means to an end.
• Isolation: By replacing a real dependency (like a database, a web service, or a complex component), a stub allows you to test a single unit of code in isolation. This makes tests faster, more reliable, and easier to write and debug.
• Canned Responses: A stub's core feature is providing "canned" or predefined answers. For example, a stub for a user repository might be configured to always return a user object with the name "John Doe" whenever its getUser method is called.

Example (Pseudocode)

Imagine you are testing a DiscountCalculator that depends on a ProductService to get a product's price.

// The dependency we want to replace
interface ProductService {
  Price getPrice(productId);
}

// The system under test (SUT)
class DiscountCalculator {
  ProductService service;
  Price calculate(productId) {
    Price originalPrice = service.getPrice(productId);
    // Apply a 10% discount
    return originalPrice * 0.90;
  }
}

// The test using a stub
test "should apply 10% discount" {
  // 1. Create the stub with a canned response
  stub ProductServiceStub implements ProductService {
    Price getPrice(productId) {
      return 100.00; // Always return 100.00
    }
  }

  // 2. Inject the stub into the SUT
  DiscountCalculator calculator = new DiscountCalculator(new ProductServiceStub());

  // 3. Call the method and assert the result
  Price finalPrice = calculator.calculate("some-product");
  assert(finalPrice == 90.00);
}

In this example, the ProductServiceStub allows us to test the DiscountCalculator's logic without needing a real ProductService, which might involve slow network or database calls.