Bad state: no method stub was called from within `when()`. was a real method called, or perhaps an extension method?

Explanation of the error “bad state: no method stub was called from within `when()`. was a real method called, or perhaps an extension method?”

This error message is related to the usage of stubbing methods in a testing environment, especially when using frameworks like MockK, Mockito, or other similar libraries.

The error message suggests that a method stub (using `when()`) is expected to be called, but it wasn’t actually invoked. This can happen due to several reasons:

1. The method stub is never called: Double-check if the expected method is actually called in the test code or application code. Make sure the method name, parameters, and their types match correctly.
2. Incorrect import: Ensure that the correct imports are included for the stubbing methods. For example, if using MockK, ensure that you have imported `io.mockk.every` or `io.mockk.` where `` represents the actual method you are stubbing.
3. Method overloading: If the class contains overloaded methods with the same name, ensure that the correct signature is matched when stubbing the method. Use specific argument matchers (like `eq()`, `any()` etc.) to differentiate between overloaded methods.
4. Extension method confusion: If using extension methods provided by the testing framework, ensure that the extension methods are imported correctly. Also, make sure that the extension methods are used properly with the correct syntax.

Let’s take an example using the MockK library to illustrate the resolution of this error:

    
      // Assuming a class with a method to be stubbed
      class MyClass {
          fun myMethod(): String {
              return "Original method"
          }
      }
      
      // Test code where method stub is expected to be called
      import io.mockk.every
      import io.mockk.mockk
      import io.mockk.spyk
      import org.junit.Test
      
      class MyTestClass {
          @Test
          fun testSomething() {
              // Create a mock or spy object
              val myObject = mockk()
              
              // Stub the method
              every { myObject.myMethod() } returns "Stubbed response"
              
              // Perform tests or actions
              
              // Verify the expectations
          }
      }
    
  

In the example above, we create a mock object of the class `MyClass` using MockK and stub the `myMethod()` with a specific return value using `every { myObject.myMethod() } returns “Stubbed response”`. If the error “bad state: no method stub was called from within `when()`. was a real method called, or perhaps an extension method?” occurs, we need to verify the causes mentioned earlier and ensure that the `myMethod()` is actually invoked somewhere in the test code.