How does thenThrow() handle exceptions differently in void methods compared to non-void methods

Non-Void Methods

For non-void methods, `when().thenThrow()` is the standard approach to mock exceptions. This method is used when you want a specific method to throw an exception when it is called. The syntax involves setting up a mock object, specifying the method call with `when()`, and then defining the exception to be thrown using `thenThrow()`.

Here's an example:

java
import static org.mockito.Mockito.*;
import org.junit.jupiter.api.Test;

class MyServiceTest {
    @Test
    void testThrowException() {
        MyClass mockClass = mock(MyClass.class);
        when(mockClass.someMethod()).thenThrow(new RuntimeException("An error occurred"));
        assertThrows(RuntimeException.class, () -> {
            mockClass.someMethod();
        });
    }
}

In this case, `someMethod()` is a non-void method, and `when().thenThrow()` is used to simulate an exception.

Void Methods

For void methods, you cannot use `when().thenThrow()` because void methods do not return a value, which is necessary for the `when()` method to work. Instead, Mockito provides the `doThrow()` method to handle exceptions in void methods.

Here's how you can use `doThrow()`:

java
import static org.mockito.Mockito.*;
import org.junit.jupiter.api.Test;

class MyServiceTest {
    @Test
    void testThrowExceptionInVoidMethod() {
        MyClass mockClass = mock(MyClass.class);
        doThrow(new IllegalArgumentException("Invalid argument")).when(mockClass).voidMethod();
        assertThrows(IllegalArgumentException.class, () -> {
            mockClass.voidMethod();
        });
    }
}

In this example, `voidMethod()` is a void method, and `doThrow()` is used to simulate an exception when it is called.

Key Differences

- Return Type: Non-void methods return a value, allowing `when().thenThrow()` to be used. Void methods do not return a value, requiring `doThrow()` instead.
- Syntax: The syntax for setting up the mock differs slightly between `when().thenThrow()` and `doThrow().when()`.
- Use Cases: `when().thenThrow()` is suitable for methods that return values, while `doThrow()` is necessary for methods without return values.

In summary, `thenThrow()` is not directly applicable to void methods due to their lack of return type, necessitating the use of `doThrow()` for mocking exceptions in such methods.