when thenReturn
- Java course
- Advanced
- Lesson 51/82
- 5 min
Learn behavior imitation with when(...).thenReturn().
when thenReturn
With when(mock).thenReturn(value) you define what a mocked method returns for a given input. The test scenario is then fully under control.
UserRepository repo = mock(UserRepository.class);
when(repo.findById(1)).thenReturn(new User("Alex"));
User u = repo.findById(1);
// u.getName() equals "Alex"This reproduces a specific scenario without a real database. You can define different return values for different inputs. To throw an exception, use thenThrow.
Throwing exceptions with thenThrow
To make a mocked method throw an exception, use thenThrow. This is an easy way to test error scenarios.
when(repo.findById(99))
.thenThrow(new UserNotFoundException(99));The calling code catches the exception with try/catch and verifies the error flow. This lets you test the error path without producing a real error. Error handling code is thus safely tested.
Practice Build stub logic by hand
Define a Discount class with a rateFor(String code) method. The method uses a Map<String, Integer> to return the discount for a code; for unknown codes it returns 0. In a StubDemo class write a main method; add put("SUMMER", 20) and put("WINTER", 15) to the map; print the result of rateFor("SUMMER").
Starter code
import java.util.*;
// Define the Discount class (rateFor method uses a Map)
// Write main in StubDemo; add codes to the map and print rateFor's resultSample solution
import java.util.*;
class Discount {
Map<String, Integer> table = new HashMap<>();
int rateFor(String code) {
return table.getOrDefault(code, 0);
}
}
class StubDemo {
public static void main(String[] args) {
Discount d = new Discount();
d.table.put("SUMMER", 20);
d.table.put("WINTER", 15);
System.out.println(d.rateFor("SUMMER"));
}
}Expected output
20
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Match stubbing methods
| Term | Meaning |
|---|---|
thenReturn |
Returns a specific value |
thenThrow |
Throws an exception |
thenAnswer |
Produces a custom answer |
thenCallRealMethod |
Calls the real method |
2 Complete the thenReturn call
when(repo.findById(1))
.____(new User("Ayşe"));when(repo.findById(1))
.thenReturn(new User("Ayşe"));Explanation: thenReturn returns the specific value.
3 What does when(repo.findById(1)).thenReturn(...) do?
- Defines a return value for the findById(1) call on the repo mock Correct answer
- Deletes the findById method
- Calls the findById method
- Turns repo into a real object
Explanation: This defines a return value for the findById(1) call.