Use assertThat(values).isSorted() to check ascending natural order, or assertThat(values).isSortedAccordingTo(comparator) when the required order is defined by a comparator. Both methods verify the existing list; they do not sort or mutate it.
Choose the assertion that matches the required order
| Assertion | Ordering rule | Use it when |
|---|---|---|
isSorted() |
Ascending natural order | The elements have suitable, mutually compatible Comparable semantics. |
isSortedAccordingTo(comparator) |
The order defined by the supplied Comparator |
The requirement is reverse order or a domain-specific ordering. |
The method contracts below are documented in the AssertJ Core 3.27.7 AbstractListAssert API.
Assert ascending natural order with isSorted()
For values whose natural order is the expected ascending order, call isSorted() on the list assertion:
import static org.assertj.core.api.Assertions.assertThat;
List<Integer> values = List.of(1, 2, 3);
assertThat(values).isSorted();The list elements must be mutually comparable. It is not enough for each element to implement Comparable if values of different runtime types cannot compare with one another. For domain objects, use this assertion only when their natural ordering expresses the requirement under test.
Check a comparator-defined order
Pass the expected ordering directly to isSortedAccordingTo. This makes the rule visible at the assertion, including when checking descending order:
import static org.assertj.core.api.Assertions.assertThat;
import static java.util.Comparator.reverseOrder;
List<Integer> values = List.of(3, 2, 1);
assertThat(values).isSortedAccordingTo(reverseOrder());A comparator can also state a business ordering for objects:
Rank #2
Comparator<Person> byLastName = Comparator.comparing(Person::lastName);
assertThat(people).isSortedAccordingTo(byLastName);Use the same comparator semantics that define the production requirement. If equal primary keys must appear in a deterministic order, add an appropriate tie-breaker to the comparator; sortedness under the primary comparator alone does not specify the order among elements it considers equal.
Understand empty, singleton, and invalid inputs
- Empty list: Both assertions consider an empty list sorted.
- One element: Natural-order checking requires the element type to support the documented
Comparablerequirement; comparator-based checking requires the element to be compatible with the supplied comparator. - Null list: The assertion fails with
AssertionError. - Null comparator: Passing one to
isSortedAccordingTothrowsNullPointerException. - Incompatible elements: If values cannot be compared under the selected natural order or comparator, the sortedness check fails.
- Unsorted list: When the documented order condition is not met, the assertion fails with
AssertionError.
Do not confuse usingComparator with the sorting comparator
usingComparator(...) configures comparison behavior for incoming assertion checks. It is not the way to specify the order to test. For sortedness under an explicit ordering, pass the comparator to isSortedAccordingTo(comparator), as documented by the AssertJ 3.27.7 list assertion API.
Do these 3 things before closing this tab:
1Fix the driver behind crashes, sound loss and screen glitches2Clear out junk files and repair common Windows errors3Scan for outdated or missing drivers - takes under a minuteKeep the assertion readable in a fluent chain
Both sortedness methods return the fluent assertion object, so they can be followed by other assertions when the sequence remains clear:
assertThat(values)
.isNotEmpty()
.isSorted();Choose a chain that reflects the requirement rather than adding checks that merely repeat it.
Rank #4
Related array assertions have a separate contract
AssertJ provides sorting assertions for arrays as well, but the array API is related rather than interchangeable with the List contract. Consult the AssertJ Core 3.27.7 ArraySortedAssert API before applying List-specific edge-case behavior to an array.
For runnable project examples, the AssertJ examples repository describes executable assertion examples; its main branch follows the latest released modules and can change over time.
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