Checking that a List is not empty in Hamcrest

Question

I was wondering if anyone knew of a way to check if a List is empty using assertThat() and Matchers?

Best way I could see just use JUnit:

assertFalse(list.isEmpty());

But I was hoping that there was some way to do this in Hamcrest.

Solution 1

Well there's always

assertThat(list.isEmpty(), is(false));

... but I'm guessing that's not quite what you meant :)

Alternatively:

assertThat((Collection)list, is(not(empty())));

empty() is a static in the Matchers class. Note the need to cast the list to Collection, thanks to Hamcrest 1.2's wonky generics.

The following imports can be used with hamcrest 1.3

import static org.hamcrest.Matchers.empty;
import static org.hamcrest.core.Is.is;
import static org.hamcrest.core.IsNot.*;

Solution 2

This is fixed in Hamcrest 1.3. The below code compiles and does not generate any warnings:

// given
List<String> list = new ArrayList<String>();
// then
assertThat(list, is(not(empty())));

But if you have to use older version - instead of bugged empty() you could use:

hasSize(greaterThan(0))
(import static org.hamcrest.number.OrderingComparison.greaterThan; or
import static org.hamcrest.Matchers.greaterThan;)

Example:

// given
List<String> list = new ArrayList<String>();
// then
assertThat(list, hasSize(greaterThan(0)));

The most important thing about above solutions is that it does not generate any warnings. The second solution is even more useful if you would like to estimate minimum result size.

Solution 3

If you're after readable fail messages, you can do without hamcrest by using the usual assertEquals with an empty list:

assertEquals(new ArrayList<>(0), yourList);

E.g. if you run

assertEquals(new ArrayList<>(0), Arrays.asList("foo", "bar");

you get

java.lang.AssertionError
Expected :[]
Actual   :[foo, bar]

Comments

[Fabricio Lemos]

For a better solution, vote for: code.google.com/p/hamcrest/issues/detail?id=97

[rafalmag]

@FabricioLemos issue#97 seems to be resolved and merget to master git branch. Lets hope it will be soon in next hamcrest release.

[andyb]

@rafalmag Good spot. Will be good to fix all my not-so-readable assertions when v1.3 is released

[skaffman]

I find that Hamcrest code looks much nicer if you change your syntax highlighting to make the parenthesis invisible...

[Ian Dallas]

Your second assertThat() gives me the following error: The method assertThat(T, Matcher<T>) in the type Assert is not applicable for the arguments (List<Integer>, Matcher<Collection<Object>>)

[skaffman]

@tkeE2036: That's Hamcrest's broken generics at work. Sometimes you need to cast to make it compile, e.g. assertThat((Collection)list, is(not(empty())));

[yegor256]

could you please give a link to Matchers class where this empty() method is defined? org.hamcrest:hamcrest-all:1.1 doesn't have it.

[skaffman]

@yegor256: Hamcrest is at v1.2 now.

[rafalmag]

@skaffman : in this solution you still get compile warnings, because you do not use generics (and in fact you cant...)

[dzieciou]

Is this really easier to read than simply assertFalse(list.isEmpty)? I always believed DSL should remove boilerplate code instead of introducing it.

[Brad Cupit]

@dzieciou it gives you a better error message when the test fails. So instead of expected true but got false you get something like expected empty but got [1, 2, 3]

[earcam]

If you prefer no unchecked conversion, and are willing to give up the static import, then you can add the generics to the method, like: assertThat(list, Matchers.<String>empty()) (assuming list is a collection of Strings)

[Christian García]

Note that with the second assertThat, the test will pass if the list is null, so the test will assert the list is not empty when it actually is (well, actually it's null. There could be a long metaphysical discussion about if a null list can be considered empty, though)

[Dave]

It is really nice to see what was left in the supposedly empty list!

[Admin]

@Ian this answer is outdated and there are better answers that should be accepted now instead.

[rafalmag]

@rogerdpack Here you go. I added the 1.3 style example. :)

[meustrus]

I would upvote this 1000 times if I could for the sole reason that unlike most Java answers, this answer actually includes the import statements you need to make the solution work.

[IIRed-DeathII]

In kotlin you won't be able to use "is" since it's a reserved word. You need to use it between grave accents: `is` -> `is`(not(empty())

[kelgwiin]

the lib is already fixed, use assertThat(someList, not(empty())); is enough