Assertj-core: Iterables#assertEndsWith should not fail if sequence is empty

[fbiville]

Summary

Any sequence always ends with an empty sequence. That is always true.

While this may seem silly, the current behaviour limits the usefulness of property-based tests (I was playing around with junit-quickcheck when I noticed the error).

Example

...
        <dependency>
            <groupId>org.assertj</groupId>
            <artifactId>assertj-core</artifactId>
            <version>2.8.0</version>
            <scope>test</scope>
        </dependency>
        <dependency>
            <groupId>com.pholser</groupId>
            <artifactId>junit-quickcheck-generators</artifactId>
            <version>0.7</version>
            <scope>test</scope>
        </dependency>
...

public final class Lists {

    public static final List<String> prepend(String head, String... tail) {
        List<String> result = new ArrayList<>(1 + tail.length);
        result.add(head);
        result.addAll(asList(tail));
        return result;
    }
}

@RunWith(JUnitQuickcheck.class)
public class ListsTest {

    @Property
    public void prepends_head_to_list(String head, ArrayList<String> strings) {
        String[] tail = strings.toArray(new String[strings.size()]);

        List<String> result = Lists.prepend(head, tail);

        assertThat(result)
                .hasSize(1 + tail.length)
                .startsWith(head)
                .endsWith(tail);
    }
}

That will yield:

java.lang.AssertionError: Property prepends_head_to_list falsified via shrinking: actual is not empty
Shrunken args: [, []]
Original failure message: [actual is not empty]
Original args: [, []]
Seeds: [-6556694662598446822, -5206529275569188621]


    at org.assertj.core.internal.CommonValidations.failIfEmptySinceActualIsNotEmpty(CommonValidations.java:92)
    at org.assertj.core.internal.Iterables.commonCheckThatIterableAssertionSucceeds(Iterables.java:598)
    at org.assertj.core.internal.Iterables.assertEndsWith(Iterables.java:579)
    at org.assertj.core.api.AbstractIterableAssert.endsWith(AbstractIterableAssert.java:354)
    at org.assertj.core.api.ListAssert.endsWith(ListAssert.java:112)
    at io.xxx.internal.collection.ListsTest.prepends_head_to_list(ListsTest.java:39)
    at sun.reflect.GeneratedMethodAccessor1.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:497)
    at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:50)
    at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
    at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:47)
    at com.pholser.junit.quickcheck.runner.PropertyVerifier$2.evaluate(PropertyVerifier.java:104)
    at com.pholser.junit.quickcheck.runner.PropertyVerifier$1.evaluate(PropertyVerifier.java:76)
    at com.pholser.junit.quickcheck.runner.PropertyVerifier.verify(PropertyVerifier.java:68)
    at com.pholser.junit.quickcheck.runner.ShrinkNode.verifyProperty(ShrinkNode.java:98)
    at com.pholser.junit.quickcheck.runner.Shrinker.shrink(Shrinker.java:82)
    at com.pholser.junit.quickcheck.runner.PropertyStatement.shrink(PropertyStatement.java:158)
    at com.pholser.junit.quickcheck.runner.PropertyStatement.lambda$property$38(PropertyStatement.java:132)
    at com.pholser.junit.quickcheck.runner.PropertyVerifier$1.evaluate(PropertyVerifier.java:86)
    at com.pholser.junit.quickcheck.runner.PropertyVerifier.verify(PropertyVerifier.java:68)
    at com.pholser.junit.quickcheck.runner.PropertyStatement.verifyProperty(PropertyStatement.java:108)
    at com.pholser.junit.quickcheck.runner.PropertyStatement.evaluate(PropertyStatement.java:92)
    at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:325)
    at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:78)
    at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:57)
    at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)
    at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)
    at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)
    at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)
    at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)
    at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
    at org.junit.runner.JUnitCore.run(JUnitCore.java:137)
    at com.intellij.junit4.JUnit4IdeaTestRunner.startRunnerWithArgs(JUnit4IdeaTestRunner.java:68)
    at com.intellij.rt.execution.junit.IdeaTestRunner$Repeater.startRunnerWithArgs(IdeaTestRunner.java:51)
    at com.intellij.rt.execution.junit.JUnitStarter.prepareStreamsAndStart(JUnitStarter.java:242)
    at com.intellij.rt.execution.junit.JUnitStarter.main(JUnitStarter.java:70)
Caused by: java.lang.AssertionError: actual is not empty
    at org.assertj.core.internal.CommonValidations.failIfEmptySinceActualIsNotEmpty(CommonValidations.java:92)
    at org.assertj.core.internal.Iterables.commonCheckThatIterableAssertionSucceeds(Iterables.java:598)
    at org.assertj.core.internal.Iterables.assertEndsWith(Iterables.java:579)
    at org.assertj.core.api.AbstractIterableAssert.endsWith(AbstractIterableAssert.java:354)
    at org.assertj.core.api.ListAssert.endsWith(ListAssert.java:112)
    at io.xxx.internal.collection.ListsTest.prepends_head_to_list(ListsTest.java:39)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:497)
    at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:50)
    at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
    at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:47)
    at com.pholser.junit.quickcheck.runner.PropertyVerifier$2.evaluate(PropertyVerifier.java:104)
    at com.pholser.junit.quickcheck.runner.PropertyVerifier$1.evaluate(PropertyVerifier.java:76)
    ... 17 more

Java 8 specific ?

Nope.

[joel-costigliola]

@fbiville you have a point, but it's a bit weird to allow this assertion:

assertThat(list(1,2,3)).endsWith()

I'm not yet fully convinced to allow that ... I still need to think about it.

I wonder if there is a mathematical definition of endsWith to setlle this, I could not find one but I only spent 5min looking.

[fbiville]

I was more thinking of assertThat(list(1,2,3)).endsWith(array) where array.length == 0.
If the initial method supported only varargs, what about introducing instead:

• endsWith(first: T, rest: T...)
• endsWith(T[])

The first overload keeps the "comfort" of using varargs while avoiding the weird endsWith() and the second would happily allow endsWith(myEmptyArray).

[fbiville]

@joel-costigliola Haskell Data.List#isSuffixOf example (try it with https://repl.it/languages/haskell)

 $> :m + Data.List
 $> isSuffixOf "c" "abc"
=> True
 $> isSuffixOf "a" "abc"
=> False
 $> isSuffixOf "" "string is just a sequence of chars"
=> True
 $> isSuffixOf "" ""
=> True

[joel-costigliola]

Just to make it clear, I would be happy with adding your suggestion to solve the issue:

• endsWith(first: T, rest: T...)
  
  
  
  • endsWith(T[])
  
  

[fbiville]

Once https://github.com/joel-costigliola/assertj-core/pull/1074 is merged, I'm thinking of adding the junit-quickcheck example with this assertion in assertj-examples.

WDYT @joel-costigliola? Would this example be too complex?

[joel-costigliola]

happy to have some examples with junit-quickcheck - I like property based testing :)