Hamcrest Matcher在字符串列表中没有重复项？

Question

I would like a simple hamcrest matcher for finding duplicates of an object in a List<String> . This is what I wrote

for (QuizEntity quiz : quizzes)
            for (QuestionEntity question : quiz.getQuestions())
                Assert.assertThat("There should be no duplicate questions", 1, Matchers.equalTo(Collections.frequency(questions, question.getQuestion())));

Unfortunately I got this output, which isn't descriptive enough. Any

java.lang.AssertionError: There should be no duplicate questions
Expected: <20>
     but: was <1>

Answer 1

There's actually an implemented method that does exactly this - doesNotHaveDuplicates

 Assertions.assertThat(list).doesNotHaveDuplicates();

you could take quizzes and map it using Java 8 to the questions and assert there are no duplicates

Answer 2

Another option is to use hasDistinctElements() matcher from Cirneco library (that at the end is an Hamcrest extension).

It can be used as follows:

Assert.assertThat("There should be no duplicate questions", questions, CirnecoMatchersJ7.hasDistinctElements());

while I suggest to do

import static CirnecoMatchersJ7.*;

to make it more human readable

assertThat("There should be no duplicate questions", questions, hasDistinctElements());

Cirneco also has some fluent representation, ie

given(questions).withReason("There should be no duplicate questions").assertThat(hasDistinctElements());

---

The dependency can be imported as follows:

<dependency>
  <groupId>it.ozimov</groupId>
  <artifactId>java7-hamcrest-matchers</artifactId>
  <version>0.7.0</version>
</dependency>

Answer 3

Replaced

Assert.assertThat("There should be no duplicate questions", 1, Matchers.equalTo(Collections.frequency(questions, question.getQuestion())));

with

Assert.assertThat("Question '" + question.getQuestion() + "' should not be duplicated", 1, Matchers.equalTo(Collections.frequency(questions, question.getQuestion())));

Answer 4

I implemented my own matcher for that. Please note the error message did not work correctly to me (displayed another entry than the duplicate one). I share the code with the community (please link and upvote if you want to use in your product)

The code

public static class DistinctMatcher<T> extends BaseMatcher<T>
{
    private final Set<T> theSet = new HashSet<>();

    public static <T> Matcher<? super T> distinct()
    {
        return new DistinctMatcher<T>();
    }

    @Override
    public void describeTo(Description description)
    {
        description.appendText("has distinct values");
    }

    @Override
    public void describeMismatch(Object item, Description description)
    {
        description.appendText("element found twice: ").appendValue(item);
    }

    @SuppressWarnings("unchecked")
    @Override
    public boolean matches(Object arg0)
    {
        return theSet.add((T) arg0);
    }

}

How it works

By maintaining a stateful HashSet , the matcher is instantiated once and fed by each item of the iteration. According to Set.add method, it tells you whether the set contains or not the item already, so matching is immediate.

Usage: (I tested it on a not-nested collection)

assertThat(quizzes,(everyItem(hasProperty("questions",everyItem(hasProperty("question",is(distinct())))))));

Performance considerations

The above matcher is not suitable for unconstrained lazy collections (eg iterator over billions of records) because the Set holds a permanent reference to the object. One could easily modify the matcher to own only the hashCode of the object, but should keep in mind memory requirements.

Any other set-based solution suffers the same issue