Map equality using Hamcrest

Sometimes Map.equals() is enough. But sometimes you don't know the types of Maps is returned by code under tests, so you don't know if .equals() will properly compare that map of unknown type returned by code with map constructed by you. Or you don't want to bind your code with such tests.

Additionally, constructing a map separately to compare the result with it is IMHO not very elegant:

Map<MyKey, MyValue> actual = methodUnderTest();

Map<MyKey, MyValue> expected = new HashMap<MyKey, MyValue>();
expected.put(new MyKey(1), new MyValue(10));
expected.put(new MyKey(2), new MyValue(20));
expected.put(new MyKey(3), new MyValue(30));
assertThat(actual, equalTo(expected));

I prefer using machers:

import static org.hamcrest.Matchers.hasEntry;

Map<MyKey, MyValue> actual = methodUnderTest();
assertThat(actual, allOf(
                      hasSize(3), // make sure there are no extra key/value pairs in map
                      hasEntry(new MyKey(1), new MyValue(10)),
                      hasEntry(new MyKey(2), new MyValue(20)),
                      hasEntry(new MyKey(3), new MyValue(30))
));

I have to define hasSize() myself:

public static <K, V> Matcher<Map<K, V>> hasSize(final int size) {
    return new TypeSafeMatcher<Map<K, V>>() {
        @Override
        public boolean matchesSafely(Map<K, V> kvMap) {
            return kvMap.size() == size;
        }

        @Override
        public void describeTo(Description description) {
            description.appendText(" has ").appendValue(size).appendText(" key/value pairs");
        }
    };
}

And there is another variant of hasEntry() that takes matchers as parameters instead of exact values of key and value. This can be useful in case you need something other than equality testing of every key and value.

The shortest way I've come up with is two statements:

assertThat( affA.entrySet(), everyItem(isIn(affB.entrySet())));
assertThat( affB.entrySet(), everyItem(isIn(affA.entrySet())));

But you can probably also do:

assertThat(affA.entrySet(), equalTo(affB.entrySet()));

depending on the implementations of the maps, and sacrificing the clarity of the difference report: that would just tell you that there is a difference, while the statement above would also tell you which one.

UPDATE: actually there is one statement that works independently of the collection types:

assertThat(affA.entrySet(), both(everyItem(isIn(affB.entrySet()))).and(containsInAnyOrder(affB.entrySet())));

I favor using Guava ImmutableMap. They support Map.equals() and are easy to construct. The only trick is to explicitly specify type parameters, since hamcrest will assume the ImmutableMap type.

assertThat( actualValue,
            Matchers.<Map<String, String>>equalTo( ImmutableMap.of(
                "key1", "value",
                "key2", "other-value"
) ) );