Map

An object that maps keys to values. A map cannot contain duplicate keys; each key can map to at most one value.

TreeMap

public class TreeMapTest {
    private TreeMap<String, String> collection;

    @Before
    public void setUp() throws Exception {
        collection = new  TreeMap<>();
    }

    @After
    public void tearDown() throws Exception {
        collection.clear();
    }

    @Ignore
    @Test
    public void itNotGuaranteesToTheIterationOrder() throws Exception {
    }

    @Ignore
    @Test
    public void itSortElementsByDefault() throws Exception {
    }

    @Test
    public void itCanHaveDuplicateElement() throws Exception {
        // arrange
        String firstKey = "key";
        String secondKey = "key2";

        // act
        collection.put(firstKey, "fake");
        collection.put(secondKey, "fake");

        // assert
        assertThat("it should have 2 elements", collection.size(), is(2));
        assertThat("its first element is same as second", collection.get(firstKey), is(collection.get(secondKey)));
        assertThat("its first element is same instance as second", collection.get(firstKey), sameInstance(collection.get(secondKey)));
    }

    @Test
    public void itCanHaveNullElement() throws Exception {
        // arrange
        String firstKey = "key";

        // act
        collection.put(firstKey, null);

        // assert
        assertThat("it should have 1 elements", collection.size(), is(1));
        assertThat("it has null element", collection.get(firstKey), is(nullValue()));
    }

    @Test(expected = NullPointerException.class)
    public void itCanNotHaveNullKey() throws Exception {
        // act
        collection.put(null, "element");
    }
}

ArrayMap

public class ArrayMapTest {
    private ArrayMap<String, String> collection;

    @Before
    public void setUp() throws Exception {
        collection = new  ArrayMap<>();
    }

    @After
    public void tearDown() throws Exception {
        collection.clear();
    }

    @Ignore
    @Test
    public void itNotGuaranteesToTheIterationOrder() throws Exception {
    }

    @Ignore
    @Test
    public void itSortElementsByDefault() throws Exception {
    }

    @Test
    public void itCanHaveDuplicateElement() throws Exception {
        // arrange
        String firstKey = "key";
        String secondKey = "key2";

        // act
        collection.put(firstKey, "fake");
        collection.put(secondKey, "fake");

        // assert
        assertThat("it should have 2 elements", collection.size(), is(2));
        assertThat("its first element is same as second", collection.get(firstKey), is(collection.get(secondKey)));
        assertThat("its first element is same instance as second", collection.get(firstKey), sameInstance(collection.get(secondKey)));
    }

    @Test
    public void itCanHaveNullElement() throws Exception {
        // arrange
        String firstKey = "key";

        // act
        collection.put(firstKey, null);

        // assert
        assertThat("it should have 1 elements", collection.size(), is(1));
        assertThat("it has null element", collection.get(firstKey), is(nullValue()));
    }

    @Test
    public void itCanHaveNullKey() throws Exception {
        // arrange
        String element = "element";

        // act
        collection.put(null, element);

        // assert
        assertThat("it should have 1 elements", collection.size(), is(1));
        assertThat("it has element with null key", collection.get(null), is(element));
    }
}

HashMap

public class HashMapTest {
    private HashMap<String, String> collection;

    @Before
    public void setUp() throws Exception {
        collection = new HashMap<>();
    }

    @After
    public void tearDown() throws Exception {
        collection.clear();
    }

    @Ignore
    @Test
    public void itNotGuaranteesToTheIterationOrder() throws Exception {
    }

    @Ignore
    @Test
    public void itSortElementsByDefault() throws Exception {
    }

    @Test
    public void itCanHaveDuplicateElement() throws Exception {
        // arrange
        String firstKey = "key";
        String secondKey = "key2";

        // act
        collection.put(firstKey, "fake");
        collection.put(secondKey, "fake");

        // assert
        assertThat("it should have 2 elements", collection.size(), is(2));
        assertThat("its first element is same as second", collection.get(firstKey), is(collection.get(secondKey)));
        assertThat("its first element is same instance as second", collection.get(firstKey), sameInstance(collection.get(secondKey)));
    }

    @Test
    public void itCanHaveNullElement() throws Exception {
        // arrange
        String firstKey = "key";

        // act
        collection.put(firstKey, null);

        // assert
        assertThat("it should have 1 elements", collection.size(), is(1));
        assertThat("it has null element", collection.get(firstKey), is(nullValue()));
    }

    @Test
    public void itCanHaveNullKey() throws Exception {
        // arrange
        String element = "element";

        // act
        collection.put(null, element);

        // assert
        assertThat("it should have 1 elements", collection.size(), is(1));
        assertThat("it has element with null key", collection.get(null), is(element));
    }
}

LinkedHashMap

public class LinkedHashMapTest {
    private LinkedHashMap<String, String> collection;

    @Before
    public void setUp() throws Exception {
        collection = new LinkedHashMap<>();
    }

    @After
    public void tearDown() throws Exception {
        collection.clear();
    }

    @Ignore
    @Test
    public void itNotGuaranteesToTheIterationOrder() throws Exception {
    }

    @Ignore
    @Test
    public void itSortElementsByDefault() throws Exception {
    }

    @Test
    public void itCanHaveDuplicateElement() throws Exception {
        // arrange
        String firstKey = "key";
        String secondKey = "key2";

        // act
        collection.put(firstKey, "fake");
        collection.put(secondKey, "fake");

        // assert
        assertThat("it should have 2 elements", collection.size(), is(2));
        assertThat("its first element is same as second", collection.get(firstKey), is(collection.get(secondKey)));
        assertThat("its first element is same instance as second", collection.get(firstKey), sameInstance(collection.get(secondKey)));
    }

    @Test
    public void itCanHaveNullElement() throws Exception {
        // arrange
        String firstKey = "key";

        // act
        collection.put(firstKey, null);

        // assert
        assertThat("it should have 1 elements", collection.size(), is(1));
        assertThat("it has null element", collection.get(firstKey), is(nullValue()));
    }

    @Test
    public void itCanHaveNullKey() throws Exception {
        // arrange
        String element = "element";

        // act
        collection.put(null, element);

        // assert
        assertThat("it should have 1 elements", collection.size(), is(1));
        assertThat("it has element with null key", collection.get(null), is(element));
    }
}