FlattenerTest.java

import org.junit.Before;
import org.junit.Test;


import static java.util.Arrays.asList;
import static java.util.Collections.emptyList;
import static java.util.Collections.singletonList;
import static org.junit.Assert.assertEquals;

public class FlattenerTest {

    private Flattener flattener;

    @Before
    public void setUp() {
        flattener = new Flattener();
    }

    @Test
    public void testFlatListIsPreserved() {
        assertEquals(asList(0, '1', "two"), flattener.flatten(asList(0, '1', "two")));
    }

    @Test
    public void testASingleLevelOfNestingWithNoNulls() {
        assertEquals(
            asList(1, '2', 3, 4, 5, "six", "7", 8),
            flattener.flatten(asList(1, asList('2', 3, 4, 5, "six", "7"), 8)));
    }

    @Test
    public void testFiveLevelsOfNestingWithNoNulls() {
        assertEquals(
            asList(0, '2', 2, "three", '8', 100, "four", 50, "-2"),
            flattener.flatten(asList(0, 
                              '2', 
                              asList(asList(2, "three"), 
                                     '8', 
                                     100, 
                                     "four",
                                     singletonList(singletonList(singletonList(50)))), "-2")));
    }

    @Test
    public void testSixLevelsOfNestingWithNoNulls() {
        assertEquals(
            asList("one", '2', 3, '4', 5, "six", 7, "8"),
            flattener.flatten(asList("one", 
                                     asList('2', 
                                            singletonList(singletonList(3)),
                                                          asList('4', 
                                                                 singletonList(singletonList(5))), "six", 7), "8")));
    }

    @Test
    public void testSixLevelsOfNestingWithNulls() {
        assertEquals(
            asList("0", 2, "two", '3', "8", "one hundred", "negative two"),
            flattener.flatten(asList("0",
                                     2,
                                     asList(asList("two", '3'),
                                            "8",
                                            singletonList(singletonList("one hundred")),
                                            null, 
                                            singletonList(singletonList(null))), 
                                                          "negative two")));
    }

    @Test
    public void testNestedListsFullOfNullsOnly() {
        assertEquals(emptyList(),
                     flattener.flatten(asList(null, 
                                              singletonList(singletonList(singletonList(null))),
                                                            null, 
                                                            null, 
                                                            asList(asList(null, null), null), null)));
    }
}