Java generics, convert nested list to nested array

Question

I have to interface to some old code that is using nested arrays, hence I have to convert my nested Lists to nested arrays.

I thought to myself that such feature should be already implemented in some existing library. I searched on Google/StackOverflow but nothing surfaced. I have checked Apache Commons and Guava but I did not find equivalent feature.

Here's my code, my concern is this can be done in a more simple way. Any idea, suggestions, improvements are warmly welcomed.

@SuppressWarnings("rawtypes")
public static <E> Class getNestedListClass(List<List<E>> list){
    if (list == null) {
        return null;
    }

    for (final List<E> innerList: list) {
        if (innerList == null || innerList.isEmpty()) {
            continue;
        }
        return innerList.get(0).getClass();
    }
    return null;
}

@SuppressWarnings({ "unchecked", "rawtypes" })
public static <E> E[][] convertNestedList(List<List<E>> list){
    final Class<?> clazz = getNestedListClass(list);
    if (clazz == null) {
        return null;
    }
    final E[][] result = (E[][]) Array.newInstance(clazz, list.size(), 0);

    for (int i = 0; i < list.size(); i++) {    
        final List<E> innerList = list.get(i);
        if (innerList == null) {
            continue;
        }
        result[i] = innerList.toArray((E[]) Array.newInstance(clazz, innerList.size()));
    }
    return result;
}


@Test
public void testConvertNestedList() {
    final List<List<String>> list = new ArrayList<List<String>>();
    list.add(new ArrayList<String>());
    list.get(list.size() - 1).add("0.0");
    list.get(list.size() - 1).add("0.1");
    list.get(list.size() - 1).add("0.2");
    list.add(new ArrayList<String>());
    list.get(list.size() - 1).add("1.0");
    list.get(list.size() - 1).add("1.1");
    list.get(list.size() - 1).add("1.2");
    list.get(list.size() - 1).add("1.3");
    list.add(new ArrayList<String>());
    list.get(list.size() - 1).add("2.0");
    list.get(list.size() - 1).add("2.1");

    final String[][] array = convertNestedList(list);

    final String[][] expected = new String[][] { 
            { "0.0", "0.1", "0.2" }, 
            { "1.0", "1.1", "1.2", "1.3" },
            { "2.0", "2.1" } };
    assertArrayEquals(expected, array);

}

EDIT added some test cases:

@Test
public void testConvertNestedListCaseNull() throws NetLineMarketException {
    List<List<Object>> list = null;
    Object[][] array = ArrayUtils.convertNestedList(list);
    Object[][] expected = null;
    assertArrayEquals(expected, array);
}

@Test
public void testConvertNestedListCaseNullElement() throws NetLineMarketException {
    List<List<Object>> list = new ArrayList<List<Object>>();
    list.add(null);
    Object[][] array = ArrayUtils.convertNestedList(list);
    Object[][] expected = new Object[][] { null };
    assertArrayEquals(expected, array);
}

@Test
public void testConvertNestedListCaseNullElement2() throws NetLineMarketException {
    List<List<Object>> list = new ArrayList<List<Object>>();
    list.add(new ArrayList<Object>());
    list.get(0).add(null);
    Object[][] array = ArrayUtils.convertNestedList(list);
    Object[][] expected = new Object[][] { { null } };
    assertArrayEquals(expected, array);
}

@Test
public void testConvertNestedListCaseEmpty() throws NetLineMarketException {
    List<List<Object>> list = new ArrayList<List<Object>>();
    Object[][] array = ArrayUtils.convertNestedList(list);
    Object[][] expected = new Object[][] { };
    assertArrayEquals(expected, array);
}

@Test
public void testConvertNestedListCaseString() throws NetLineMarketException {
    final String string = "string";
    List<List<Object>> list = new ArrayList<List<Object>>();
    list.add(new ArrayList<Object>());
    list.get(0).add(string);
    Object[][] array = ArrayUtils.convertNestedList(list);
    Object[][] expected = new Object[][] { { string } };
    assertArrayEquals(expected, array);
}

@Test(expected=MyException.class)
public void testConvertNestedListCaseMix() throws NetLineMarketException {
    final String string = "string";
    List<List<Object>> list = new ArrayList<List<Object>>();
    list.add(new ArrayList<Object>());
    list.get(0).add(string);
    list.get(0).add(Boolean.TRUE);
    Object[][] array = ArrayUtils.convertNestedList(list);
}

Answer 1

There are some problems with your code. For example:

• List<Object>(null) is converted to null instead of Object[] { null }
• List<Object>("string") is converted to String[] { "string" } instead of Object[] { "string" }
• List<Object>("string", Boolean.TRUE) produces a run-time error.

My suggestion:

public static <T> T[][] toArray(List<List<T>> list, Class<T> clazz) {
    if (list == null) {
        return null;
    }
    List<T[]> temp = new ArrayList<>();
    @SuppressWarnings("unchecked")
    T[] inner = (T[]) Array.newInstance(clazz, 0);
    for (List<T> sublist : list) {
        temp.add(sublist == null ? null : sublist.toArray(inner));
    }
    @SuppressWarnings("unchecked")
    T[][] outer = (T[][]) Array.newInstance(inner.getClass(), temp.size());
    return temp.toArray(outer);
}