[英]Java generics, convert nested list to nested array
我必須接口到一些使用嵌套數組的舊代碼,因此必須將嵌套列表轉換為嵌套數組。
我以為自己應該已經在某些現有庫中實現了此功能。 我在Google / StackOverflow上進行了搜索,但沒有發現任何問題。 我檢查了Apache Commons和Guava,但沒有找到等效的功能。
這是我的代碼,我擔心的是,這可以通過更簡單的方式完成。 任何想法,建議,改進都將受到熱烈歡迎。
@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添加了一些測試用例:
@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);
}
您的代碼存在一些問題。 例如:
List<Object>(null)
轉換為null
而不是Object[] { null }
List<Object>("string")
轉換為String[] { "string" }
而不是Object[] { "string" }
List<Object>("string", Boolean.TRUE)
產生運行時錯誤。 我的建議:
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);
}
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