感謝作者能翻譯並且分享這麼優質的文章。不過文章中有一處表述欠妥:
如果希望避免這兩個坑,請改用這個方式
Collections.addAll(arraylist, array);
從字面上理解,作者的意思應該是指避免上述的兩個坑,定長
&& 修改原數據會影響list裏的對象
。
欠妥的主要是後者:修改原數據,list中的值也會變
環境:
java version "1.8.0_25"
Java(TM) SE Runtime Environment (build 1.8.0_25-b18)
Java HotSpot(TM) 64-Bit Server VM (build 25.25-b02, mixed mode)
Collections.addAll方法的源碼如下所示:
@SafeVarargs
public static <T> boolean addAll(Collection<? super T> c, T... elements) {
boolean result = false;
for (T element : elements)
result |= c.add(element);
return result;
}
Arrays.asList方法相關源碼如下所示:
@SafeVarargs
@SuppressWarnings("varargs")
public static <T> List<T> asList(T... a) {
return new ArrayList<>(a);// 內部私有類
}
ArrayList(E[] array) {
a = Objects.requireNonNull(array);
}
public static <T> T requireNonNull(T obj) {
if (obj == null)
throw new NullPointerException();
return obj;
}
從源碼來看,不管是Collections.addAll,還是Arrays.asList,均是引用傳遞,修改source object會影響到list裏的target object。下面是我測試的代碼:
public static void main(String[] args) {
User user_1 = new User();
user_1.put("name", "user_1");
user_1.put("value", "v_1");
User user_2 = new User();
user_2.put("name", "user_2");
user_2.put("value", "v_2");
List<User> asList = Arrays.asList(user_1, user_2);
List<User> arraylist = new ArrayList<>();
Collections.addAll(arraylist, user_1, user_2);
user_1.put("value", "v_a_1");
user_2.put("value", "v_a_2");
System.out.println("-------- asList --------");
System.out.println(JsonKit.toJson(asList));
System.out.println("\n-------- arraylist --------");
System.out.println(JsonKit.toJson(arraylist));
}
運行結果
-------- asList --------
[{"name":"user_1","value":"v_a_1"},{"name":"user_2","value":"v_a_2"}]
-------- arraylist --------
[{"name":"user_1","value":"v_a_1"},{"name":"user_2","value":"v_a_2"}]
結論,不管是Collections.addAll還是Arrays.asList,修改原始數據時,均會影響到List裏的數據