一、 前言
先交代该问题出现的背景,博主最近在业务中,需要把List以逗号为分隔符,转为字符串存到本地文件中,同样也需要读取这个字符串转回List,于是自然而然想到的是:
String pkgsString = "aaa,bbb,ccc,ddd,eee";
String[] pkgs = pkgsString.split(",");
List<String> pkgList = Arrays.asList(pkgs);
这么看来,没毛病,然后我要对List进行增删操作,为了避免出现并发修改异常(ConcurrentModificationException),此处使用迭代器:
Iterator<String> it = pkgList.iterator();
while (it.hasNext()) {
String pkg = it.next();
if ("aaa".equals(pkg)) {
it.remove();
}
}
再一看,也没毛病,然后一运行,报错,抛出UnsupportedOperationException异常,不支持的操作?Arrays.asList()返回的不是一个ArrayList吗,ArrayList中获取的迭代器怎么会不支持remove操作?于是开始检查源码,发现了这个大坑。
二、 Arrays.asList()为什么坑
jdk中的源码是:(注释全部贴上来了,因为很关键)
/**
* Returns a fixed-size list backed by the specified array. (Changes to
* the returned list "write through" to the array.) This method acts
* as bridge between array-based and collection-based APIs, in
* combination with {@link Collection#toArray}. The returned list is
* serializable and implements {@link RandomAccess}.
*
* <p>This method also provides a convenient way to create a fixed-size
* list initialized to contain several elements:
* <pre>
* List<String> stooges = Arrays.asList("Larry", "Moe", "Curly");
* </pre>
*
* @param <T> the class of the objects in the array
* @param a the array by which the list will be backed
* @return a list view of the specified array
*/
public static <T> List<T> asList(T... a) {
return new ArrayList<>(a);
}
表面上看,这不就是我们熟悉的ArrayList吗?怎么会出问题,点进去看:
/**
* @serial include
*/
private static class ArrayList<E> extends AbstractList<E>
implements RandomAccess, java.io.Serializable
{
private static final long serialVersionUID = -2764017481108945198L;
private final E[] a;
ArrayList(E[] array) {
a = Objects.requireNonNull(array);
}
@Override
public int size() {
return a.length;
}
@Override
public Object[] toArray() {
return a.clone();
}
@Override
@SuppressWarnings("unchecked")
public <T> T[] toArray(T[] a) {
int size = size();
if (a.length < size)
return Arrays.copyOf(this.a, size,
(Class<? extends T[]>) a.getClass());
System.arraycopy(this.a, 0, a, 0, size);
if (a.length > size)
a[size] = null;
return a;
}
@Override
public E get(int index) {
return a[index];
}
@Override
public E set(int index, E element) {
E oldValue = a[index];
a[index] = element;
return oldValue;
}
@Override
public int indexOf(Object o) {
E[] a = this.a;
if (o == null) {
for (int i = 0; i < a.length; i++)
if (a[i] == null)
return i;
} else {
for (int i = 0; i < a.length; i++)
if (o.equals(a[i]))
return i;
}
return -1;
}
@Override
public boolean contains(Object o) {
return indexOf(o) != -1;
}
@Override
public Spliterator<E> spliterator() {
return Spliterators.spliterator(a, Spliterator.ORDERED);
}
@Override
public void forEach(Consumer<? super E> action) {
Objects.requireNonNull(action);
for (E e : a) {
action.accept(e);
}
}
@Override
public void replaceAll(UnaryOperator<E> operator) {
Objects.requireNonNull(operator);
E[] a = this.a;
for (int i = 0; i < a.length; i++) {
a[i] = operator.apply(a[i]);
}
}
@Override
public void sort(Comparator<? super E> c) {
Arrays.sort(a, c);
}
}
这是一个位于Arrays类中的自定义的静态内部类,同样继承与AbstractList,仔细看源码会发现,没有
重写add和remove方法,那么当迭代器调用add和remove时,便调用的AbstractList的方法,再来到AbstractList中查看:
public void add(int index, E element) {
throw new UnsupportedOperationException();
}
public E remove(int index) {
throw new UnsupportedOperationException();
}
巨坑,果然是巨坑!
三、 解决方案
再看刚才Arrays.asList()的注释,说得很清楚了,“This method acts as bridge between array-based and collection-based APIs”,这是个两种不同数据结构之间的桥梁,既然是桥梁,包含了桥梁必要的功能就行了,所以缺少了一些别的方法,我们不应该把这个伪ArrayList当做真ArrayList来使用,通过真ArrayList的构造方法,构造一个真ArrayList,便能解决该问题:
List<String> pkgList = new ArrayList<>(Arrays.asList(pkgs));
四、 心得体会
-
作为一名coder,请保持好奇心,对不确定的东西,多去深入探究,挖掘知识,查漏补缺。
-
对于jdk源码,还是要多花点心思去学习,这里的学问太深了。
-
不能想当然地带着侥幸心理来开发,不报错不抛异常,并不代表代码就没问题,只是一些隐患还未暴露出来,若不及时纠正,将来可能在项目中酿成灾难。