Quote:

Alibaba said Java development specification to use tools Arrays. The asList () method to convert the array collection, cannot use the modify the related methods of collection, its the add/remove/clear method will throw an UnsupportedOperationException (), let’s take a look at Here’s why this happens.

Problem analysis:

Let’s do a test

public static void main(String[] args) {
       List<String> list = Arrays.asList("a"."b"."c");
       // list.clear();
       // list.remove("a");
       // list.add("g");
   }
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The annotated three lines can be uncommented separately, and the run does produce the exception described in the specification. Let’s take a look at what arrays.aslist () does.

public static <T> List<T> asList(T... a) {
       return new ArrayList<>(a);
   }
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It looks like a normal method, but in fact when you click on an ArrayList, it turns out that an ArrayList is not an ArrayList.

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(a) {
           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;
       }
       //
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It’s an inner class of Arrays. And this inner class doesn’t have add,clear, and remove methods, so the exception actually comes from AbstractList.

   public void add(int index, E element) {
       throw new UnsupportedOperationException();
   }

   public E remove(int index) {
      throw new UnsupportedOperationException();
  }
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Click on it to see where the exception was thrown, and the bottom layer of clear will also call remove so it will throw an exception.

Conclusion:

1.Arrays. AsList () 2. If arrays.aslist () is used, it’s best not to use its collections; 3. List List = new ArrayList<>(array.asList (“a”, “b”, “c”))