ArrayList Iterator remove java.lang.UnsupportedOperationException

在使用Arrays.asList()后调用add，remove这些method时出现 java.lang.UnsupportedOperationException异常。这是由于Arrays.asList() 返回java.util.Arrays$ArrayList， 而不是ArrayList。Arrays$ArrayList和ArrayList都是继承AbstractList，remove，add等 method在AbstractList中是默认throw UnsupportedOperationException而且不作任何操作。ArrayList override这些method来对list进行操作，但是Arrays$ArrayList没有override remove()，add()等，所以throw UnsupportedOperationException。

例子：

package com.test;

import java.util.Arrays;
import java.util.List;

public class TestUnsupported {
  public static void main(String[] args) {
        String[] s = {
            "one", "two", "three", "four", "five",
            "six", "seven", "eight", "nine", "ten",
          };

List a = Arrays.asList(s);
        System.out.println(
          "a.contains(" + s[0] + ") = " +
          a.contains(s[0]));
        a.add("eleven"); // Unsupported
        a.remove(s[0]); // Unsupported
      }
}

运行后，抛出异常如下：

Exception in thread "main" java.lang.UnsupportedOperationException
 at java.util.AbstractList.add(AbstractList.java:151)
 at java.util.AbstractList.add(AbstractList.java:89)
 at com.test.TestUnsupported.main(TestUnsupported.java:28)

解决方法是转换为ArrayList

List arrayList = new ArrayList(a);

或者直接这么写  List arrayList = new ArrayList(Arrays.asList(s));

参考

When you call Arrays.asList it does not return a java.util.ArrayList. It
returns a java.util.Arrays$ArrayList which is an immutable list. You
cannot add to it and you cannot remove from it.

If you want a mutable list built from your array you will have to loop
over the array yourself and add each element into the list in turn.

Even then your code won't work because you'll get an
IndexOutOfBoundsException as you remove the elements from the list in
the for loop. There are two options: use an Iterator which allows you to
remove from the list as you iterate over it (my recommendation
as it makes the code easier to maintain) or loop backwards over the
loop removing from the last one downwards (harder to read).

You are using AbstractList. ArrayList and Arrays$ArrayList are both
types of AbstractList. That's why you get UnsupportedOperationException:
Arrays$ArrayList does not override remove(int) so the method is called
on the superclass, AbstractList, which is what
is throwing the exception because this method is not implemented on
that class (the reason being to allow you to build immutable
subclasses).