Why Arrays.asList() Throws UnsupportedOperationException When Modifying the Resulting List

Alibaba's Java development guidelines warn that the utility method Arrays.asList() should not be used with collection‑modifying operations because its add , remove and clear methods throw UnsupportedOperationException .

Problem Analysis

We start with a simple test:

public static void main(String[] args) {
    List
list = Arrays.asList("a", "b", "c");
    // list.clear();
    // list.remove("a");
    // list.add("g");
}

Uncommenting any of the three lines and running the program reproduces the exception described in the guideline.

To understand why, we look at the implementation of Arrays.asList() :

public static
List
asList(T... a) {
    return new ArrayList<>(a);
}

Although the method appears to return a normal ArrayList , the actual class instantiated is an internal static class inside java.util.Arrays , not the public java.util.ArrayList we normally use.

private static class ArrayList
extends AbstractList
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[] toArray(T[] a) {
        int size = size();
        if (a.length < size)
            return (T[]) Arrays.copyOf(this.a, size, (Class
) a.getClass());
        System.arraycopy(this.a, 0, a, 0, size);
        if (a.length > size) a[size] = null;
        return a;
    }
    // other methods omitted
}

This internal class does not implement add , remove or clear ; those methods are inherited from AbstractList and simply throw UnsupportedOperationException :

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

public E remove(int index) {
    throw new UnsupportedOperationException();
}

Therefore, any attempt to modify the list returned by Arrays.asList() results in the exception.

Summary

Do not misuse Arrays.asList() ; it is backed by a fixed‑size array.

If you need a mutable list, avoid calling collection‑modifying methods on the result.

Wrap the result in a real ArrayList , e.g., List<String> list = new ArrayList<>(Arrays.asList("a", "b", "c")); , to obtain a mutable list.

Reference: https://juejin.im/post/5d10e52ee51d454f6f16ec11