java使用Arrays asList需要注意了

java Arrays.asList

Arrays.asList相信搞java的基本都用过，但有些坑不一定踩过

坑一

Arrays.asList返回的List不能进行add和remove操作，为什么呢，来看看源码就明白了

@SafeVarargs
@SuppressWarnings("varargs")
public static <T> List<T> asList(T... a) {
    return new ArrayList<>(a);
}

在这里看，似乎没什么问题，返回一个ArrayList，那怎么就不能进行add和remove操作了呢,且看下面代码

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);
    }
}

原来Arrays.asList调用以后生成的ArrayList其实是Arrays的内部类对象，这个类继承了AbstractList，这个List的抽象类对get和remove方法都是需要子类自己实现的，来看看AbstractList的add和remove方法的代码

public E remove(int index) {
    throw new UnsupportedOperationException();
}
public void add(int index, E element) {
    throw new UnsupportedOperationException();
}
public boolean add(E e) {
    add(size(), e);
    return true;
}