Collections.emptyList()引发的java.lang.UnsupportedOperationException

缘由:

private static List getEmptyList() {
return Collections.emptyList();
}

public static void main(String[] args) {
try {
List list1 = new ArrayList<>(Arrays.asList(1,2,3,4,5));
List list2 = getEmptyList();
list2.addAll(list1);
System.out.println(list1.toString());
} catch (Exception e) {
e.printStackTrace();
}
}

此方法会抛出以下异常:

java.lang.UnsupportedOperationException
at java.util.AbstractList.add(Unknown Source)
at java.util.AbstractList.add(Unknown Source)
at java.util.AbstractCollection.addAll(Unknown Source)

...

原因在于

ArrayList里面有addAll的实现, 详见源码

    public boolean addAll(Collection c) {
        Object[] a = c.toArray();
        int numNew = a.length;
        ensureCapacityInternal(size + numNew);  // Increments modCount
        System.arraycopy(a, 0, elementData, size, numNew);
        size += numNew;
        return numNew != 0;
    }

而emptyList里面没有, 详见源码

/**
     * @serial include
     */
    private static class EmptyList
        extends AbstractList
        implements RandomAccess, Serializable {
        private static final long serialVersionUID = 8842843931221139166L;


        public Iterator iterator() {
            return emptyIterator();
        }
        public ListIterator listIterator() {
            return emptyListIterator();
        }


        public int size() {return 0;}
        public boolean isEmpty() {return true;}


        public boolean contains(Object obj) {return false;}
        public boolean containsAll(Collection c) { return c.isEmpty(); }


        public Object[] toArray() { return new Object[0]; }


        public T[] toArray(T[] a) {
            if (a.length > 0)
                a[0] = null;
            return a;
        }


        public E get(int index) {
            throw new IndexOutOfBoundsException("Index: "+index);
        }


        public boolean equals(Object o) {
            return (o instanceof List) && ((List)o).isEmpty();
        }


        public int hashCode() { return 1; }


        // Preserves singleton property
        private Object readResolve() {
            return EMPTY_LIST;
        }
    }

但是, 如果我们这么写就可以避免这个异常

private static List getEmptyList() {
return Collections.emptyList();
}

public static void main(String[] args) {
try {
List list1 = new ArrayList<>(Arrays.asList(1,2,3,4,5));
List list2 = getEmptyList();
list1.addAll(list2);
System.out.println(list1.toString());
} catch (Exception e) {
e.printStackTrace();
}
}

程序可以输出

[1, 2, 3, 4, 5]

所以以后用 Collections.emptyList() 的时候要注意addAll的操作