一、问题描述

  实习时写过下面一段代码：

Collection<ISynchro> allISynchros = synchroCache.values(); // synchroCache是HashMap
Collection<ISynchro> npcISynchros = synchroNpcCache.values(); // synchroNpcCache是HashMap
allISynchros.addAll(npcISynchros);

  运行时抛出java.lang.UnsupportedOperationException异常。

二、原因分析

  Collection接口的实现与继承关系如下图：

  AbstractCollection是Java集合框架中Collection接口的直接实现类，Collection的子接口的实现类大都继承AbstractCollection，比如AbstractList和AbstractSet。
  AbstractCollection实现了几个方法，也定义了几个抽象方法留给子类实现，因此是一个抽象类。两个抽象方法：

    public abstract Iterator<E> iterator();
    public abstract int size();

  此外，注意add()和addAll()方法的实现:

/**
     * {@inheritDoc}
     *
     * <p>This implementation always throws an
     * <tt>UnsupportedOperationException</tt>.
     *
     * @throws UnsupportedOperationException {@inheritDoc}
     * @throws ClassCastException            {@inheritDoc}
     * @throws NullPointerException          {@inheritDoc}
     * @throws IllegalArgumentException      {@inheritDoc}
     * @throws IllegalStateException         {@inheritDoc}
     */
    public boolean add(E e) {
        throw new UnsupportedOperationException();
    }

/**
     * {@inheritDoc}
     *
     * <p>This implementation iterates over the specified collection, and adds
     * each object returned by the iterator to this collection, in turn.
     *
     * <p>Note that this implementation will throw an
     * <tt>UnsupportedOperationException</tt> unless <tt>add</tt> is
     * overridden (assuming the specified collection is non-empty).
     *
     * @throws UnsupportedOperationException {@inheritDoc}
     * @throws ClassCastException            {@inheritDoc}
     * @throws NullPointerException          {@inheritDoc}
     * @throws IllegalArgumentException      {@inheritDoc}
     * @throws IllegalStateException         {@inheritDoc}
     *
     * @see #add(Object)
     */
    public boolean addAll(Collection<? extends E> c) {
        boolean modified = false;
        for (E e : c)
            if (add(e))
                modified = true;
        return modified;
    }

  如果调用AbstractCollection的add()或addAll()方法，将抛出Unsupported OperationException异常。因此如果子类需要add的功能，需要重写add()方法。
  文章开头的代码中synchroCache和synchroNpcCache均为HashMap,查看源代码，发现HashMap的values()方法实现如下：

/**
     * Returns a {@link Collection} view of the values contained in this map.
     * The collection is backed by the map, so changes to the map are
     * reflected in the collection, and vice-versa.  If the map is
     * modified while an iteration over the collection is in progress
     * (except through the iterator's own <tt>remove</tt> operation),
     * the results of the iteration are undefined.  The collection
     * supports element removal, which removes the corresponding
     * mapping from the map, via the <tt>Iterator.remove</tt>,
     * <tt>Collection.remove</tt>, <tt>removeAll</tt>,
     * <tt>retainAll</tt> and <tt>clear</tt> operations.  It does not
     * support the <tt>add</tt> or <tt>addAll</tt> operations.
     *
     * @return a view of the values contained in this map
     */
    public Collection<V> values() {
        Collection<V> vs = values;
        if (vs == null) {
            vs = new Values();
            values = vs;
        }
        return vs;
    }

    final class Values extends AbstractCollection<V> {
        public final int size()                 { return size; }
        public final void clear()               { HashMap.this.clear(); }
        public final Iterator<V> iterator()     { return new ValueIterator(); }
        public final boolean contains(Object o) { return containsValue(o); }
        public final Spliterator<V> spliterator() {
            return new ValueSpliterator<>(HashMap.this, 0, -1, 0, 0);
        }
        public final void forEach(Consumer<? super V> action) {
            Node<K,V>[] tab;
            if (action == null)
                throw new NullPointerException();
            if (size > 0 && (tab = table) != null) {
                int mc = modCount;
                for (int i = 0; i < tab.length; ++i) {
                    for (Node<K,V> e = tab[i]; e != null; e = e.next)
                        action.accept(e.value);
                }
                if (modCount != mc)
                    throw new ConcurrentModificationException();
            }
        }
    }

  可以看出values()方法返回的vs属于Values内部类，该类继承自AbstractCollection，而且并未重写add()方法。因此如果对返回值调用addAll()或者add()方法，依然会抛出UnsupportedOperationException异常。因此今后在实际使用中要避免HashMap.values().add()&HashMap.values.addAll()。

三、解决方案

  那么如果需要合并两个AbstractCollection该如何操作？
  方案（1）将Collection转List，首先尝试强转的方法：

List<ISynchro> allISynchros = (List<ISynchro>)synchroCache.values();

  强转报错java.lang.ClassCastException。因为Collection是List的父接口，而强制类型转换只能完成向上转型，即子类对象转为父类对象，反过来不行，因为子类有的方法父类可能没有。但将List向上转型为Collection是可行的。
  后来看到ArrayList有个构造函数：

/**
     * Constructs a list containing the elements of the specified
     * collection, in the order they are returned by the collection's
     * iterator.
     *
     * @param c the collection whose elements are to be placed into this list
     * @throws NullPointerException if the specified collection is null
     */
    public ArrayList(Collection<? extends E> c) {
        elementData = c.toArray();
        if ((size = elementData.length) != 0) {
            // c.toArray might (incorrectly) not return Object[] (see 6260652)
            if (elementData.getClass() != Object[].class)
                elementData = Arrays.copyOf(elementData, size, Object[].class);
        } else {
            // replace with empty array.
            this.elementData = EMPTY_ELEMENTDATA;
        }
    }

  可以接受一个Collection的参数，返回一个List，实现了将Collection转换为List，满足要求：

List<ISynchro> allISynchros = new ArrayList<ISynchro>(synchroCache.values());
List<ISynchro> npcISynchros = new ArrayList<ISynchro>(synchroNpcCache.values());

之后再调用ArrayList的addAll()方法即可。ArrayList的add()方法是重写的，实现了add功能，不会抛异常：

allISynchros.addAll(npcISynchros);

  方案（2）使用CollectionUtils.union(collection1，collection2)方法：

Collection<ISynchro> allISynchros = synchroCache.values();
Collection<ISynchro> npcISynchros = synchroNpcCache.values();
CollectionsUtils.union(allISynchros, npcISynchros);