java.lang.UnsupportedOperationException分析及解决

    今天在写业务的时候，需要对从数据库返回的List集合根据对象属性进行排序，那么常规的做法就是使用Collections的sort方法，实现Compartor接口重写compare方法，或者直接使用该list的sort方法，但是无论使用那种方法都遇到了这样的报错原因： 

java.lang.UnsupportedOperationException: null
    at java.util.Collections$UnmodifiableList.sort(Collections.java:1333) ~[na:1.8.0_331]
    at java.util.Collections.sort(Collections.java:177) ~[na:1.8.0_331]

     无法支持的操作，再看报错第二行UnmodifiableList，这是一个不可变的集合，它继承 UnmodifiableCollection类，UnmodifiableCollection 中涉及到元素改动（新增、删除、清空…）的方法都直接抛出 UnsupportedOperationException 异常，并不改动元素；Iterator 中涉及到元素修改的方法也一样不进行元素的改动。

    那么是不是把这个集合变成可变的集合问题是不就解决了呢？

list = new ArrayList<>(list);

     在排序前给这个list再次封装，问题得以解决。

static class UnmodifiableList<E> extends UnmodifiableCollection<E>
                                  implements List<E> {
        private static final long serialVersionUID = -283967356065247728L;

        final List<? extends E> list;

        UnmodifiableList(List<? extends E> list) {
            super(list);
            this.list = list;
        }

        public boolean equals(Object o) {return o == this || list.equals(o);}
        public int hashCode()           {return list.hashCode();}

        public E get(int index) {return list.get(index);}
        public E set(int index, E element) {
            throw new UnsupportedOperationException();
        }
        public void add(int index, E element) {
            throw new UnsupportedOperationException();
        }
        public E remove(int index) {
            throw new UnsupportedOperationException();
        }
        public int indexOf(Object o)            {return list.indexOf(o);}
        public int lastIndexOf(Object o)        {return list.lastIndexOf(o);}
        public boolean addAll(int index, Collection<? extends E> c) {
            throw new UnsupportedOperationException();
        }

        @Override
        public void replaceAll(UnaryOperator<E> operator) {
            throw new UnsupportedOperationException();
        }
        @Override
        public void sort(Comparator<? super E> c) {
            throw new UnsupportedOperationException();
        }

        public ListIterator<E> listIterator()   {return listIterator(0);}

        public ListIterator<E> listIterator(final int index) {
            return new ListIterator<E>() {
                private final ListIterator<? extends E> i
                    = list.listIterator(index);

                public boolean hasNext()     {return i.hasNext();}
                public E next()              {return i.next();}
                public boolean hasPrevious() {return i.hasPrevious();}
                public E previous()          {return i.previous();}
                public int nextIndex()       {return i.nextIndex();}
                public int previousIndex()   {return i.previousIndex();}

                public void remove() {
                    throw new UnsupportedOperationException();
                }
                public void set(E e) {
                    throw new UnsupportedOperationException();
                }
                public void add(E e) {
                    throw new UnsupportedOperationException();
                }

                @Override
                public void forEachRemaining(Consumer<? super E> action) {
                    i.forEachRemaining(action);
                }
            };
        }

        public List<E> subList(int fromIndex, int toIndex) {
            return new UnmodifiableList<>(list.subList(fromIndex, toIndex));
        }

        /**
         * UnmodifiableRandomAccessList instances are serialized as
         * UnmodifiableList instances to allow them to be deserialized
         * in pre-1.4 JREs (which do not have UnmodifiableRandomAccessList).
         * This method inverts the transformation.  As a beneficial
         * side-effect, it also grafts the RandomAccess marker onto
         * UnmodifiableList instances that were serialized in pre-1.4 JREs.
         *
         * Note: Unfortunately, UnmodifiableRandomAccessList instances
         * serialized in 1.4.1 and deserialized in 1.4 will become
         * UnmodifiableList instances, as this method was missing in 1.4.
         */
        private Object readResolve() {
            return (list instanceof RandomAccess
                    ? new UnmodifiableRandomAccessList<>(list)
                    : this);
        }
    }

    这里从UnmodifiableList的源码来分析：除了equals、hashCode、get、indexOf、lastIndexOf等方法外，其他方法都会抛出UnsupportedOperationException()异常，而这些方法的共同点都是返回该list的属性，说明UnmodifiableList是一个只读的集合，因此对这个List不能进行添加或删除元素等操作。

    如果需要将list转化为不可变的集合，Collections也提供了这样的方法，最终转化为UnmodifiableList。

  public static <T> List<T> unmodifiableList(List<? extends T> list) {
        return (list instanceof RandomAccess ?
                new UnmodifiableRandomAccessList<>(list) :
                new UnmodifiableList<>(list));
    }

    同样的 Collections 集合类中的 unmodifiableMap、unmodifiableSet也是只读内部类。它们可以在多线程环境下使用，或者在需要保护列表免受修改的场景下使用。