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: nullat 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。