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java.lang.UnsupportedOperationException分析及解決辦法

更新時(shí)間：2024年03月01日 09:13:30 作者：仙草不加料

日常開(kāi)發(fā)中我遇到j(luò)ava.lang.UnsupportedOperationException:異常兩次了,下面這篇文章主要給對(duì)大家介紹了關(guān)于java.lang.UnsupportedOperationException分析及解決辦法,需要的朋友可以參考下

今天在寫(xiě)業(yè)務(wù)的時(shí)候，需要對(duì)從數(shù)據(jù)庫(kù)返回的List集合根據(jù)對(duì)象屬性進(jìn)行排序，那么常規(guī)的做法就是使用Collections的sort方法，實(shí)現(xiàn)Compartor接口重寫(xiě)compare方法，或者直接使用該list的sort方法，但是無(wú)論使用那種方法都遇到了這樣的報(bào)錯(cuò)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]

無(wú)法支持的操作，再看報(bào)錯(cuò)第二行UnmodifiableList，這是一個(gè)不可變的集合，它繼承 UnmodifiableCollection類，UnmodifiableCollection 中涉及到元素改動(dòng)（新增、刪除、清空…）的方法都直接拋出 UnsupportedOperationException 異常，并不改動(dòng)元素；Iterator 中涉及到元素修改的方法也一樣不進(jìn)行元素的改動(dòng)。

那么是不是把這個(gè)集合變成可變的集合問(wèn)題是不就解決了呢？

list = new ArrayList<>(list);

在排序前給這個(gè)list再次封裝，問(wèn)題得以解決。

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);
        }
    }

這里從UnmodifiableList的源碼來(lái)分析：除了equals、hashCode、get、indexOf、lastIndexOf等方法外，其他方法都會(huì)拋出UnsupportedOperationException()異常，而這些方法的共同點(diǎn)都是返回該list的屬性，說(shuō)明UnmodifiableList是一個(gè)只讀的集合，因此對(duì)這個(gè)List不能進(jìn)行添加或刪除元素等操作。

如果需要將list轉(zhuǎn)化為不可變的集合，Collections也提供了這樣的方法，最終轉(zhuǎn)化為UnmodifiableList。

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

同樣的 Collections 集合類中的 unmodifiableMap、unmodifiableSet也是只讀內(nèi)部類。它們可以在多線程環(huán)境下使用，或者在需要保護(hù)列表免受修改的場(chǎng)景下使用。

總結(jié)

到此這篇關(guān)于java.lang.UnsupportedOperationException分析及解決辦法的文章就介紹到這了,更多相關(guān)java.lang.UnsupportedOperationException內(nèi)容請(qǐng)搜索腳本之家以前的文章或繼續(xù)瀏覽下面的相關(guān)文章希望大家以后多多支持腳本之家！

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