使用Arrays.asList可能引發(fā)的UnsupportedOperationException異常

首先看一個(gè)簡單的示例demo,使用集合(Set)對(duì)列表(List)去重

import java.util.*;

public class Test {

    public static void main(String[] args) {
        //使用Arrays類構(gòu)造一個(gè)列表
        String[] sourceArr = {"ii", "aa", "ee", "ii"};
        List<String> sourceList = Arrays.asList(sourceArr);

        //去重
        List<String> result = duplicateHandler(sourceList);

        //輸出
        result.forEach(item -> {
            System.out.print(item + ",");
        });
    }

    /**
     * 直接使用集合去重
     * @param source 需要去重的列表
     * @return 去重后的列表
     */
    public static List<String> duplicateHandler(List<String> source) {
        Set<String> util = new HashSet<>(source);
        source.clear();  //①
        source.addAll(util);
        return source;
    }
}

結(jié)果,在執(zhí)行到①處發(fā)生異常java.lang.UnsupportedOperationException。這就有丶奇怪,為什么能編譯通過,運(yùn)行時(shí)卻發(fā)生異常,難道ArrayList.clear方法內(nèi)部有什么玄機(jī)?追蹤一下Arrays.asList的JDK源碼:

/**
     * Returns a fixed-size list backed by the specified array.  (Changes to
     * the returned list "write through" to the array.)  This method acts
     * as bridge between array-based and collection-based APIs, in
     * combination with {@link Collection#toArray}.  The returned list is
     * serializable and implements {@link RandomAccess}.
     *
     * <p>This method also provides a convenient way to create a fixed-size
     * list initialized to contain several elements:
     * <pre>
     *     List&lt;String&gt; stooges = Arrays.asList("Larry", "Moe", "Curly");
     * </pre>
     *
     * @param <T> the class of the objects in the array
     * @param a the array by which the list will be backed
     * @return a list view of the specified array
     */
    @SafeVarargs
    @SuppressWarnings("varargs")
    public static <T> List<T> asList(T... a) {
        return new ArrayList<>(a);
    }

    /**
     * @serial include
     */
    private static class ArrayList<E> extends AbstractList<E>
        implements RandomAccess, java.io.Serializable
    {
        private static final long serialVersionUID = -2764017481108945198L;
        private final E[] a;

        ArrayList(E[] array) {
            a = Objects.requireNonNull(array);
        }

        @Override
        public int size() {
            return a.length;
        }

        @Override
        public Object[] toArray() {
            return a.clone();
        }

        @Override
        @SuppressWarnings("unchecked")
        public <T> T[] toArray(T[] a) {
            int size = size();
            if (a.length < size)
                return Arrays.copyOf(this.a, size,
                                     (Class<? extends T[]>) a.getClass());
            System.arraycopy(this.a, 0, a, 0, size);
            if (a.length > size)
                a[size] = null;
            return a;
        }

        @Override
        public E get(int index) {
            return a[index];
        }

        @Override
        public E set(int index, E element) {
            E oldValue = a[index];
            a[index] = element;
            return oldValue;
        }

        @Override
        public int indexOf(Object o) {
            E[] a = this.a;
            if (o == null) {
                for (int i = 0; i < a.length; i++)
                    if (a[i] == null)
                        return i;
            } else {
                for (int i = 0; i < a.length; i++)
                    if (o.equals(a[i]))
                        return i;
            }
            return -1;
        }

        @Override
        public boolean contains(Object o) {
            return indexOf(o) != -1;
        }

        @Override
        public Spliterator<E> spliterator() {
            return Spliterators.spliterator(a, Spliterator.ORDERED);
        }

        @Override
        public void forEach(Consumer<? super E> action) {
            Objects.requireNonNull(action);
            for (E e : a) {
                action.accept(e);
            }
        }

        @Override
        public void replaceAll(UnaryOperator<E> operator) {
            Objects.requireNonNull(operator);
            E[] a = this.a;
            for (int i = 0; i < a.length; i++) {
                a[i] = operator.apply(a[i]);
            }
        }

        @Override
        public void sort(Comparator<? super E> c) {
            Arrays.sort(a, c);
        }
    }

驚喜的發(fā)現(xiàn):
Arrays.asList返回并的并不是我們熟悉的java.util.ArrayList,而是Arrays的一個(gè)同名內(nèi)部類——java.util.Arrays.ArrayList。這個(gè)ArrayList同樣繼承了AbstractList,但是并沒有重寫clear、add等方法。所以,在①處調(diào)用的實(shí)際上是父類AbstractList的clear方法,然后我們看父類的JDK源碼:

    /**
     * Removes all of the elements from this list (optional operation).
     * The list will be empty after this call returns.
     *
     * <p>This implementation calls {@code removeRange(0, size())}.
     *
     * <p>Note that this implementation throws an
     * {@code UnsupportedOperationException} unless {@code remove(int
     * index)} or {@code removeRange(int fromIndex, int toIndex)} is
     * overridden.
     *
     * @throws UnsupportedOperationException if the {@code clear} operation
     *         is not supported by this list
     */
    public void clear() {
        removeRange(0, size());
    }

注意注釋的這一句:Note that this implementation throws an{@code UnsupportedOperationException} unless {@code remove(int index)} or {@code removeRange(int fromIndex, int toIndex)} is overridden.意思是說,除非remove方法或removeRange方法被重寫,否則會(huì)拋出UnsupportedOperationException異常。

找到了原因,下一個(gè)問題就是怎樣解決。還好java.util.ArrayList提供了接收Collection類型參數(shù)的構(gòu)造方法,而java.util.Arrays.ArrayList繼承自AbstractList,當(dāng)然也是Collection的實(shí)現(xiàn)類,因此我們可以將java.util.Arrays.ArrayList轉(zhuǎn)為java.util.ArrayList,再進(jìn)行下面的clear、add等操作。示例demo如下:

public class Test {

    public static void main(String[] args) {
        //使用Arrays類構(gòu)造一個(gè)列表,這個(gè)列表的類型是java.util.Arrays.ArrayList
        String[] sourceArr = {"ii", "aa", "ee", "ii"};
        List<String> sourceList = Arrays.asList(sourceArr);

        //去重
        List<String> result = duplicateHandler(sourceList);

        //輸出
        result.forEach(item -> {
            System.out.print(item + ",");
        });
    }

    /**
     * 做類型轉(zhuǎn)換后再使用集合去重
     * @param source 需要去重的列表,它的類型是java.util.Arrays.ArrayList
     * @return 去重后的列表
     */
    public static List<String> duplicateHandler(List<String> source) {
        //將參數(shù)轉(zhuǎn)換為java.util.ArrayList類型
        source = new ArrayList<>(source);

        //去重
        Set<String> util = new HashSet<>(source);
        source.clear();
        source.addAll(util);
        return source;
    }
}
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