ArrayList的add方法报错： java.lang.UnsupportedOperationException

雨点打透心脏的1/2处 2022-05-21 07:44 333阅读 0赞

今天修改以前一个同事的代码，其中有一句是我新增的代码：

private static List<String> exceptionAPIs = new ArrayList<>();
    
    //调用某些方法，处理数据
    
    File file = new File(filePath);
    try {
        BufferedReader reader = new BufferedReader(new FileReader(file));
        String line = reader.readLine();
        while (!StringUtils.isEmpty(line)) {
            list.add(line.trim());
            line = reader.readLine();
        }
    } catch (IOException e) {
        e.printStackTrace();
    }

结果却报了下面这个错误：

Caused by: java.lang.UnsupportedOperationException
        at java.util.AbstractList.add (AbstractList.java:148)
        at java.util.AbstractList.add (AbstractList.java:108)

从错误信息上看到，list的类型发生了变化，不在是java.util.ArrayList，而是java.util.AbstractList，找到这个类的add方法：

/** * { @inheritDoc} * * <p>This implementation always throws an * { @code UnsupportedOperationException}. * * @throws UnsupportedOperationException { @inheritDoc} * @throws ClassCastException { @inheritDoc} * @throws NullPointerException { @inheritDoc} * @throws IllegalArgumentException { @inheritDoc} * @throws IndexOutOfBoundsException { @inheritDoc} */
        public void add(int index, E element) {
            throw new UnsupportedOperationException();
        }

AbstractList是个抽象类，在执行我新写的代码之前，唯一的操作就是：

list = Arrays.asList(members.toArray(new String[]{}))

应该是这个方法的实现，改变了list的类型，找到这个方法：

/**
         * 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<String> 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);
        }

返回的数据看起来是ArrayList，找到这个ArrayList，在Arrays中，是个内部类：

private static class ArrayList<E> extends AbstractList<E> implements RandomAccess, java.io.Serializable

这个内部类实现上面的抽象类AbstractList，但是没有重写add方法，所以执行的还是抽象类中的add方法，而这个方法只会抛出异常 java.lang.UnsupportedOperationException，所以，解决这个问题只要把list的类型改回来就行了；  
两种简单方式：

list = new ArrayList<>();
    list = new ArrayList<>(list);