【List.toArray()避免强转报错】List泛型使用List.toArray()转数组的正确操作方式

r囧r小猫 2021-09-24 08:50 280阅读 0赞

常见错误

List<String> list = new ArrayList<>();
    list.add("你大爷");
    list.add("你二爷");
    
    //会报错 java.lang.ClassCastException: java.lang.Object[] cannot be cast to java.lang.String[]
    String[] strs= (String[]) list.toArray();
    
    //正确方式
    String[] strs2= list.toArray(new String[0]);

参考阿里手册

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源码参考

/**
         * Returns an array containing all of the elements in this list in proper
         * sequence (from first to last element); the runtime type of the returned
         * array is that of the specified array.  If the list fits in the
         * specified array, it is returned therein.  Otherwise, a new array is
         * allocated with the runtime type of the specified array and the size of
         * this list.
         *
         * <p>If the list fits in the specified array with room to spare
         * (i.e., the array has more elements than the list), the element in
         * the array immediately following the end of the collection is set to
         * <tt>null</tt>.  (This is useful in determining the length of the
         * list <i>only</i> if the caller knows that the list does not contain
         * any null elements.)
         *
         * @param a the array into which the elements of the list are to
         *          be stored, if it is big enough; otherwise, a new array of the
         *          same runtime type is allocated for this purpose.
         * @return an array containing the elements of the list
         * @throws ArrayStoreException if the runtime type of the specified array
         *         is not a supertype of the runtime type of every element in
         *         this list
         * @throws NullPointerException if the specified array is null
         */
        @SuppressWarnings("unchecked")
        public <T> T[] toArray(T[] a) {
            if (a.length < size)
                // Make a new array of a's runtime type, but my contents:
                return (T[]) Arrays.copyOf(elementData, size, a.getClass());//这里指定了返回类型
            System.arraycopy(elementData, 0, a, 0, size);
            if (a.length > size)
                a[size] = null;
            return a;
        }

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