Kotlin中Arraylist无法使用clear()，addA

• 在改写一个方法过程中，发现好端端的Arraylist无法清除与addAll()

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• 查看mList定义，习惯了java中的写法

// 数据list
    protected var mList: List<M> = ArrayList()

• 查看kotlin中List定义，代码如下，果然没有clear()，和addAll();

/**
 * A generic ordered collection of elements. Methods in this interface support only read-only access to the list;
 * read/write access is supported through the [MutableList] interface.
 * @param E the type of elements contained in the list. The list is covariant on its element type.
 */
public interface List<out E> : Collection<E> {
    // Query Operations
    override val size: Int

    override fun isEmpty(): Boolean
    override fun contains(element: @UnsafeVariance E): Boolean
    override fun iterator(): Iterator<E>

    // Bulk Operations
    override fun containsAll(elements: Collection<@UnsafeVariance E>): Boolean

    // Positional Access Operations
    /**
     * Returns the element at the specified index in the list.
     */
    public operator fun get(index: Int): E

    // Search Operations
    /**
     * Returns the index of the first occurrence of the specified element in the list, or -1 if the specified
     * element is not contained in the list.
     */
    public fun indexOf(element: @UnsafeVariance E): Int

    /**
     * Returns the index of the last occurrence of the specified element in the list, or -1 if the specified
     * element is not contained in the list.
     */
    public fun lastIndexOf(element: @UnsafeVariance E): Int

    // List Iterators
    /**
     * Returns a list iterator over the elements in this list (in proper sequence).
     */
    public fun listIterator(): ListIterator<E>

    /**
     * Returns a list iterator over the elements in this list (in proper sequence), starting at the specified [index].
     */
    public fun listIterator(index: Int): ListIterator<E>

    // View
    /**
     * Returns a view of the portion of this list between the specified [fromIndex] (inclusive) and [toIndex] (exclusive).
     * The returned list is backed by this list, so non-structural changes in the returned list are reflected in this list, and vice-versa.
     *
     * Structural changes in the base list make the behavior of the view undefined.
     */
    public fun subList(fromIndex: Int, toIndex: Int): List<E>
}

• 注意到kotlin中ArrayList继承自MutableList，查看源码果然有clear(),和addAll()的定义。

/**
 * A generic ordered collection of elements that supports adding and removing elements.
 * @param E the type of elements contained in the list. The mutable list is invariant on its element type.
 */
public interface MutableList<E> : List<E>, MutableCollection<E> {
    // Modification Operations
    override fun add(element: E): Boolean

    override fun remove(element: E): Boolean

    // Bulk Modification Operations
    override fun addAll(elements: Collection<E>): Boolean

    /**
     * Inserts all of the elements in the specified collection [elements] into this list at the specified [index].
     *
     * @return `true` if the list was changed as the result of the operation.
     */
    public fun addAll(index: Int, elements: Collection<E>): Boolean

    override fun removeAll(elements: Collection<E>): Boolean
    override fun retainAll(elements: Collection<E>): Boolean
    override fun clear(): Unit

    // Positional Access Operations
    /**
     * Replaces the element at the specified position in this list with the specified element.
     *
     * @return the element previously at the specified position.
     */
    public operator fun set(index: Int, element: E): E

    /**
     * Inserts an element into the list at the specified [index].
     */
    public fun add(index: Int, element: E): Unit

    /**
     * Removes an element at the specified [index] from the list.
     *
     * @return the element that has been removed.
     */
    public fun removeAt(index: Int): E

    // List Iterators
    override fun listIterator(): MutableListIterator<E>

    override fun listIterator(index: Int): MutableListIterator<E>

    // View
    override fun subList(fromIndex: Int, toIndex: Int): MutableList<E>
}

-故问题得到解决，重新定义mList变量，问题得到解决。完~

// 数据list
    protected var mList: MutableList<M> = ArrayList()