List.java

/*
 * Copyright (c) 1997, 2018, Oracle and/or its affiliates. All rights reserved.
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.  Oracle designates this
 * particular file as subject to the "Classpath" exception as provided
 * by Oracle in the LICENSE file that accompanied this code.
 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
 * or visit www.oracle.com if you need additional information or have any
 * questions.
 */

package java.util;

import java.util.function.UnaryOperator;

/**
 * An ordered collection (also known as a <i>sequence</i>).  The user of this
 * interface has precise control over where in the list each element is
 * inserted.  The user can access elements by their integer index (position in
 * the list), and search for elements in the list.<p>
 *
 * Unlike sets, lists typically allow duplicate elements.  More formally,
 * lists typically allow pairs of elements {@code e1} and {@code e2}
 * such that {@code e1.equals(e2)}, and they typically allow multiple
 * null elements if they allow null elements at all.  It is not inconceivable
 * that someone might wish to implement a list that prohibits duplicates, by
 * throwing runtime exceptions when the user attempts to insert them, but we
 * expect this usage to be rare.<p>
 *
 * The {@code List} interface places additional stipulations, beyond those
 * specified in the {@code Collection} interface, on the contracts of the
 * {@code iterator}, {@code add}, {@code remove}, {@code equals}, and
 * {@code hashCode} methods.  Declarations for other inherited methods are
 * also included here for convenience.<p>
 *
 * The {@code List} interface provides four methods for positional (indexed)
 * access to list elements.  Lists (like Java arrays) are zero based.  Note
 * that these operations may execute in time proportional to the index value
 * for some implementations (the {@code LinkedList} class, for
 * example). Thus, iterating over the elements in a list is typically
 * preferable to indexing through it if the caller does not know the
 * implementation.<p>
 *
 * The {@code List} interface provides a special iterator, called a
 * {@code ListIterator}, that allows element insertion and replacement, and
 * bidirectional access in addition to the normal operations that the
 * {@code Iterator} interface provides.  A method is provided to obtain a
 * list iterator that starts at a specified position in the list.<p>
 *
 * The {@code List} interface provides two methods to search for a specified
 * object.  From a performance standpoint, these methods should be used with
 * caution.  In many implementations they will perform costly linear
 * searches.<p>
 *
 * The {@code List} interface provides two methods to efficiently insert and
 * remove multiple elements at an arbitrary point in the list.<p>
 *
 * Note: While it is permissible for lists to contain themselves as elements,
 * extreme caution is advised: the {@code equals} and {@code hashCode}
 * methods are no longer well defined on such a list.
 *
 * <p>Some list implementations have restrictions on the elements that
 * they may contain.  For example, some implementations prohibit null elements,
 * and some have restrictions on the types of their elements.  Attempting to
 * add an ineligible element throws an unchecked exception, typically
 * {@code NullPointerException} or {@code ClassCastException}.  Attempting
 * to query the presence of an ineligible element may throw an exception,
 * or it may simply return false; some implementations will exhibit the former
 * behavior and some will exhibit the latter.  More generally, attempting an
 * operation on an ineligible element whose completion would not result in
 * the insertion of an ineligible element into the list may throw an
 * exception or it may succeed, at the option of the implementation.
 * Such exceptions are marked as "optional" in the specification for this
 * interface.
 *
 * <h2><a id="unmodifiable">Unmodifiable Lists</a></h2>
 * <p>The {@link List#of(Object...) List.of} and
 * {@link List#copyOf List.copyOf} static factory methods
 * provide a convenient way to create unmodifiable lists. The {@code List}
 * instances created by these methods have the following characteristics:
 *
 * <ul>
 * <li>They are <a href="Collection.html#unmodifiable"><i>unmodifiable</i></a>. Elements cannot
 * be added, removed, or replaced. Calling any mutator method on the List
 * will always cause {@code UnsupportedOperationException} to be thrown.
 * However, if the contained elements are themselves mutable,
 * this may cause the List's contents to appear to change.
 * <li>They disallow {@code null} elements. Attempts to create them with
 * {@code null} elements result in {@code NullPointerException}.
 * <li>They are serializable if all elements are serializable.
 * <li>The order of elements in the list is the same as the order of the
 * provided arguments, or of the elements in the provided array.
 * <li>They are <a href="../lang/doc-files/ValueBased.html">value-based</a>.
 * Callers should make no assumptions about the identity of the returned instances.
 * Factories are free to create new instances or reuse existing ones. Therefore,
 * identity-sensitive operations on these instances (reference equality ({@code ==}),
 * identity hash code, and synchronization) are unreliable and should be avoided.
 * <li>They are serialized as specified on the
 * <a href="{@docRoot}/serialized-form.html#java.util.CollSer">Serialized Form</a>
 * page.
 * </ul>
 *
 * <p>This interface is a member of the
 * <a href="{@docRoot}/java.base/java/util/package-summary.html#CollectionsFramework">
 * Java Collections Framework</a>.
 *
 * @param <E> the type of elements in this list
 *
 * @author  Josh Bloch
 * @author  Neal Gafter
 * @see Collection
 * @see Set
 * @see ArrayList
 * @see LinkedList
 * @see Vector
 * @see Arrays#asList(Object[])
 * @see Collections#nCopies(int, Object)
 * @see Collections#EMPTY_LIST
 * @see AbstractList
 * @see AbstractSequentialList
 * @since 1.2
 */
// 线性表接口，其实现可以是顺序存储结构，也可以是链式存储结构
public interface List<E> extends Collection<E> {
    
    /*▼ 存值 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Appends the specified element to the end of this list (optional
     * operation).
     *
     * <p>Lists that support this operation may place limitations on what
     * elements may be added to this list.  In particular, some
     * lists will refuse to add null elements, and others will impose
     * restrictions on the type of elements that may be added.  List
     * classes should clearly specify in their documentation any restrictions
     * on what elements may be added.
     *
     * @param e element to be appended to this list
     * @return {@code true} (as specified by {@link Collection#add})
     * @throws UnsupportedOperationException if the {@code add} operation
     *         is not supported by this list
     * @throws ClassCastException if the class of the specified element
     *         prevents it from being added to this list
     * @throws NullPointerException if the specified element is null and this
     *         list does not permit null elements
     * @throws IllegalArgumentException if some property of this element
     *         prevents it from being added to this list
     */
    // 将元素e追加到当前线性表中
    boolean add(E e);
    
    /**
     * Inserts the specified element at the specified position in this list
     * (optional operation).  Shifts the element currently at that position
     * (if any) and any subsequent elements to the right (adds one to their
     * indices).
     *
     * @param index index at which the specified element is to be inserted
     * @param element element to be inserted
     * @throws UnsupportedOperationException if the {@code add} operation
     *         is not supported by this list
     * @throws ClassCastException if the class of the specified element
     *         prevents it from being added to this list
     * @throws NullPointerException if the specified element is null and
     *         this list does not permit null elements
     * @throws IllegalArgumentException if some property of the specified
     *         element prevents it from being added to this list
     * @throws IndexOutOfBoundsException if the index is out of range
     *         ({@code index < 0 || index > size()})
     */
    // 将元素element添加到线性表index处
    void add(int index, E element);
    
    /**
     * Appends all of the elements in the specified collection to the end of
     * this list, in the order that they are returned by the specified
     * collection's iterator (optional operation).  The behavior of this
     * operation is undefined if the specified collection is modified while
     * the operation is in progress.  (Note that this will occur if the
     * specified collection is this list, and it's nonempty.)
     *
     * @param c collection containing elements to be added to this list
     * @return {@code true} if this list changed as a result of the call
     * @throws UnsupportedOperationException if the {@code addAll} operation
     *         is not supported by this list
     * @throws ClassCastException if the class of an element of the specified
     *         collection prevents it from being added to this list
     * @throws NullPointerException if the specified collection contains one
     *         or more null elements and this list does not permit null
     *         elements, or if the specified collection is null
     * @throws IllegalArgumentException if some property of an element of the
     *         specified collection prevents it from being added to this list
     * @see #add(Object)
     */
    // 将指定容器中的元素追加到当前线性表中
    boolean addAll(Collection<? extends E> c);
    
    /**
     * Inserts all of the elements in the specified collection into this
     * list at the specified position (optional operation).  Shifts the
     * element currently at that position (if any) and any subsequent
     * elements to the right (increases their indices).  The new elements
     * will appear in this list in the order that they are returned by the
     * specified collection's iterator.  The behavior of this operation is
     * undefined if the specified collection is modified while the
     * operation is in progress.  (Note that this will occur if the specified
     * collection is this list, and it's nonempty.)
     *
     * @param index index at which to insert the first element from the
     *              specified collection
     * @param c collection containing elements to be added to this list
     * @return {@code true} if this list changed as a result of the call
     * @throws UnsupportedOperationException if the {@code addAll} operation
     *         is not supported by this list
     * @throws ClassCastException if the class of an element of the specified
     *         collection prevents it from being added to this list
     * @throws NullPointerException if the specified collection contains one
     *         or more null elements and this list does not permit null
     *         elements, or if the specified collection is null
     * @throws IllegalArgumentException if some property of an element of the
     *         specified collection prevents it from being added to this list
     * @throws IndexOutOfBoundsException if the index is out of range
     *         ({@code index < 0 || index > size()})
     */
    // 将指定容器中的元素添加到当前线性表的index处
    boolean addAll(int index, Collection<? extends E> c);
    
    /*▲ 存值 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 取值 ████████████████████████████████████████████████████████████████████████████████┓ */
    
     /**
     * Returns the element at the specified position in this list.
     *
     * @param index index of the element to return
     * @return the element at the specified position in this list
     * @throws IndexOutOfBoundsException if the index is out of range
     *         ({@code index < 0 || index >= size()})
     */
    // 获取指定索引处的元素
    E get(int index);
    
    /*▲ 取值 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 移除 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Removes the element at the specified position in this list (optional
     * operation).  Shifts any subsequent elements to the left (subtracts one
     * from their indices).  Returns the element that was removed from the
     * list.
     *
     * @param index the index of the element to be removed
     * @return the element previously at the specified position
     * @throws UnsupportedOperationException if the {@code remove} operation
     *         is not supported by this list
     * @throws IndexOutOfBoundsException if the index is out of range
     *         ({@code index < 0 || index >= size()})
     */
    // 移除索引index处的元素，返回被移除的元素
    E remove(int index);
    
    /**
     * Removes the first occurrence of the specified element from this list,
     * if it is present (optional operation).  If this list does not contain
     * the element, it is unchanged.  More formally, removes the element with
     * the lowest index {@code i} such that
     * {@code Objects.equals(o, get(i))}
     * (if such an element exists).  Returns {@code true} if this list
     * contained the specified element (or equivalently, if this list changed
     * as a result of the call).
     *
     * @param o element to be removed from this list, if present
     * @return {@code true} if this list contained the specified element
     * @throws ClassCastException if the type of the specified element
     *         is incompatible with this list
     * (<a href="Collection.html#optional-restrictions">optional</a>)
     * @throws NullPointerException if the specified element is null and this
     *         list does not permit null elements
     * (<a href="Collection.html#optional-restrictions">optional</a>)
     * @throws UnsupportedOperationException if the {@code remove} operation
     *         is not supported by this list
     */
    // 移除指定的元素，返回值指示是否移除成功
    boolean remove(Object o);
    
    /**
     * Removes from this list all of its elements that are contained in the
     * specified collection (optional operation).
     *
     * @param c collection containing elements to be removed from this list
     * @return {@code true} if this list changed as a result of the call
     * @throws UnsupportedOperationException if the {@code removeAll} operation
     *         is not supported by this list
     * @throws ClassCastException if the class of an element of this list
     *         is incompatible with the specified collection
     * (<a href="Collection.html#optional-restrictions">optional</a>)
     * @throws NullPointerException if this list contains a null element and the
     *         specified collection does not permit null elements
     *         (<a href="Collection.html#optional-restrictions">optional</a>),
     *         or if the specified collection is null
     * @see #remove(Object)
     * @see #contains(Object)
     */
    // (匹配则移除)移除当前线性表中所有与给定容器中的元素匹配的元素
    boolean removeAll(Collection<?> c);
    
    /**
     * Retains only the elements in this list that are contained in the
     * specified collection (optional operation).  In other words, removes
     * from this list all of its elements that are not contained in the
     * specified collection.
     *
     * @param c collection containing elements to be retained in this list
     * @return {@code true} if this list changed as a result of the call
     * @throws UnsupportedOperationException if the {@code retainAll} operation
     *         is not supported by this list
     * @throws ClassCastException if the class of an element of this list
     *         is incompatible with the specified collection
     * (<a href="Collection.html#optional-restrictions">optional</a>)
     * @throws NullPointerException if this list contains a null element and the
     *         specified collection does not permit null elements
     *         (<a href="Collection.html#optional-restrictions">optional</a>),
     *         or if the specified collection is null
     * @see #remove(Object)
     * @see #contains(Object)
     */
    // (不匹配则移除)移除当前线性表中所有与给定容器中的元素不匹配的元素
    boolean retainAll(Collection<?> c);
    
    /**
     * Removes all of the elements from this list (optional operation).
     * The list will be empty after this call returns.
     *
     * @throws UnsupportedOperationException if the {@code clear} operation
     *         is not supported by this list
     */
    // 清空当前线性表中的元素
    void clear();
    
    /*▲ 移除 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 替换 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Replaces the element at the specified position in this list with the
     * specified element (optional operation).
     *
     * @param index index of the element to replace
     * @param element element to be stored at the specified position
     * @return the element previously at the specified position
     * @throws UnsupportedOperationException if the {@code set} operation
     *         is not supported by this list
     * @throws ClassCastException if the class of the specified element
     *         prevents it from being added to this list
     * @throws NullPointerException if the specified element is null and
     *         this list does not permit null elements
     * @throws IllegalArgumentException if some property of the specified
     *         element prevents it from being added to this list
     * @throws IndexOutOfBoundsException if the index is out of range
     *         ({@code index < 0 || index >= size()})
     */
    // 将index处的元素更新为element，并返回旧元素
    E set(int index, E element);
    
    /**
     * Replaces each element of this list with the result of applying the
     * operator to that element.  Errors or runtime exceptions thrown by
     * the operator are relayed to the caller.
     *
     * @implSpec
     * The default implementation is equivalent to, for this {@code list}:
     * <pre>{@code
     *     final ListIterator<E> li = list.listIterator();
     *     while (li.hasNext()) {
     *         li.set(operator.apply(li.next()));
     *     }
     * }</pre>
     *
     * If the list's list-iterator does not support the {@code set} operation
     * then an {@code UnsupportedOperationException} will be thrown when
     * replacing the first element.
     *
     * @param operator the operator to apply to each element
     * @throws UnsupportedOperationException if this list is unmodifiable.
     *         Implementations may throw this exception if an element
     *         cannot be replaced or if, in general, modification is not
     *         supported
     * @throws NullPointerException if the specified operator is null or
     *         if the operator result is a null value and this list does
     *         not permit null elements
     *         (<a href="Collection.html#optional-restrictions">optional</a>)
     * @since 1.8
     */
    // 更新当前线性表中所有元素，更新策略由operator决定
    default void replaceAll(UnaryOperator<E> operator) {
        Objects.requireNonNull(operator);
        final ListIterator<E> li = this.listIterator();
        while (li.hasNext()) {
            li.set(operator.apply(li.next()));
        }
    }
    
    /*▲ 替换 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 包含查询 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Returns {@code true} if this list contains the specified element.
     * More formally, returns {@code true} if and only if this list contains
     * at least one element {@code e} such that
     * {@code Objects.equals(o, e)}.
     *
     * @param o element whose presence in this list is to be tested
     * @return {@code true} if this list contains the specified element
     * @throws ClassCastException if the type of the specified element
     *         is incompatible with this list
     * (<a href="Collection.html#optional-restrictions">optional</a>)
     * @throws NullPointerException if the specified element is null and this
     *         list does not permit null elements
     * (<a href="Collection.html#optional-restrictions">optional</a>)
     */
    // 判断当前线性表中是否包含指定的元素
    boolean contains(Object o);
    
    /**
     * Returns {@code true} if this list contains all of the elements of the
     * specified collection.
     *
     * @param  c collection to be checked for containment in this list
     * @return {@code true} if this list contains all of the elements of the
     *         specified collection
     * @throws ClassCastException if the types of one or more elements
     *         in the specified collection are incompatible with this
     *         list
     * (<a href="Collection.html#optional-restrictions">optional</a>)
     * @throws NullPointerException if the specified collection contains one
     *         or more null elements and this list does not permit null
     *         elements
     *         (<a href="Collection.html#optional-restrictions">optional</a>),
     *         or if the specified collection is null
     * @see #contains(Object)
     */
    // 判读指定容器中的元素是否都包含在当前线性表中
    boolean containsAll(Collection<?> c);
    
    /*▲ 包含查询 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 定位 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Returns the index of the first occurrence of the specified element
     * in this list, or -1 if this list does not contain the element.
     * More formally, returns the lowest index {@code i} such that
     * {@code Objects.equals(o, get(i))},
     * or -1 if there is no such index.
     *
     * @param o element to search for
     * @return the index of the first occurrence of the specified element in
     *         this list, or -1 if this list does not contain the element
     * @throws ClassCastException if the type of the specified element
     *         is incompatible with this list
     *         (<a href="Collection.html#optional-restrictions">optional</a>)
     * @throws NullPointerException if the specified element is null and this
     *         list does not permit null elements
     *         (<a href="Collection.html#optional-restrictions">optional</a>)
     */
    // 返回指定元素的正序索引(正序查找首个匹配的元素)
    int indexOf(Object o);
    
    /**
     * Returns the index of the last occurrence of the specified element
     * in this list, or -1 if this list does not contain the element.
     * More formally, returns the highest index {@code i} such that
     * {@code Objects.equals(o, get(i))},
     * or -1 if there is no such index.
     *
     * @param o element to search for
     * @return the index of the last occurrence of the specified element in
     *         this list, or -1 if this list does not contain the element
     * @throws ClassCastException if the type of the specified element
     *         is incompatible with this list
     *         (<a href="Collection.html#optional-restrictions">optional</a>)
     * @throws NullPointerException if the specified element is null and this
     *         list does not permit null elements
     *         (<a href="Collection.html#optional-restrictions">optional</a>)
     */
    // 返回指定元素的逆序索引(逆序查找首个匹配的元素)
    int lastIndexOf(Object o);
    
    /*▲ 定位 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 视图 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Returns a view of the portion of this list between the specified
     * {@code fromIndex}, inclusive, and {@code toIndex}, exclusive.  (If
     * {@code fromIndex} and {@code toIndex} are equal, the returned list is
     * empty.)  The returned list is backed by this list, so non-structural
     * changes in the returned list are reflected in this list, and vice-versa.
     * The returned list supports all of the optional list operations supported
     * by this list.<p>
     *
     * This method eliminates the need for explicit range operations (of
     * the sort that commonly exist for arrays).  Any operation that expects
     * a list can be used as a range operation by passing a subList view
     * instead of a whole list.  For example, the following idiom
     * removes a range of elements from a list:
     * <pre>{@code
     *      list.subList(from, to).clear();
     * }</pre>
     * Similar idioms may be constructed for {@code indexOf} and
     * {@code lastIndexOf}, and all of the algorithms in the
     * {@code Collections} class can be applied to a subList.<p>
     *
     * The semantics of the list returned by this method become undefined if
     * the backing list (i.e., this list) is <i>structurally modified</i> in
     * any way other than via the returned list.  (Structural modifications are
     * those that change the size of this list, or otherwise perturb it in such
     * a fashion that iterations in progress may yield incorrect results.)
     *
     * @param fromIndex low endpoint (inclusive) of the subList
     * @param toIndex high endpoint (exclusive) of the subList
     * @return a view of the specified range within this list
     * @throws IndexOutOfBoundsException for an illegal endpoint index value
     *         ({@code fromIndex < 0 || toIndex > size ||
     *         fromIndex > toIndex})
     */
    // 返回[fromIndex, toIndex)之间的元素的视图
    List<E> subList(int fromIndex, int toIndex);
    
    
    /**
     * Returns an array containing all of the elements in this list in proper
     * sequence (from first to last element).
     *
     * <p>The returned array will be "safe" in that no references to it are
     * maintained by this list.  (In other words, this method must
     * allocate a new array even if this list is backed by an array).
     * The caller is thus free to modify the returned array.
     *
     * <p>This method acts as bridge between array-based and collection-based
     * APIs.
     *
     * @return an array containing all of the elements in this list in proper
     *         sequence
     * @see Arrays#asList(Object[])
     */
    // 以数组形式返回当前List中的元素
    Object[] toArray();
    
    /**
     * 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 list is set to {@code null}.
     * (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.)
     *
     * <p>Like the {@link #toArray()} method, this method acts as bridge between
     * array-based and collection-based APIs.  Further, this method allows
     * precise control over the runtime type of the output array, and may,
     * under certain circumstances, be used to save allocation costs.
     *
     * <p>Suppose {@code x} is a list known to contain only strings.
     * The following code can be used to dump the list into a newly
     * allocated array of {@code String}:
     *
     * <pre>{@code
     *     String[] y = x.toArray(new String[0]);
     * }</pre>
     *
     * Note that {@code toArray(new Object[0])} is identical in function to
     * {@code toArray()}.
     *
     * @param a the array into which the elements of this 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 this 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
     */
    // 将当前List中的元素存入数组a后返回，需要将List中的元素转换为T类型
    <T> T[] toArray(T[] a);
    
    /*▲ 视图 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 迭代 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Returns an iterator over the elements in this list in proper sequence.
     *
     * @return an iterator over the elements in this list in proper sequence
     */
    // 返回当前线性表的一个迭代器
    Iterator<E> iterator();
    
    /**
     * Returns a list iterator over the elements in this list (in proper
     * sequence).
     *
     * @return a list iterator over the elements in this list (in proper
     *         sequence)
     */
    // 返回当前线性表的一个增强的迭代器，且设定下一个待遍历元素为索引0处的元素
    ListIterator<E> listIterator();
    
    /**
     * Returns a list iterator over the elements in this list (in proper
     * sequence), starting at the specified position in the list.
     * The specified index indicates the first element that would be
     * returned by an initial call to {@link ListIterator#next next}.
     * An initial call to {@link ListIterator#previous previous} would
     * return the element with the specified index minus one.
     *
     * @param index index of the first element to be returned from the
     *        list iterator (by a call to {@link ListIterator#next next})
     * @return a list iterator over the elements in this list (in proper
     *         sequence), starting at the specified position in the list
     * @throws IndexOutOfBoundsException if the index is out of range
     *         ({@code index < 0 || index > size()})
     */
    // 返回当前线性表的一个增强的迭代器，且设定下一个待遍历元素为索引index处的元素
    ListIterator<E> listIterator(int index);
    
    /**
     * Creates a {@link Spliterator} over the elements in this list.
     *
     * <p>The {@code Spliterator} reports {@link Spliterator#SIZED} and
     * {@link Spliterator#ORDERED}.  Implementations should document the
     * reporting of additional characteristic values.
     *
     * @implSpec
     * The default implementation creates a
     * <em><a href="Spliterator.html#binding">late-binding</a></em>
     * spliterator as follows:
     * <ul>
     * <li>If the list is an instance of {@link RandomAccess} then the default
     *     implementation creates a spliterator that traverses elements by
     *     invoking the method {@link List#get}.  If such invocation results or
     *     would result in an {@code IndexOutOfBoundsException} then the
     *     spliterator will <em>fail-fast</em> and throw a
     *     {@code ConcurrentModificationException}.
     *     If the list is also an instance of {@link AbstractList} then the
     *     spliterator will use the list's {@link AbstractList#modCount modCount}
     *     field to provide additional <em>fail-fast</em> behavior.
     * <li>Otherwise, the default implementation creates a spliterator from the
     *     list's {@code Iterator}.  The spliterator inherits the
     *     <em>fail-fast</em> of the list's iterator.
     * </ul>
     *
     * @implNote
     * The created {@code Spliterator} additionally reports
     * {@link Spliterator#SUBSIZED}.
     *
     * @return a {@code Spliterator} over the elements in this list
     * @since 1.8
     */
    // 返回一个可分割的迭代器
    @Override
    default Spliterator<E> spliterator() {
        if (this instanceof RandomAccess) {
            return new AbstractList.RandomAccessSpliterator<>(this);
        } else {
            return Spliterators.spliterator(this, Spliterator.ORDERED);
        }
    }
    
    /*▲ 迭代 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 杂项 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Returns the number of elements in this list.  If this list contains
     * more than {@code Integer.MAX_VALUE} elements, returns
     * {@code Integer.MAX_VALUE}.
     *
     * @return the number of elements in this list
     */
    // 返回当前线性表的元素数量
    int size();
    
    /**
     * Returns {@code true} if this list contains no elements.
     *
     * @return {@code true} if this list contains no elements
     */
    // 判断当前线性表是否为空
    boolean isEmpty();
    
    /**
     * Sorts this list according to the order induced by the specified
     * {@link Comparator}.  The sort is <i>stable</i>: this method must not
     * reorder equal elements.
     *
     * <p>All elements in this list must be <i>mutually comparable</i> using the
     * specified comparator (that is, {@code c.compare(e1, e2)} must not throw
     * a {@code ClassCastException} for any elements {@code e1} and {@code e2}
     * in the list).
     *
     * <p>If the specified comparator is {@code null} then all elements in this
     * list must implement the {@link Comparable} interface and the elements'
     * {@linkplain Comparable natural ordering} should be used.
     *
     * <p>This list must be modifiable, but need not be resizable.
     *
     * @implSpec
     * The default implementation obtains an array containing all elements in
     * this list, sorts the array, and iterates over this list resetting each
     * element from the corresponding position in the array. (This avoids the
     * n<sup>2</sup> log(n) performance that would result from attempting
     * to sort a linked list in place.)
     *
     * @implNote
     * This implementation is a stable, adaptive, iterative mergesort that
     * requires far fewer than n lg(n) comparisons when the input array is
     * partially sorted, while offering the performance of a traditional
     * mergesort when the input array is randomly ordered.  If the input array
     * is nearly sorted, the implementation requires approximately n
     * comparisons.  Temporary storage requirements vary from a small constant
     * for nearly sorted input arrays to n/2 object references for randomly
     * ordered input arrays.
     *
     * <p>The implementation takes equal advantage of ascending and
     * descending order in its input array, and can take advantage of
     * ascending and descending order in different parts of the same
     * input array.  It is well-suited to merging two or more sorted arrays:
     * simply concatenate the arrays and sort the resulting array.
     *
     * <p>The implementation was adapted from Tim Peters's list sort for Python
     * (<a href="http://svn.python.org/projects/python/trunk/Objects/listsort.txt">
     * TimSort</a>).  It uses techniques from Peter McIlroy's "Optimistic
     * Sorting and Information Theoretic Complexity", in Proceedings of the
     * Fourth Annual ACM-SIAM Symposium on Discrete Algorithms, pp 467-474,
     * January 1993.
     *
     * @param c the {@code Comparator} used to compare list elements.
     *          A {@code null} value indicates that the elements'
     *          {@linkplain Comparable natural ordering} should be used
     * @throws ClassCastException if the list contains elements that are not
     *         <i>mutually comparable</i> using the specified comparator
     * @throws UnsupportedOperationException if the list's list-iterator does
     *         not support the {@code set} operation
     * @throws IllegalArgumentException
     *         (<a href="Collection.html#optional-restrictions">optional</a>)
     *         if the comparator is found to violate the {@link Comparator}
     *         contract
     * @since 1.8
     */
    // 使用指定的比较器对当前线性表内的元素进行排序
    @SuppressWarnings({"unchecked", "rawtypes"})
    default void sort(Comparator<? super E> c) {
        Object[] a = this.toArray();
        Arrays.sort(a, (Comparator) c);
        ListIterator<E> i = this.listIterator();
        for (Object e : a) {
            i.next();
            i.set((E) e);
        }
    }
    
    /*▲ 杂项 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 只读对象 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Returns an <a href="#unmodifiable">unmodifiable List</a> containing the elements of
     * the given Collection, in its iteration order. The given Collection must not be null,
     * and it must not contain any null elements. If the given Collection is subsequently
     * modified, the returned List will not reflect such modifications.
     *
     * @implNote
     * If the given Collection is an <a href="#unmodifiable">unmodifiable List</a>,
     * calling copyOf will generally not create a copy.
     *
     * @param <E> the {@code List}'s element type
     * @param coll a {@code Collection} from which elements are drawn, must be non-null
     * @return a {@code List} containing the elements of the given {@code Collection}
     * @throws NullPointerException if coll is null, or if it contains any nulls
     * @since 10
     */
    static <E> List<E> copyOf(Collection<? extends E> coll) {
        return ImmutableCollections.listCopy(coll);
    }
    
    
    /**
     * Returns an unmodifiable list containing zero elements.
     *
     * See <a href="#unmodifiable">Unmodifiable Lists</a> for details.
     *
     * @param <E> the {@code List}'s element type
     * @return an empty {@code List}
     *
     * @since 9
     */
    static <E> List<E> of() {
        return ImmutableCollections.emptyList();
    }
    
    /**
     * Returns an unmodifiable list containing one element.
     *
     * See <a href="#unmodifiable">Unmodifiable Lists</a> for details.
     *
     * @param <E> the {@code List}'s element type
     * @param e1 the single element
     * @return a {@code List} containing the specified element
     * @throws NullPointerException if the element is {@code null}
     *
     * @since 9
     */
    static <E> List<E> of(E e1) {
        return new ImmutableCollections.List12<>(e1);
    }
    
    /**
     * Returns an unmodifiable list containing two elements.
     *
     * See <a href="#unmodifiable">Unmodifiable Lists</a> for details.
     *
     * @param <E> the {@code List}'s element type
     * @param e1 the first element
     * @param e2 the second element
     * @return a {@code List} containing the specified elements
     * @throws NullPointerException if an element is {@code null}
     *
     * @since 9
     */
    static <E> List<E> of(E e1, E e2) {
        return new ImmutableCollections.List12<>(e1, e2);
    }
    
    /**
     * Returns an unmodifiable list containing three elements.
     *
     * See <a href="#unmodifiable">Unmodifiable Lists</a> for details.
     *
     * @param <E> the {@code List}'s element type
     * @param e1 the first element
     * @param e2 the second element
     * @param e3 the third element
     * @return a {@code List} containing the specified elements
     * @throws NullPointerException if an element is {@code null}
     *
     * @since 9
     */
    static <E> List<E> of(E e1, E e2, E e3) {
        return new ImmutableCollections.ListN<>(e1, e2, e3);
    }
    
    /**
     * Returns an unmodifiable list containing four elements.
     *
     * See <a href="#unmodifiable">Unmodifiable Lists</a> for details.
     *
     * @param <E> the {@code List}'s element type
     * @param e1 the first element
     * @param e2 the second element
     * @param e3 the third element
     * @param e4 the fourth element
     * @return a {@code List} containing the specified elements
     * @throws NullPointerException if an element is {@code null}
     *
     * @since 9
     */
    static <E> List<E> of(E e1, E e2, E e3, E e4) {
        return new ImmutableCollections.ListN<>(e1, e2, e3, e4);
    }
    
    /**
     * Returns an unmodifiable list containing five elements.
     *
     * See <a href="#unmodifiable">Unmodifiable Lists</a> for details.
     *
     * @param <E> the {@code List}'s element type
     * @param e1 the first element
     * @param e2 the second element
     * @param e3 the third element
     * @param e4 the fourth element
     * @param e5 the fifth element
     * @return a {@code List} containing the specified elements
     * @throws NullPointerException if an element is {@code null}
     *
     * @since 9
     */
    static <E> List<E> of(E e1, E e2, E e3, E e4, E e5) {
        return new ImmutableCollections.ListN<>(e1, e2, e3, e4, e5);
    }
    
    /**
     * Returns an unmodifiable list containing six elements.
     *
     * See <a href="#unmodifiable">Unmodifiable Lists</a> for details.
     *
     * @param <E> the {@code List}'s element type
     * @param e1 the first element
     * @param e2 the second element
     * @param e3 the third element
     * @param e4 the fourth element
     * @param e5 the fifth element
     * @param e6 the sixth element
     * @return a {@code List} containing the specified elements
     * @throws NullPointerException if an element is {@code null}
     *
     * @since 9
     */
    static <E> List<E> of(E e1, E e2, E e3, E e4, E e5, E e6) {
        return new ImmutableCollections.ListN<>(e1, e2, e3, e4, e5, e6);
    }
    
    /**
     * Returns an unmodifiable list containing seven elements.
     *
     * See <a href="#unmodifiable">Unmodifiable Lists</a> for details.
     *
     * @param <E> the {@code List}'s element type
     * @param e1 the first element
     * @param e2 the second element
     * @param e3 the third element
     * @param e4 the fourth element
     * @param e5 the fifth element
     * @param e6 the sixth element
     * @param e7 the seventh element
     * @return a {@code List} containing the specified elements
     * @throws NullPointerException if an element is {@code null}
     *
     * @since 9
     */
    static <E> List<E> of(E e1, E e2, E e3, E e4, E e5, E e6, E e7) {
        return new ImmutableCollections.ListN<>(e1, e2, e3, e4, e5, e6, e7);
    }
    
    /**
     * Returns an unmodifiable list containing eight elements.
     *
     * See <a href="#unmodifiable">Unmodifiable Lists</a> for details.
     *
     * @param <E> the {@code List}'s element type
     * @param e1 the first element
     * @param e2 the second element
     * @param e3 the third element
     * @param e4 the fourth element
     * @param e5 the fifth element
     * @param e6 the sixth element
     * @param e7 the seventh element
     * @param e8 the eighth element
     * @return a {@code List} containing the specified elements
     * @throws NullPointerException if an element is {@code null}
     *
     * @since 9
     */
    static <E> List<E> of(E e1, E e2, E e3, E e4, E e5, E e6, E e7, E e8) {
        return new ImmutableCollections.ListN<>(e1, e2, e3, e4, e5, e6, e7, e8);
    }
    
    /**
     * Returns an unmodifiable list containing nine elements.
     *
     * See <a href="#unmodifiable">Unmodifiable Lists</a> for details.
     *
     * @param <E> the {@code List}'s element type
     * @param e1 the first element
     * @param e2 the second element
     * @param e3 the third element
     * @param e4 the fourth element
     * @param e5 the fifth element
     * @param e6 the sixth element
     * @param e7 the seventh element
     * @param e8 the eighth element
     * @param e9 the ninth element
     * @return a {@code List} containing the specified elements
     * @throws NullPointerException if an element is {@code null}
     *
     * @since 9
     */
    static <E> List<E> of(E e1, E e2, E e3, E e4, E e5, E e6, E e7, E e8, E e9) {
        return new ImmutableCollections.ListN<>(e1, e2, e3, e4, e5, e6, e7, e8, e9);
    }
    
    /**
     * Returns an unmodifiable list containing ten elements.
     *
     * See <a href="#unmodifiable">Unmodifiable Lists</a> for details.
     *
     * @param <E> the {@code List}'s element type
     * @param e1 the first element
     * @param e2 the second element
     * @param e3 the third element
     * @param e4 the fourth element
     * @param e5 the fifth element
     * @param e6 the sixth element
     * @param e7 the seventh element
     * @param e8 the eighth element
     * @param e9 the ninth element
     * @param e10 the tenth element
     * @return a {@code List} containing the specified elements
     * @throws NullPointerException if an element is {@code null}
     *
     * @since 9
     */
    static <E> List<E> of(E e1, E e2, E e3, E e4, E e5, E e6, E e7, E e8, E e9, E e10) {
        return new ImmutableCollections.ListN<>(e1, e2, e3, e4, e5, e6, e7, e8, e9, e10);
    }
    
    /**
     * Returns an unmodifiable list containing an arbitrary number of elements.
     * See <a href="#unmodifiable">Unmodifiable Lists</a> for details.
     *
     * @apiNote
     * This method also accepts a single array as an argument. The element type of
     * the resulting list will be the component type of the array, and the size of
     * the list will be equal to the length of the array. To create a list with
     * a single element that is an array, do the following:
     *
     * <pre>{@code
     *     String[] array = ... ;
     *     List<String[]> list = List.<String[]>of(array);
     * }</pre>
     *
     * This will cause the {@link List#of(Object) List.of(E)} method
     * to be invoked instead.
     *
     * @param <E> the {@code List}'s element type
     * @param elements the elements to be contained in the list
     * @return a {@code List} containing the specified elements
     * @throws NullPointerException if an element is {@code null} or if the array is {@code null}
     *
     * @since 9
     */
    @SafeVarargs
    @SuppressWarnings("varargs")
    static <E> List<E> of(E... elements) {
        switch (elements.length) { // implicit null check of elements
            case 0:
                return ImmutableCollections.emptyList();
            case 1:
                return new ImmutableCollections.List12<>(elements[0]);
            case 2:
                return new ImmutableCollections.List12<>(elements[0], elements[1]);
            default:
                return new ImmutableCollections.ListN<>(elements);
        }
    }
    
    /*▲ 只读对象 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /**
     * Compares the specified object with this list for equality.  Returns
     * {@code true} if and only if the specified object is also a list, both
     * lists have the same size, and all corresponding pairs of elements in
     * the two lists are <i>equal</i>.  (Two elements {@code e1} and
     * {@code e2} are <i>equal</i> if {@code Objects.equals(e1, e2)}.)
     * In other words, two lists are defined to be
     * equal if they contain the same elements in the same order.  This
     * definition ensures that the equals method works properly across
     * different implementations of the {@code List} interface.
     *
     * @param o the object to be compared for equality with this list
     * @return {@code true} if the specified object is equal to this list
     */
    boolean equals(Object o);
    
    /**
     * Returns the hash code value for this list.  The hash code of a list
     * is defined to be the result of the following calculation:
     * <pre>{@code
     *     int hashCode = 1;
     *     for (E e : list)
     *         hashCode = 31*hashCode + (e==null ? 0 : e.hashCode());
     * }</pre>
     * This ensures that {@code list1.equals(list2)} implies that
     * {@code list1.hashCode()==list2.hashCode()} for any two lists,
     * {@code list1} and {@code list2}, as required by the general
     * contract of {@link Object#hashCode}.
     *
     * @return the hash code value for this list
     * @see Object#equals(Object)
     * @see #equals(Object)
     */
    int hashCode();
    
}