[数据结构/Java/ListUtils] 深入解读：List(有序列表)

0 序：研究背景

• 2025-07，在某项目中，遭遇了对海量（百万级 / 千万级）有序列表数据的转换处理。

结合此项目的诉求，经过实际验证，ArrayList 比 LinkedList 的性能高出了 20%+ 的读取性能。

1 概述：List(有序列表)

• 在 Java 中，List 是一个接口（java.util.List），属于 Java 集合框架（Collection Framework）的一部分。

主要用于存储有序、可重复的元素集合。
它是 Collection 接口的子接口，相比 Set（无序、不可重复），List 更强调元素的【顺序】和【可访问性】。

源码: java.util.List

/*
 * Copyright (c) 1997, 2020, 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>The lists and their {@link #subList(int, int) subList} views implement the
 * {@link RandomAccess} interface.
 * <li>They are <a href="../lang/doc-files/ValueBased.html">value-based</a>.
 * Programmers should treat instances that are {@linkplain #equals(Object) equal}
 * as interchangeable and should not use them for synchronization, or
 * unpredictable behavior may occur. For example, in a future release,
 * synchronization may fail. Callers should make no assumptions about the
 * identity of the returned instances. Factories are free to
 * create new instances or reuse existing ones.
 * <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
 */

public interface List<E> extends Collection<E> {

Query Operations

• int size()
• boolean isEmpty()
• boolean contains(Object o)
• Iterator<E> iterator()
• Object[] toArray()
• <T> T[] toArray(T[] a)

    // Query Operations

    /**
     * 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
     */
    int size();

    /**
     * Returns {@code true} if this list contains no elements.
     *
     * @return {@code true} if this list contains no elements
     */
    boolean isEmpty();

    /**
     * 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>)
     */
    boolean contains(Object o);

    /**
     * Returns an iterator over the elements in this list in proper sequence.
     *
     * @return an iterator over the elements in this list in proper sequence
     */
    Iterator<E> iterator();

    /**
     * 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[])
     */
    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
     */
    <T> T[] toArray(T[] a);

Modification Operations

• boolean add(E e)
• boolean remove(Object o)
• boolean containsAll(Collection<?> c)
• boolean addAll(Collection<? extends E> c)
• boolean addAll(int index, Collection<? extends E> c)
• boolean removeAll(Collection<?> c)
• boolean retainAll(Collection<?> c)
• default void replaceAll(UnaryOperator<E> operator)
• default void sort(Comparator<? super E> c)
• void clear()

    // Modification Operations

    /**
     * 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
     */
    boolean add(E e);

    /**
     * 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
     */
    boolean remove(Object o);


    // Bulk Modification Operations

    /**
     * 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)
     */
    boolean containsAll(Collection<?> c);

    /**
     * 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)
     */
    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()})
     */
    boolean addAll(int index, Collection<? extends E> c);

    /**
     * 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)
     */
    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)
     */
    boolean retainAll(Collection<?> c);

    /**
     * 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
     */
    default void replaceAll(UnaryOperator<E> operator) {
        Objects.requireNonNull(operator);
        final ListIterator<E> li = this.listIterator();
        while (li.hasNext()) {
            li.set(operator.apply(li.next()));
        }
    }

    /**
     * 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
     */
    @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);
        }
    }

    /**
     * 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
     */
    void clear();

Comparison and hashing

• boolean equals(Object o)
• int hashCode()
• E get(int index)
• E set(int index, E element)
• void add(int index, E element)
• E remove(int index)

    // Comparison and hashing

    /**
     * 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();


    // Positional Access Operations

    /**
     * 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()})
     */
    E get(int index);

    /**
     * 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()})
     */
    E set(int index, E element);

    /**
     * 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()})
     */
    void add(int index, E element);

    /**
     * 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()})
     */
    E remove(int index);

Search Operations

• int indexOf(Object o)
• int lastIndexOf(Object o)

    // Search Operations

    /**
     * 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>)
     */
    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>)
     */
    int lastIndexOf(Object o);

List Iterators

• ListIterator<E> listIterator()
• ListIterator<E> listIterator(int index)

    // List Iterators

    /**
     * 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)
     */
    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()})
     */
    ListIterator<E> listIterator(int index);

View

• 方法列表

List<E> subList(int fromIndex, int toIndex)
default Spliterator<E> spliterator()
static <E> List<E> of()
static <E> List<E> of(E e1)
static <E> List<E> of(E e1, E e2)
static <E> List<E> of(E e1, E e2, E e3)
static <E> List<E> of(E e1, E e2, E e3, E e4)
static <E> List<E> of(E e1, E e2, E e3, E e4, E e5)
static <E> List<E> of(E e1, E e2, E e3, E e4, E e5, E e6)
static <E> List<E> of(E e1, E e2, E e3, E e4, E e5, E e6, E e7)
static <E> List<E> of(E e1, E e2, E e3, E e4, E e5, E e6, E e7, E e8)
static <E> List<E> of(E e1, E e2, E e3, E e4, E e5, E e6, E e7, E e8, E e9)
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)
static <E> List<E> of(E... elements)
static <E> List<E> copyOf(Collection<? extends E> coll)

• 源码

    // View

    /**
     * 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})
     */
    List<E> subList(int fromIndex, int toIndex);

    /**
     * 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
     */
    @Override
    default Spliterator<E> spliterator() {
        if (this instanceof RandomAccess) {
            return new AbstractList.RandomAccessSpliterator<>(this);
        } else {
            return Spliterators.spliterator(this, Spliterator.ORDERED);
        }
    }

    /**
     * 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
     */
    @SuppressWarnings("unchecked")
    static <E> List<E> of() {
        return (List<E>) ImmutableCollections.EMPTY_LIST;
    }

    /**
     * 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 ImmutableCollections.listFromTrustedArray(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 ImmutableCollections.listFromTrustedArray(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 ImmutableCollections.listFromTrustedArray(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 ImmutableCollections.listFromTrustedArray(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 ImmutableCollections.listFromTrustedArray(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 ImmutableCollections.listFromTrustedArray(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 ImmutableCollections.listFromTrustedArray(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 ImmutableCollections.listFromTrustedArray(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:
                @SuppressWarnings("unchecked")
                var list = (List<E>) ImmutableCollections.EMPTY_LIST;
                return list;
            case 1:
                return new ImmutableCollections.List12<>(elements[0]);
            case 2:
                return new ImmutableCollections.List12<>(elements[0], elements[1]);
            default:
                return ImmutableCollections.listFromArray(elements);
        }
    }

    /**
     * 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);
    }
}

核心特点

1. 有序性：元素按插入顺序保存，每个元素都有对应的索引（从 0 开始），可通过索引访问元素。
2. 可重复性：允许存储重复元素（即 equals() 方法返回 true 的元素）。
3. 动态大小：与数组不同，List 的大小可以动态调整，无需预先指定容量。

常用实现类

List 是接口，无法直接实例化，需使用其实现类，常见的有：

• ArrayList：基于动态数组实现，查询效率高（通过索引直接访问），增删元素（尤其是中间位置）效率较低，线程不安全。
• LinkedList：基于双向链表实现，增删元素（尤其是首尾位置）效率高，查询效率较低（需遍历链表），线程不安全，还实现了 Queue 接口，可作为队列使用。
• Vector：古老的动态数组实现，线程安全（方法加了 synchronized），但效率较低，现已很少使用。
• Stack：继承自 Vector，模拟栈结构（后进先出），但推荐使用 Deque 接口的实现类（如 ArrayDeque）替代。

• 小结

List 适合需要按顺序存储、频繁访问元素（如通过索引）或允许重复元素的场景，根据操作需求选择 ArrayList（查询多）或 LinkedList（增删多）作为实现类。

常用方法

List 继承了 Collection 的方法，并新增了基于索引的操作：

1. 添加元素：

   • add(E e)：在末尾添加元素。
   • add(int index, E e)：在指定索引插入元素。
   • addAll(Collection<? extends E> c)：添加另一个集合的所有元素。

2. 获取元素：

   • get(int index)：返回指定索引的元素。

3. 修改元素：

   • set(int index, E e)：替换指定索引的元素，返回被替换的旧元素。

4. 删除元素：

   • remove(int index)：删除指定索引的元素，返回被删除的元素。
   • remove(Object o)：删除第一个匹配的元素（基于 equals()）。

5. 查询索引：

   • indexOf(Object o)：返回元素首次出现的索引，不存在则返回 -1。
   • lastIndexOf(Object o)：返回元素最后出现的索引。

6. 子列表：

   • subList(int fromIndex, int toIndex)：返回从 fromIndex（包含）到 toIndex（不包含）的子列表（视图，修改会影响原列表）。

7. 其他：

   • size()：返回元素数量。
   • isEmpty()：判断是否为空。
   • contains(Object o)：判断是否包含指定元素。
   • clear()：清空所有元素。

示例代码

import java.util.ArrayList;
import java.util.List;

public class ListExample {
    public static void main(String[] args) {
        List<String> list = new ArrayList<>();
        list.add("Java");
        list.add("Python");
        list.add("Java"); // 允许重复

        System.out.println(list.get(0)); // 输出：Java
        System.out.println(list.indexOf("Java")); // 输出：0

        list.set(1, "C++"); // 替换索引1的元素
        list.remove(2); // 删除索引2的元素

        for (String s : list) {
            System.out.println(s); // 输出：Java、C++
        }
    }
}

M FAQ for List(有序列表)

Q:说说你自己对 ArrayList 的理解？

很多面试官喜欢这样子开头，考察面试同学对 ArrayList 有没有总结经验，介于 ArrayList 内容很多，建议先回答总体架构，再从某个细节出发作为突破口，比如这样：
ArrayList 底层数据结构是个数组，其 API 都做了一层对数组底层访问的封装，比如说 add 方法的过程是……（这里可以引用我们在 ArrayList 源码解析中 add 的过程）。

一般面试官看你回答得井井有条，并且没啥漏洞的话，基本就不会深究了，这样面试的主动权就掌握在自己手里面了，如果你回答得支支吾吾，那么面试官可能就会开启自己面试的套路了。

Q:说说你自己对 LinkedList 的理解也是同样套路。

Q: 扩容类问题

Q:ArrayList 无参数构造器构造，现在 add 一个值进去，此时数组的大小是多少，下一次扩容前最大可用大小是多少？

答：此处数组的大小是 1，下一次扩容前最大可用大小是 10，因为 ArrayList 第一次扩容时，是有默认值的，默认值是 10，在第一次 add 一个值进去时，数组的可用大小被扩容到 10 了。

Q:如果我连续往 list 里面新增值，增加到第 11 个的时候，数组的大小是多少？

答：这里的考查点就是扩容的公式，当增加到 11 的时候，此时我们希望数组的大小为 11，但实际上数组的最大容量只有 10，不够了就需要扩容，扩容的公式是：oldCapacity + (oldCapacity>> 1)，oldCapacity 表示数组现有大小，目前场景计算公式是：10 + 10 ／2 = 15，然后我们发现 15 已经够用了，所以数组的大小会被扩容到 15。

Q:数组初始化，被加入一个值后，如果我使用 addAll 方法，一下子加入 15 个值，那么最终数组的大小是多少？

答：第一题中我们已经计算出来数组在加入一个值后，实际大小是 1，最大可用大小是 10 ，现在需要一下子加入 15 个值，那我们期望数组的大小值就是 16，此时数组最大可用大小只有 10，明显不够，需要扩容，扩容后的大小是：10 + 10 ／2 = 15，这时候发现扩容后的大小仍然不到我们期望的值 16，这时候源码中有一种策略如下：

// newCapacity 本次扩容的大小，minCapacity 我们期望的数组最小大小
// 如果扩容后的值 < 我们的期望值，我们的期望值就等于本次扩容的大小
if (newCapacity - minCapacity < 0)
    newCapacity = minCapacity;

所以最终数组扩容后的大小为 16。

Q:现在我有一个很大的数组需要拷贝，原数组大小是 5k，请问如何快速拷贝？

答：因为原数组比较大，如果新建新数组的时候，不指定数组大小的话，就会频繁扩容，频繁扩容就会有大量拷贝的工作，造成拷贝的性能低下，所以回答说新建数组时，指定新数组的大小为 5k 即可。

Q:为什么说扩容会消耗性能？

答：扩容底层使用的是 System.arraycopy 方法，会把原数组的数据全部拷贝到新数组上，所以性能消耗比较严重。

Q:源码扩容过程有什么值得借鉴的地方？

答：有两点：

是扩容的思想值得学习，通过自动扩容的方式，让使用者不用关心底层数据结构的变化，封装得很好，1.5 倍的扩容速度，可以让扩容速度在前期缓慢上升，在后期增速较快，大部分工作中要求数组的值并不是很大，所以前期增长缓慢有利于节省资源，在后期增速较快时，也可快速扩容。

扩容过程中，有数组大小溢出的意识，比如要求扩容后的数组大小，不能小于 0，不能大于 Integer 的最大值。
这两点在我们平时设计和写代码时都可以借鉴。

Q:删除类问题

Q:有一个 ArrayList，数据是 2、3、3、3、4，中间有三个 3，现在我通过 for (int i=0;i<list.size ();i++) 的方式，想把值是 3 的元素删除，请问可以删除干净么？最终删除的结果是什么，为什么？删除代码如下：

List<String> list = new ArrayList<String>() {{
  add("2");
  add("3");
  add("3");
  add("3");
  add("4");
}};
for (int i = 0; i < list.size(); i++) {
  if (list.get(i).equals("3")) {
    list.remove(i);
  }
}

答：不能删除干净，最终删除的结果是 2、3、4，有一个 3 删除不掉，原因我们看下图：

图片描述从图中我们可以看到，每次删除一个元素后，该元素后面的元素就会往前移动，而此时循环的 i 在不断地增长，最终会使每次删除 3 的后一个 3 被遗漏，导致删除不掉。

Q:还是上面的 ArrayList 数组，我们通过增强 for 循环进行删除，可以么？

答：不可以，会报错。因为增强 for 循环过程其实调用的就是迭代器的 next () 方法，当你调用 list#remove () 方法进行删除时，modCount 的值会 +1，而这时候迭代器中的 expectedModCount 的值却没有变，导致在迭代器下次执行 next () 方法时，expectedModCount != modCount 就会报 ConcurrentModificationException 的错误。

Q:还是上面的数组，如果删除时使用 Iterator.remove () 方法可以删除么，为什么？

答：可以的，因为 Iterator.remove () 方法在执行的过程中，会把最新的 modCount 赋值给 expectedModCount，这样在下次循环过程中，modCount 和 expectedModCount 两者就会相等。

Q:以上三个问题对于 LinkedList 也是同样的结果么？

答：是的，虽然 LinkedList 底层结构是双向链表，但对于上述三个问题，结果和 ArrayList 是一致的。

Q:对比类问题

Q:ArrayList 和 LinkedList 有何不同？

答：可以先从底层数据结构开始说起，然后以某一个方法为突破口深入，比如：最大的不同是两者底层的数据结构不同，ArrayList 底层是数组，LinkedList 底层是双向链表，两者的数据结构不同也导致了操作的 API 实现有所差异，拿新增实现来说，ArrayList 会先计算并决定是否扩容，然后把新增的数据直接赋值到数组上，而 LinkedList 仅仅只需要改变插入节点和其前后节点的指向位置关系即可。

Q:ArrayList 和 LinkedList 应用场景有何不同

答：ArrayList 更适合于快速的查找匹配，不适合频繁新增删除，像工作中经常会对元素进行匹配查询的场景比较合适，LinkedList 更适合于经常新增和删除，对查询反而很少的场景。

Q:ArrayList 和 LinkedList 两者有没有最大容量

答：ArrayList 有最大容量的，为 Integer 的最大值，大于这个值 JVM 是不会为数组分配内存空间的，LinkedList 底层是双向链表，理论上可以无限大。但源码中，LinkedList 实际大小用的是 int 类型，这也说明了 LinkedList 不能超过 Integer 的最大值，不然会溢出。

Q:ArrayList 和 LinkedList 是如何对 null 值进行处理的

答：ArrayList 允许 null 值新增，也允许 null 值删除。删除 null 值时，是从头开始，找到第一值是 null 的元素删除；LinkedList 新增删除时对 null 值没有特殊校验，是允许新增和删除的。

Q:ArrayList 和 LinedList 是线程安全的么，为什么？

答：当两者作为非共享变量时，比如说仅仅是在方法里面的局部变量时，是没有线程安全问题的，只有当两者是共享变量时，才会有线程安全问题。主要的问题点在于多线程环境下，所有线程任何时刻都可对数组和链表进行操作，这会导致值被覆盖，甚至混乱的情况。

如果有线程安全问题，在迭代的过程中，会频繁报 ConcurrentModificationException 的错误，意思是在我当前循环的过程中，数组或链表的结构被其它线程修改了。

Q:如何解决线程安全问题？

Java 源码中推荐使用 Collections#synchronizedList 进行解决，Collections#synchronizedList 的返回值是 List 的每个方法都加了 synchronized 锁，保证了在同一时刻，数组和链表只会被一个线程所修改，或者采用 CopyOnWriteArrayList 并发 List 来解决，这个类我们后面会说。

Q:其它类型题目

Q:你能描述下双向链表么？

答：如果和面试官面对面沟通的话，你可以去画一下，可以把 《LinkedList 源码解析》中的 LinkedList 的结构画出来，如果是电话面试，可以这么描述：双向链表中双向的意思是说前后节点之间互相有引用，链表的节点我们称为 Node。Node 有三个属性组成：其前一个节点，本身节点的值，其下一个节点，假设 A、B 节点相邻，A 节点的下一个节点就是 B，B 节点的上一个节点就是 A，两者互相引用，在链表的头部节点，我们称为头节点。头节点的前一个节点是 null，尾部称为尾节点，尾节点的后一个节点是 null，如果链表数据为空的话，头尾节点是同一个节点，本身是 null，指向前后节点的值也是 null。

Q:描述下双向链表的新增和删除?

答：如果是面对面沟通，最好可以直接画图，如果是电话面试，可以这么描述：

新增：我们可以选择从链表头新增，也可以选择从链表尾新增，如果是从链表尾新增的话，直接把当前节点追加到尾节点之后，本身节点自动变为尾节点。

删除：把删除节点的后一个节点的 prev 指向其前一个节点，把删除节点的前一个节点的 next 指向其后一个节点，最后把删除的节点置为 null 即可。

F 案例实践

CASE 深入比对：ArrayList vs. LinkedList

• ArrayList 基于数组实现，适合随机访问；LinkedList 基于双向链表实现，适合频繁的插入和删除操作。

存储结构

• ArrayList

• 基于动态数组实现，内存连续，支持快速随机访问。

• LinkedList

• 基于双向链表实现，每个节点包含前后指针，插入和删除效率较高，但随机访问性能较差。

插入与删除效率

• ArrayList

• 在插入或删除时需要移动元素，效率较低，尤其是头部和中间位置操作。

• LinkedList

• 通过调整指针即可完成插入和删除，效率较高，尤其在头部操作时表现优异。

随机访问效率

• ArrayList

• 由于底层是数组，支持 O(1) 的随机访问。

• LinkedList

• 需要从头或尾遍历链表，随机访问效率为 O(n)，性能远低于 ArrayList。

迭代性能

• ArrayList 的连续存储特性使其在迭代时性能更优，得益于 CPU 缓存优化。
• LinkedList 由于需要频繁跳转指针，迭代性能稍逊。

内存占用

• ArrayList 仅存储元素本身，内存占用较低。
• LinkedList 每个节点需额外存储两个指针（前驱和后继），内存占用更高。

有序性

• ArrayList 和 LinkedList 都是 有序集合，它们的主要区别在于底层实现和性能特点，而不是存储顺序是否有序。

    /**
     * 验证 array list 的有序性: 有序
     */
    @Test
    public void arrayListTest(){
        String [] numbers = {"1", "2", "3", "4", "5", "6", "7", "8", "9", "10"};

        ArrayList<String> list = new ArrayList();
        Arrays.stream(numbers).forEach( element -> {
            list.add(element);
        });
        list.stream().forEach( element -> {
            log.info( "{}", element );
        });//"1", "2", "3", "4", "5", "6", "7", "8", "9", "10"
    }

• 【延申】 Map 集合

• HashMap： 无序
• TreeMap： 有序
• LinkedHashMap: 有序

最佳实践

• 当需要频繁随机访问或存储大量数据时，推荐使用 ArrayList。

ArrayList 在随机访问和迭代性能上表现优异，适合读多写少的场景

• 若需要频繁在头部插入或删除元素，LinkedList 更为适合。

LinkedList 在插入和删除操作上更高效，适合写多读少或需要频繁头部操作的场景。

根据具体需求选择合适的数据结构是关键。

CASE List数据结构的增强设计：循环队列(FixedArrayList)

import org.jetbrains.annotations.NotNull;

import java.util.ArrayList;
import java.util.Collection;
import java.util.Iterator;
import java.util.Objects;
import java.util.function.Consumer;


/**
 * 循环队列实现
 * 主要功能：
 * 1.固定队列长度
 * 2.队列先进先出，当写入时队列已满则移除第一次写入的数据
 * 3.可以通过下标获取或更新指定队列的数据
 * 4.当队列为空队列时重新开始计算
 *
 * @param <T>
 */
public class FixedArrayList<T> {
    private int currentWritePos = 0;
    private int currentReadPos = 0;
    private int length = 5;
    private ArrayList<T> arr;

    /**
     * 初始长度队列
     *
     * @param length
     */
    public FixedArrayList(int length) {
        this.length = length;
        this.arr = new ArrayList<>(length);
        for (int i = 0; i < length; i++) {
            this.arr.add(null);
        }
    }

    /**
     * 默认长度队列
     */
    public FixedArrayList() {
        this.arr = new ArrayList<>(this.length);
        for (int i = 0; i < this.length; i++) {
            this.arr.add(null);
        }
    }

    /**
     * 获取位置
     *
     * @param pos
     * @return
     */
    private int getRealPosition(int pos) {
        return pos % this.length;
    }

    /**
     * 入队列
     *
     * @param obj
     */
    public void push(T obj) {
        if (isFull()) {
            this.currentReadPos = this.currentWritePos + 1 - this.length;
        }
        this.arr.set(this.getRealPosition(this.currentWritePos), obj);

        this.currentWritePos++;
    }

    /**
     * 出队列
     */
    public boolean pop() {
        if (isEmpty()) {
            this.currentWritePos = 0;
            this.currentReadPos = 0;
            return false;
        } else {
            this.arr.set(this.getRealPosition(this.currentReadPos), null);
            this.currentReadPos++;
            return true;
        }
    }

    /**
     * 获取队列指定位置元素
     *
     * @param i
     * @return
     */
    public T get(int i) {
        return this.arr.get(this.getRealPosition(this.currentReadPos + i));
    }

    /**
     * 更新队列指定位置元素
     *
     * @param i
     * @param obj
     */
    public void set(int i, T obj) {
        this.arr.set(this.getRealPosition(this.currentReadPos + i), obj);
    }


    /**
     * 检测队列是否已满
     *
     * @return
     */
    public boolean isFull() {
        return this.currentWritePos == this.currentReadPos + this.length;
    }


    /**
     * 检测队列是否全部不为null
     *
     * @return
     */
    public boolean isAllNotNull() {
        for (T t : this.arr) {
            if (t == null) {
                return false;
            }
        }
        return true;
    }


    /**
     * 检测队列是否为空
     *
     * @return
     */
    public boolean isEmpty() {
        return this.currentReadPos == this.currentWritePos;
    }

    /**
     * 清空队列
     */
    public void clean() {
        this.currentWritePos = 0;
        this.currentReadPos = 0;
        for (int i = 0; i < this.length; i++) {
            this.arr.set(i, null);
        }
    }

    /**
     * 获取当队列满时的队列最后一个元素
     */
    public T getLastElement() {
        return this.arr.get(this.arr.size() - 1);
    }

    /**
     * 获取队列长度
     */
    public int getLength() {
        return this.arr.size();
    }

    /**
     * 获取队列元素个数
     */
    public int getElementLength() {
        int i = 0;
        for (T t : this.arr) {
            if (t != null) {
                i++;
            }
        }
        return i;
    }
}

CASE 基于 List<Double/Float> 对象，计算 Max/MaxIndex/Min/MinIndex/Avg 统计指标

import java.util.List;

/**
 * @author johnnyzen
 * @version v1.0
 * @create-time 2025/10/16 19:21
 * @update-time 2025/10/18 11:49
 * @description ...
 * @refrence-doc
 * @gpt-promt
 *  ListUtils Java 工具类的作用:
 *    1. 基于 List<Float> list 对象，分别计算出最小值、最小值的下标、最大值、最大值的下标、平均值。
 *    2. 若列表为 null 或空，则 min/max/average 返回 Float.NaN，下标返回 -1。
 *  请输出 ListUtils.java 的源代码文件
 */
public final class ListUtils {
    /**
     * 统计 List<Float>
     * @note
     *   1. 基于 List<Float> list 对象，分别计算出最小值、最小值的下标、最大值、最大值的下标、平均值。
     *   2. 若列表为 null 或空，则 min/max/average 返回 Float.NaN，下标返回 -1。
     * @param list
     * @return
     */
    public static Stat stat(List<Float> list) {
        if (list == null || list.isEmpty()) {
            return new Stat(null, -1, null, -1, null);
        }

        float min = Float.MAX_VALUE;
        int minIndex = -1;
        float max = -Float.MAX_VALUE;
        int maxIndex = -1;
        double sum = 0;   // 用 double 累加，减小误差

        for (int i = 0; i < list.size(); i++) {
            Float v = list.get(i);
            if (v == null) {
                continue; // 跳过 null 元素
            }
            float val = v;

            if (val < min) {
                min = val;
                minIndex = i;
            }
            if (val > max) {
                max = val;
                maxIndex = i;
            }
            sum += val;
        }

        // 全是 null 时，认为无有效数据
        if (minIndex == -1) {
            return new Stat(null, -1, null, -1, null);
        }

        float average = (float) (sum / list.size());
        return new Stat(min, minIndex, max, maxIndex, average);
    }

    public static float min(List<Float> list) {
        return stat(list).min;
    }

    public static int minIndex(List<Float> list) {
        return stat(list).minIndex;
    }

    public static float max(List<Float> list) {
        return stat(list).max;
    }

    public static int maxIndex(List<Float> list) {
        return stat(list).maxIndex;
    }

    public static float average(List<Float> list) {
        return stat(list).average;
    }

    /**
     * 统计结果封装
     */
    public static final class Stat {
        public final Float min;
        public final int minIndex;
        public final Float max;
        public final int maxIndex;
        public final Float average;

        private Stat(Float min, int minIndex, Float max, int maxIndex, Float average) {
            this.min = min;
            this.minIndex = minIndex;
            this.max = max;
            this.maxIndex = maxIndex;
            this.average = average;
        }

        @Override
        public String toString() {
            return "Stat{" +
                    "min=" + min +
                    ", minIndex=" + minIndex +
                    ", max=" + max +
                    ", maxIndex=" + maxIndex +
                    ", average=" + average +
                    '}';
        }
    }
}

CASE 基于 List<Map<String, Object>> : list对象，根据 Map 的指定 key 字段做排序

假定指定key的名称为 collectTimestamp (Long)

• 方案1: 使用 Stream API 排序（不修改原集合）

import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;

// 原集合：List<Map<String, Object>> list
List<Map<String, Object>> sortedList = list.stream()
    // 按 collectTimestamp 降序排序
    .sorted((map1, map2) -> {
        // 从 Map 中获取 collectTimestamp 的值（强制转换为 Long）
        Long time1 = (Long) map1.get("collectTimestamp");
        Long time2 = (Long) map2.get("collectTimestamp");
        // 降序排序：time2 - time1（若为升序则是 time1 - time2）
        return Long.compare(time2, time1);
    })
    // 收集为新的 List
    .collect(Collectors.toList());

• 方案2: 使用 Collections.sort () 排序（直接修改原集合）

如果希望直接修改原集合（而非生成新集合），可以使用 Collections.sort()：

import java.util.Collections;
import java.util.List;
import java.util.Map;

// 原集合：List<Map<String, Object>> list
Collections.sort(list, (map1, map2) -> {
    Long time1 = (Long) map1.get("collectTimestamp");
    Long time2 = (Long) map2.get("collectTimestamp");
    return Long.compare(time2, time1); // 降序
});

Y 推荐文献

X 参考文献

• List面试题 - CSDN