Random Access interface

I. Official Description

Let's first look at how this interface is described in java.

Location: java.util.Random Access in rt.jar

 * Marker interface used by <tt>List</tt> implementations to indicate that
 * they support fast (generally constant time) random access.  The primary
 * purpose of this interface is to allow generic algorithms to alter their
 * behavior to provide good performance when applied to either random or
 * sequential access lists.
 * <p>The best algorithms for manipulating random access lists (such as
 * <tt>ArrayList</tt>) can produce quadratic behavior when applied to
 * sequential access lists (such as <tt>LinkedList</tt>).  Generic list
 * algorithms are encouraged to check whether the given list is an
 * <tt>instanceof</tt> this interface before applying an algorithm that would
 * provide poor performance if it were applied to a sequential access list,
 * and to alter their behavior if necessary to guarantee acceptable
 * performance.
 * <p>It is recognized that the distinction between random and sequential
 * access is often fuzzy.  For example, some <tt>List</tt> implementations
 * provide asymptotically linear access times if they get huge, but constant
 * access times in practice.  Such a <tt>List</tt> implementation
 * should generally implement this interface.  As a rule of thumb, a
 * <tt>List</tt> implementation should implement this interface if,
 * for typical instances of the class, this loop:
 * <pre>
 *     for (int i=0, n=list.size(); i &lt; n; i++)
 *         list.get(i);
 * </pre>
 * runs faster than this loop:
 * <pre>
 *     for (Iterator i=list.iterator(); i.hasNext(); )
 *         i.next();
 * </pre>

In general, this is a tag interface used in List implementation classes to support fast random access (for a certain period of time). The main purpose of this interface is to allow generic algorithms to change their behavior when applied to random or sequential access lists to provide good performance.
ArrayList, an implementation class of List only, can be executed with this algorithm for + get (index), but not in LinkedList.

2. View ArrayList and LinkedList

Location: java.util.ArrayList and java.util.LinkedList in rt.jar

2.1 ArrayList

public class ArrayList<E> extends AbstractList<E>
        implements List<E>, RandomAccess, Cloneable, java.io.Serializable
    private static final long serialVersionUID = 8683452581122892189L;

2.2 LinkedList

public class LinkedList<E>
    extends AbstractSequentialList<E>
    implements List<E>, Deque<E>, Cloneable, java.io.Serializable
    transient int size = 0;

** As can be seen from the source code, ArrayList is a class that implements interfaces such as List, Random Access, Cloneable, java.io.Serializable, etc. Array sets that can be accessed, cloned and serialized randomly are implemented.
LinkedList is a class that implements interfaces such as List, Deque, Cloneable, java.io.Serializable, so it is a cloned and serialized double-ended linked list.