Vector.java

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 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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 * 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
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 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
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 * accompanied this code).
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 *
 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
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package java.util;

import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.Serializable;
import java.io.StreamCorruptedException;
import java.util.function.Consumer;
import java.util.function.Predicate;
import java.util.function.UnaryOperator;

/**
 * The {@code Vector} class implements a growable array of
 * objects. Like an array, it contains components that can be
 * accessed using an integer index. However, the size of a
 * {@code Vector} can grow or shrink as needed to accommodate
 * adding and removing items after the {@code Vector} has been created.
 *
 * <p>Each vector tries to optimize storage management by maintaining a
 * {@code capacity} and a {@code capacityIncrement}. The
 * {@code capacity} is always at least as large as the vector
 * size; it is usually larger because as components are added to the
 * vector, the vector's storage increases in chunks the size of
 * {@code capacityIncrement}. An application can increase the
 * capacity of a vector before inserting a large number of
 * components; this reduces the amount of incremental reallocation.
 *
 * <p id="fail-fast">
 * The iterators returned by this class's {@link #iterator() iterator} and
 * {@link #listIterator(int) listIterator} methods are <em>fail-fast</em>:
 * if the vector is structurally modified at any time after the iterator is
 * created, in any way except through the iterator's own
 * {@link ListIterator#remove() remove} or
 * {@link ListIterator#add(Object) add} methods, the iterator will throw a
 * {@link ConcurrentModificationException}.  Thus, in the face of
 * concurrent modification, the iterator fails quickly and cleanly, rather
 * than risking arbitrary, non-deterministic behavior at an undetermined
 * time in the future.  The {@link Enumeration Enumerations} returned by
 * the {@link #elements() elements} method are <em>not</em> fail-fast; if the
 * Vector is structurally modified at any time after the enumeration is
 * created then the results of enumerating are undefined.
 *
 * <p>Note that the fail-fast behavior of an iterator cannot be guaranteed
 * as it is, generally speaking, impossible to make any hard guarantees in the
 * presence of unsynchronized concurrent modification.  Fail-fast iterators
 * throw {@code ConcurrentModificationException} on a best-effort basis.
 * Therefore, it would be wrong to write a program that depended on this
 * exception for its correctness:  <i>the fail-fast behavior of iterators
 * should be used only to detect bugs.</i>
 *
 * <p>As of the Java 2 platform v1.2, this class was retrofitted to
 * implement the {@link List} interface, making it a member of the
 * <a href="{@docRoot}/java.base/java/util/package-summary.html#CollectionsFramework">
 * Java Collections Framework</a>.  Unlike the new collection
 * implementations, {@code Vector} is synchronized.  If a thread-safe
 * implementation is not needed, it is recommended to use {@link
 * ArrayList} in place of {@code Vector}.
 *
 * @param <E> Type of component elements
 *
 * @author Lee Boynton
 * @author Jonathan Payne
 * @see Collection
 * @see LinkedList
 * @since 1.0
 */
// 向量表：线性表的顺序存储结构，内部使用数组实现，可以看做是ArrayList的线程安全版本
public class Vector<E> extends AbstractList<E> implements List<E>, RandomAccess, Cloneable, Serializable {
    
    /**
     * The maximum size of array to allocate (unless necessary).
     * Some VMs reserve some header words in an array.
     * Attempts to allocate larger arrays may result in
     * OutOfMemoryError: Requested array size exceeds VM limit
     */
    private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;
    
    
    /**
     * The array buffer into which the components of the vector are
     * stored. The capacity of the vector is the length of this array buffer,
     * and is at least large enough to contain all the vector's elements.
     *
     * <p>Any array elements following the last element in the Vector are null.
     *
     * @serial
     */
    protected Object[] elementData; // 存储当前向量表的元素
    
    /**
     * The number of valid components in this {@code Vector} object.
     * Components {@code elementData[0]} through
     * {@code elementData[elementCount-1]} are the actual items.
     *
     * @serial
     */
    protected int elementCount; // 元素数量
    
    /**
     * The amount by which the capacity of the vector is automatically
     * incremented when its size becomes greater than its capacity.  If
     * the capacity increment is less than or equal to zero, the capacity
     * of the vector is doubled each time it needs to grow.
     *
     * @serial
     */
    protected int capacityIncrement;    // 扩容增量
    
    
    
    
    /*▼ 构造器 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Constructs an empty vector so that its internal data array
     * has size {@code 10} and its standard capacity increment is
     * zero.
     */
    public Vector() {
        this(10);
    }
    
    /**
     * Constructs an empty vector with the specified initial capacity and
     * with its capacity increment equal to zero.
     *
     * @param initialCapacity the initial capacity of the vector
     *
     * @throws IllegalArgumentException if the specified initial capacity
     *                                  is negative
     */
    public Vector(int initialCapacity) {
        this(initialCapacity, 0);
    }
    
    /**
     * Constructs an empty vector with the specified initial capacity and
     * capacity increment.
     *
     * @param initialCapacity   the initial capacity of the vector
     * @param capacityIncrement the amount by which the capacity is
     *                          increased when the vector overflows
     *
     * @throws IllegalArgumentException if the specified initial capacity
     *                                  is negative
     */
    public Vector(int initialCapacity, int capacityIncrement) {
        super();
        if(initialCapacity<0) {
            throw new IllegalArgumentException("Illegal Capacity: " + initialCapacity);
        }
        this.elementData = new Object[initialCapacity];
        this.capacityIncrement = capacityIncrement;
    }
    
    /**
     * Constructs a vector containing the elements of the specified
     * collection, in the order they are returned by the collection's
     * iterator.
     *
     * @param c the collection whose elements are to be placed into this
     *          vector
     *
     * @throws NullPointerException if the specified collection is null
     * @since 1.2
     */
    public Vector(Collection<? extends E> c) {
        elementData = c.toArray();
        elementCount = elementData.length;
        // defend against c.toArray (incorrectly) not returning Object[]
        // (see e.g. https://bugs.openjdk.java.net/browse/JDK-6260652)
        if(elementData.getClass() != Object[].class) {
            elementData = Arrays.copyOf(elementData, elementCount, Object[].class);
        }
    }
    
    /*▲ 构造器 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 存值 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Appends the specified element to the end of this Vector.
     *
     * @param e element to be appended to this Vector
     *
     * @return {@code true} (as specified by {@link Collection#add})
     *
     * @since 1.2
     */
    // 将元素e追加到当前向量表中，返回值指示是否添加成功
    public synchronized boolean add(E e) {
        modCount++;
        add(e, elementData, elementCount);
        return true;
    }
    
    /**
     * Adds the specified component to the end of this vector,
     * increasing its size by one. The capacity of this vector is
     * increased if its size becomes greater than its capacity.
     *
     * <p>This method is identical in functionality to the
     * {@link #add(Object) add(E)}
     * method (which is part of the {@link List} interface).
     *
     * @param obj the component to be added
     */
    // 将元素e追加到当前向量表中
    public synchronized void addElement(E e) {
        modCount++;
        add(e, elementData, elementCount);
    }
    
    /**
     * Inserts the specified element at the specified position in this Vector.
     * 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 ArrayIndexOutOfBoundsException if the index is out of range
     *                                        ({@code index < 0 || index > size()})
     * @since 1.2
     */
    // 将元素element添加到向量表index处
    public void add(int index, E element) {
        insertElementAt(element, index);
    }
    
    /**
     * Inserts the specified object as a component in this vector at the
     * specified {@code index}. Each component in this vector with
     * an index greater or equal to the specified {@code index} is
     * shifted upward to have an index one greater than the value it had
     * previously.
     *
     * <p>The index must be a value greater than or equal to {@code 0}
     * and less than or equal to the current size of the vector. (If the
     * index is equal to the current size of the vector, the new element
     * is appended to the Vector.)
     *
     * <p>This method is identical in functionality to the
     * {@link #add(int, Object) add(int, E)}
     * method (which is part of the {@link List} interface).  Note that the
     * {@code add} method reverses the order of the parameters, to more closely
     * match array usage.
     *
     * @param obj   the component to insert
     * @param index where to insert the new component
     *
     * @throws ArrayIndexOutOfBoundsException if the index is out of range
     *                                        ({@code index < 0 || index > size()})
     */
    // 将元素element添加到向量表index处
    public synchronized void insertElementAt(E element, int index) {
        if(index>elementCount) {
            throw new ArrayIndexOutOfBoundsException(index + " > " + elementCount);
        }
        
        modCount++;
        
        final int s = elementCount;
        
        Object[] elementData = this.elementData;
        
        // 如果向量表已满，则需要扩容
        if(s == elementData.length) {
            // 对当前向量表扩容
            elementData = grow();
        }
        
        // 移动元素
        System.arraycopy(elementData, index, elementData, index + 1, s - index);
        
        // 插入元素
        elementData[index] = element;
        
        elementCount = s + 1;
    }
    
    
    /**
     * Appends all of the elements in the specified Collection to the end of
     * this Vector, 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.
     * (This implies that the behavior of this call is undefined if the
     * specified Collection is this Vector, and this Vector is nonempty.)
     *
     * @param c elements to be inserted into this Vector
     *
     * @return {@code true} if this Vector changed as a result of the call
     *
     * @throws NullPointerException if the specified collection is null
     * @since 1.2
     */
    // 将指定容器中的元素追加到当前向量表中
    public boolean addAll(Collection<? extends E> c) {
        Object[] a = c.toArray();
        modCount++;
        
        int numNew = a.length;
        if(numNew == 0) {
            return false;
        }
        
        synchronized(this) {
            Object[] elementData = this.elementData;
            final int s = elementCount;
            if(numNew>elementData.length - s) {
                elementData = grow(s + numNew);
            }
            System.arraycopy(a, 0, elementData, s, numNew);
            elementCount = s + numNew;
            return true;
        }
    }
    
    /**
     * Inserts all of the elements in the specified Collection into this
     * Vector at the specified position.  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 the Vector
     * in the order that they are returned by the specified Collection's
     * iterator.
     *
     * @param index index at which to insert the first element from the
     *              specified collection
     * @param c     elements to be inserted into this Vector
     *
     * @return {@code true} if this Vector changed as a result of the call
     *
     * @throws ArrayIndexOutOfBoundsException if the index is out of range
     *                                        ({@code index < 0 || index > size()})
     * @throws NullPointerException           if the specified collection is null
     * @since 1.2
     */
    // 将指定容器中的元素添加到当前向量表的index处
    public synchronized boolean addAll(int index, Collection<? extends E> c) {
        if(index<0 || index>elementCount) {
            throw new ArrayIndexOutOfBoundsException(index);
        }
        
        Object[] a = c.toArray();
        modCount++;
        int numNew = a.length;
        if(numNew == 0) {
            return false;
        }
        Object[] elementData = this.elementData;
        final int s = elementCount;
        if(numNew>elementData.length - s) {
            elementData = grow(s + numNew);
        }
        
        int numMoved = s - index;
        if(numMoved>0) {
            System.arraycopy(elementData, index, elementData, index + numNew, numMoved);
        }
        System.arraycopy(a, 0, elementData, index, numNew);
        elementCount = s + numNew;
        return true;
    }
    
    /*▲ 存值 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 取值 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Returns the element at the specified position in this Vector.
     *
     * @param index index of the element to return
     *
     * @return object at the specified index
     *
     * @throws ArrayIndexOutOfBoundsException if the index is out of range
     *                                        ({@code index < 0 || index >= size()})
     * @since 1.2
     */
    // 获取指定索引处的元素
    public synchronized E get(int index) {
        if(index >= elementCount) {
            throw new ArrayIndexOutOfBoundsException(index);
        }
        
        return elementData(index);
    }
    
    /**
     * Returns the component at the specified index.
     *
     * <p>This method is identical in functionality to the {@link #get(int)}
     * method (which is part of the {@link List} interface).
     *
     * @param index an index into this vector
     *
     * @return the component at the specified index
     *
     * @throws ArrayIndexOutOfBoundsException if the index is out of range
     *                                        ({@code index < 0 || index >= size()})
     */
    // 获取指定索引处的元素
    public synchronized E elementAt(int index) {
        if(index >= elementCount) {
            throw new ArrayIndexOutOfBoundsException(index + " >= " + elementCount);
        }
        
        return elementData(index);
    }
    
    /**
     * Returns the first component (the item at index {@code 0}) of
     * this vector.
     *
     * @return the first component of this vector
     *
     * @throws NoSuchElementException if this vector has no components
     */
    // 获取向量表中首个元素
    public synchronized E firstElement() {
        if(elementCount == 0) {
            throw new NoSuchElementException();
        }
        
        return elementData(0);
    }
    
    /**
     * Returns the last component of the vector.
     *
     * @return the last component of the vector, i.e., the component at index
     * {@code size() - 1}
     *
     * @throws NoSuchElementException if this vector is empty
     */
    // 获取向量表中末尾元素
    public synchronized E lastElement() {
        if(elementCount == 0) {
            throw new NoSuchElementException();
        }
        
        return elementData(elementCount - 1);
    }
    
    /*▲ 取值 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 移除 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Removes the element at the specified position in this Vector.
     * Shifts any subsequent elements to the left (subtracts one from their
     * indices).  Returns the element that was removed from the Vector.
     *
     * @param index the index of the element to be removed
     *
     * @return element that was removed
     *
     * @throws ArrayIndexOutOfBoundsException if the index is out of range
     *                                        ({@code index < 0 || index >= size()})
     * @since 1.2
     */
    // 移除索引index处的元素，返回被移除的元素
    public synchronized E remove(int index) {
        modCount++;
        
        if(index >= elementCount) {
            throw new ArrayIndexOutOfBoundsException(index);
        }
        
        E oldValue = elementData(index);
        
        int numMoved = elementCount - index - 1;
        if(numMoved>0) {
            System.arraycopy(elementData, index + 1, elementData, index, numMoved);
        }
        
        elementData[--elementCount] = null; // Let gc do its work
        
        return oldValue;
    }
    
    /**
     * Deletes the component at the specified index. Each component in
     * this vector with an index greater or equal to the specified
     * {@code index} is shifted downward to have an index one
     * smaller than the value it had previously. The size of this vector
     * is decreased by {@code 1}.
     *
     * <p>The index must be a value greater than or equal to {@code 0}
     * and less than the current size of the vector.
     *
     * <p>This method is identical in functionality to the {@link #remove(int)}
     * method (which is part of the {@link List} interface).  Note that the
     * {@code remove} method returns the old value that was stored at the
     * specified position.
     *
     * @param index the index of the object to remove
     *
     * @throws ArrayIndexOutOfBoundsException if the index is out of range
     *                                        ({@code index < 0 || index >= size()})
     */
    // 移除索引index处的元素
    public synchronized void removeElementAt(int index) {
        if(index >= elementCount) {
            throw new ArrayIndexOutOfBoundsException(index + " >= " + elementCount);
        } else if(index<0) {
            throw new ArrayIndexOutOfBoundsException(index);
        }
        
        int j = elementCount - index - 1;
        if(j>0) {
            System.arraycopy(elementData, index + 1, elementData, index, j);
        }
        modCount++;
        elementCount--;
        elementData[elementCount] = null; /* to let gc do its work */
    }
    
    /**
     * Removes the first occurrence of the specified element in this Vector
     * If the Vector does not contain the element, it is unchanged.  More
     * formally, removes the element with the lowest index i such that
     * {@code Objects.equals(o, get(i))} (if such
     * an element exists).
     *
     * @param o element to be removed from this Vector, if present
     *
     * @return true if the Vector contained the specified element
     *
     * @since 1.2
     */
    // 移除指定的元素，返回值指示是否移除成功
    public boolean remove(Object o) {
        // 移除指定的元素，返回值指示是否移除成功，线程安全
        return removeElement(o);
    }
    
    /**
     * Removes the first (lowest-indexed) occurrence of the argument
     * from this vector. If the object is found in this vector, each
     * component in the vector with an index greater or equal to the
     * object's index is shifted downward to have an index one smaller
     * than the value it had previously.
     *
     * <p>This method is identical in functionality to the
     * {@link #remove(Object)} method (which is part of the
     * {@link List} interface).
     *
     * @param obj the component to be removed
     *
     * @return {@code true} if the argument was a component of this
     * vector; {@code false} otherwise.
     */
    // 移除指定的元素，返回值指示是否移除成功，线程安全
    public synchronized boolean removeElement(Object obj) {
        modCount++;
        int i = indexOf(obj);
        if(i >= 0) {
            removeElementAt(i);
            return true;
        }
        return false;
    }
    
    /**
     * @throws NullPointerException {@inheritDoc}
     */
    // 移除满足条件的元素，移除条件由filter决定，返回值指示是否移除成功
    @Override
    public boolean removeIf(Predicate<? super E> filter) {
        Objects.requireNonNull(filter);
        // 批量移除满足过滤条件的元素，线程安全
        return bulkRemove(filter);
    }
    
    
    /**
     * Removes from this Vector all of its elements that are contained in the
     * specified Collection.
     *
     * @param c a collection of elements to be removed from the Vector
     *
     * @return true if this Vector changed as a result of the call
     *
     * @throws ClassCastException   if the types of one or more elements
     *                              in this vector are incompatible with the specified
     *                              collection
     *                              (<a href="Collection.html#optional-restrictions">optional</a>)
     * @throws NullPointerException if this vector contains one or more null
     *                              elements and the specified collection does not support null
     *                              elements
     *                              (<a href="Collection.html#optional-restrictions">optional</a>),
     *                              or if the specified collection is null
     * @since 1.2
     */
    // (匹配则移除)移除当前向量表中所有与给定容器中的元素匹配的元素
    public boolean removeAll(Collection<?> c) {
        Objects.requireNonNull(c);
        // 批量移除满足过滤条件的元素，线程安全
        return bulkRemove(e -> c.contains(e));
    }
    
    /**
     * Retains only the elements in this Vector that are contained in the
     * specified Collection.  In other words, removes from this Vector all
     * of its elements that are not contained in the specified Collection.
     *
     * @param c a collection of elements to be retained in this Vector
     *          (all other elements are removed)
     *
     * @return true if this Vector changed as a result of the call
     *
     * @throws ClassCastException   if the types of one or more elements
     *                              in this vector are incompatible with the specified
     *                              collection
     *                              (<a href="Collection.html#optional-restrictions">optional</a>)
     * @throws NullPointerException if this vector contains one or more null
     *                              elements and the specified collection does not support null
     *                              elements
     *                              (<a href="Collection.html#optional-restrictions">optional</a>),
     *                              or if the specified collection is null
     * @since 1.2
     */
    // (不匹配则移除)移除当前向量表中所有与给定容器中的元素不匹配的元素
    public boolean retainAll(Collection<?> c) {
        Objects.requireNonNull(c);
        // 批量移除满足过滤条件的元素，线程安全
        return bulkRemove(e -> !c.contains(e));
    }
    
    
    /**
     * Removes from this list all of the elements whose index is between
     * {@code fromIndex}, inclusive, and {@code toIndex}, exclusive.
     * Shifts any succeeding elements to the left (reduces their index).
     * This call shortens the list by {@code (toIndex - fromIndex)} elements.
     * (If {@code toIndex==fromIndex}, this operation has no effect.)
     */
    // 移除当前向量表[fromIndex,toIndex]之间的元素
    protected synchronized void removeRange(int fromIndex, int toIndex) {
        modCount++;
        shiftTailOverGap(elementData, fromIndex, toIndex);
    }
    
    
    /**
     * Removes all components from this vector and sets its size to zero.
     *
     * <p>This method is identical in functionality to the {@link #clear}
     * method (which is part of the {@link List} interface).
     */
    // 移除当前向量表中的元素
    public synchronized void removeAllElements() {
        final Object[] es = elementData;
        for(int to = elementCount, i = elementCount = 0; i<to; i++) {
            es[i] = null;
        }
        modCount++;
    }
    
    
    /**
     * Removes all of the elements from this Vector.  The Vector will
     * be empty after this call returns (unless it throws an exception).
     *
     * @since 1.2
     */
    // 清空当前向量表中的元素
    public void clear() {
        removeAllElements();
    }
    
    /*▲ 移除 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 替换 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Replaces the element at the specified position in this Vector with the
     * specified element.
     *
     * @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 ArrayIndexOutOfBoundsException if the index is out of range
     *                                        ({@code index < 0 || index >= size()})
     * @since 1.2
     */
    // 将index处的元素更新为element，并返回旧元素
    public synchronized E set(int index, E element) {
        if(index >= elementCount) {
            throw new ArrayIndexOutOfBoundsException(index);
        }
        
        E oldValue = elementData(index);
        elementData[index] = element;
        return oldValue;
    }
    
    /**
     * Sets the component at the specified {@code index} of this
     * vector to be the specified object. The previous component at that
     * position is discarded.
     *
     * <p>The index must be a value greater than or equal to {@code 0}
     * and less than the current size of the vector.
     *
     * <p>This method is identical in functionality to the
     * {@link #set(int, Object) set(int, E)}
     * method (which is part of the {@link List} interface). Note that the
     * {@code set} method reverses the order of the parameters, to more closely
     * match array usage.  Note also that the {@code set} method returns the
     * old value that was stored at the specified position.
     *
     * @param obj   what the component is to be set to
     * @param index the specified index
     *
     * @throws ArrayIndexOutOfBoundsException if the index is out of range
     *                                        ({@code index < 0 || index >= size()})
     */
    // 将index处的元素更新为element
    public synchronized void setElementAt(E element, int index) {
        if(index >= elementCount) {
            throw new ArrayIndexOutOfBoundsException(index + " >= " + elementCount);
        }
        
        elementData[index] = element;
    }
    
    
    /**
     * @throws NullPointerException {@inheritDoc}
     */
    // 更新当前向量表中所有元素，更新策略由operator决定
    @Override
    public synchronized void replaceAll(UnaryOperator<E> operator) {
        Objects.requireNonNull(operator);
        final int expectedModCount = modCount;
        final Object[] es = elementData;
        final int size = elementCount;
        for(int i = 0; modCount == expectedModCount && i<size; i++) {
            es[i] = operator.apply(elementAt(es, i));
        }
        if(modCount != expectedModCount) {
            throw new ConcurrentModificationException();
        }
        modCount++;
    }
    
    /*▲ 替换 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 包含查询 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Returns {@code true} if this vector contains the specified element.
     * More formally, returns {@code true} if and only if this vector
     * contains at least one element {@code e} such that
     * {@code Objects.equals(o, e)}.
     *
     * @param o element whose presence in this vector is to be tested
     *
     * @return {@code true} if this vector contains the specified element
     */
    // 判断当前向量表中是否包含指定的元素
    public boolean contains(Object o) {
        return indexOf(o, 0) >= 0;
    }
    
    /**
     * Returns true if this Vector contains all of the elements in the
     * specified Collection.
     *
     * @param c a collection whose elements will be tested for containment
     *          in this Vector
     *
     * @return true if this Vector contains all of the elements in the
     * specified collection
     *
     * @throws NullPointerException if the specified collection is null
     */
    // 判读指定容器中的元素是否都包含在当前向量表中
    public synchronized boolean containsAll(Collection<?> c) {
        return super.containsAll(c);
    }
    
    /*▲ 包含查询 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 定位 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Returns the index of the first occurrence of the specified element
     * in this vector, or -1 if this vector 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 vector, or -1 if this vector does not contain the element
     */
    // 返回指定元素的正序索引(正序查找首个匹配的元素)
    public int indexOf(Object o) {
        return indexOf(o, 0);
    }
    
    /**
     * Returns the index of the first occurrence of the specified element in
     * this vector, searching forwards from {@code index}, or returns -1 if
     * the element is not found.
     * More formally, returns the lowest index {@code i} such that
     * {@code (i >= index && Objects.equals(o, get(i)))},
     * or -1 if there is no such index.
     *
     * @param o     element to search for
     * @param index index to start searching from
     *
     * @return the index of the first occurrence of the element in
     * this vector at position {@code index} or later in the vector;
     * {@code -1} if the element is not found.
     *
     * @throws IndexOutOfBoundsException if the specified index is negative
     * @see Object#equals(Object)
     */
    // 返回指定元素的正序索引，从index索引处开始正向查找(正序查找首个匹配的元素)
    public synchronized int indexOf(Object o, int index) {
        if(o == null) {
            for(int i = index; i<elementCount; i++) {
                if(elementData[i] == null) {
                    return i;
                }
            }
        } else {
            for(int i = index; i<elementCount; i++) {
                if(o.equals(elementData[i])) {
                    return i;
                }
            }
        }
        return -1;
    }
    
    /**
     * Returns the index of the last occurrence of the specified element
     * in this vector, or -1 if this vector 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 vector, or -1 if this vector does not contain the element
     */
    // 返回指定元素的逆序索引(逆序查找首个匹配的元素)
    public synchronized int lastIndexOf(Object o) {
        return lastIndexOf(o, elementCount - 1);
    }
    
    /**
     * Returns the index of the last occurrence of the specified element in
     * this vector, searching backwards from {@code index}, or returns -1 if
     * the element is not found.
     * More formally, returns the highest index {@code i} such that
     * {@code (i <= index && Objects.equals(o, get(i)))},
     * or -1 if there is no such index.
     *
     * @param o     element to search for
     * @param index index to start searching backwards from
     *
     * @return the index of the last occurrence of the element at position
     * less than or equal to {@code index} in this vector;
     * -1 if the element is not found.
     *
     * @throws IndexOutOfBoundsException if the specified index is greater
     *                                   than or equal to the current size of this vector
     */
    // 返回指定元素的逆序索引，从index索引处开始逆向查找(逆序查找首个匹配的元素)
    public synchronized int lastIndexOf(Object o, int index) {
        if(index >= elementCount) {
            throw new IndexOutOfBoundsException(index + " >= " + elementCount);
        }
        
        if(o == null) {
            for(int i = index; i >= 0; i--) {
                if(elementData[i] == null) {
                    return i;
                }
            }
        } else {
            for(int i = index; i >= 0; i--) {
                if(o.equals(elementData[i])) {
                    return i;
                }
            }
        }
        return -1;
    }
    
    /*▲ 定位 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 视图 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Returns an enumeration of the components of this vector. The
     * returned {@code Enumeration} object will generate all items in
     * this vector. The first item generated is the item at index {@code 0},
     * then the item at index {@code 1}, and so on. If the vector is
     * structurally modified while enumerating over the elements then the
     * results of enumerating are undefined.
     *
     * @return an enumeration of the components of this vector
     *
     * @see Iterator
     */
    // 返回向量表中所有元素的视图，支持枚举遍历
    public Enumeration<E> elements() {
        return new Enumeration<E>() {
            int count = 0;
            
            public boolean hasMoreElements() {
                return count<elementCount;
            }
            
            public E nextElement() {
                synchronized(Vector.this) {
                    if(count<elementCount) {
                        return elementData(count++);
                    }
                }
                throw new NoSuchElementException("Vector Enumeration");
            }
        };
    }
    
    
    /**
     * Returns a view of the portion of this List between fromIndex,
     * inclusive, and toIndex, exclusive.  (If fromIndex and toIndex are
     * equal, the returned List is empty.)  The returned List is backed by this
     * List, so 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 operating on a subList view
     * instead of a whole List.  For example, the following idiom
     * removes a range of elements from a List:
     * <pre>
     *      list.subList(from, to).clear();
     * </pre>
     * Similar idioms may be constructed for indexOf and lastIndexOf,
     * and all of the algorithms in the 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 the 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 if an endpoint index value is out of range
     *                                   {@code (fromIndex < 0 || toIndex > size)}
     * @throws IllegalArgumentException  if the endpoint indices are out of order
     *                                   {@code (fromIndex > toIndex)}
     */
    // 返回[fromIndex, toIndex)之间的元素的视图
    public synchronized List<E> subList(int fromIndex, int toIndex) {
        return Collections.synchronizedList(super.subList(fromIndex, toIndex), this);
    }
    
    
    /**
     * Returns an array containing all of the elements in this Vector
     * in the correct order.
     *
     * @since 1.2
     */
    public synchronized Object[] toArray() {
        return Arrays.copyOf(elementData, elementCount);
    }
    
    /**
     * Returns an array containing all of the elements in this Vector in the
     * correct order; the runtime type of the returned array is that of the
     * specified array.  If the Vector 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 Vector.
     *
     * <p>If the Vector fits in the specified array with room to spare
     * (i.e., the array has more elements than the Vector),
     * the element in the array immediately following the end of the
     * Vector is set to null.  (This is useful in determining the length
     * of the Vector <em>only</em> if the caller knows that the Vector
     * does not contain any null elements.)
     *
     * @param <T> type of array elements. The same type as {@code <E>} or a
     *            supertype of {@code <E>}.
     * @param a   the array into which the elements of the Vector are to
     *            be stored, if it is big enough; otherwise, a new array of the
     *            same runtime type is allocated for this purpose.
     *
     * @return an array containing the elements of the Vector
     *
     * @throws ArrayStoreException  if the runtime type of a, {@code <T>}, is not
     *                              a supertype of the runtime type, {@code <E>}, of every element in this
     *                              Vector
     * @throws NullPointerException if the given array is null
     * @since 1.2
     */
    @SuppressWarnings("unchecked")
    public synchronized <T> T[] toArray(T[] a) {
        if(a.length<elementCount) {
            return (T[]) Arrays.copyOf(elementData, elementCount, a.getClass());
        }
        
        System.arraycopy(elementData, 0, a, 0, elementCount);
        
        if(a.length>elementCount) {
            a[elementCount] = null;
        }
        
        return a;
    }
    
    /*▲ 视图 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 迭代 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * @throws NullPointerException {@inheritDoc}
     */
    // 遍历当前向量表中的元素，并对其应用指定的择取操作
    @Override
    public synchronized void forEach(Consumer<? super E> action) {
        Objects.requireNonNull(action);
        final int expectedModCount = modCount;
        final Object[] es = elementData;
        final int size = elementCount;
        for(int i = 0; modCount == expectedModCount && i<size; i++) {
            action.accept(elementAt(es, i));
        }
        if(modCount != expectedModCount) {
            throw new ConcurrentModificationException();
        }
    }
    
    
    /**
     * Returns an iterator over the elements in this list in proper sequence.
     *
     * <p>The returned iterator is <a href="#fail-fast"><i>fail-fast</i></a>.
     *
     * @return an iterator over the elements in this list in proper sequence
     */
    // 返回当前向量表的一个迭代器
    public synchronized Iterator<E> iterator() {
        return new Itr();
    }
    
    /**
     * 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.
     *
     * <p>The returned list iterator is <a href="#fail-fast"><i>fail-fast</i></a>.
     *
     * @throws IndexOutOfBoundsException {@inheritDoc}
     */
    // 返回当前向量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
    public synchronized ListIterator<E> listIterator(int index) {
        if(index<0 || index>elementCount) {
            throw new IndexOutOfBoundsException("Index: " + index);
        }
        
        return new ListItr(index);
    }
    
    /**
     * Returns a list iterator over the elements in this list (in proper
     * sequence).
     *
     * <p>The returned list iterator is <a href="#fail-fast"><i>fail-fast</i></a>.
     *
     * @see #listIterator(int)
     */
    // 返回当前向量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
    public synchronized ListIterator<E> listIterator() {
        return new ListItr(0);
    }
    
    
    /**
     * Creates a <em><a href="Spliterator.html#binding">late-binding</a></em>
     * and <em>fail-fast</em> {@link Spliterator} over the elements in this
     * list.
     *
     * <p>The {@code Spliterator} reports {@link Spliterator#SIZED},
     * {@link Spliterator#SUBSIZED}, and {@link Spliterator#ORDERED}.
     * Overriding implementations should document the reporting of additional
     * characteristic values.
     *
     * @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
    public Spliterator<E> spliterator() {
        return new VectorSpliterator(null, 0, -1, 0);
    }
    
    /*▲ 迭代 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 杂项 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Returns the number of components in this vector.
     *
     * @return the number of components in this vector
     */
    // 返回向量表中元素的数量
    public synchronized int size() {
        return elementCount;
    }
    
    /**
     * Tests if this vector has no components.
     *
     * @return {@code true} if and only if this vector has
     * no components, that is, its size is zero;
     * {@code false} otherwise.
     */
    // 判断当前向量表是否为空
    public synchronized boolean isEmpty() {
        return elementCount == 0;
    }
    
    // 使用指定的比较器对当前向量表内的元素进行排序
    @SuppressWarnings("unchecked")
    @Override
    public synchronized void sort(Comparator<? super E> c) {
        final int expectedModCount = modCount;
        Arrays.sort((E[]) elementData, 0, elementCount, c);
        if(modCount != expectedModCount) {
            throw new ConcurrentModificationException();
        }
        modCount++;
    }
    
    /**
     * Returns the current capacity of this vector.
     *
     * @return the current capacity (the length of its internal
     * data array, kept in the field {@code elementData}
     * of this vector)
     */
    // 返回向量表的容量
    public synchronized int capacity() {
        return elementData.length;
    }
    
    /**
     * Trims the capacity of this vector to be the vector's current
     * size. If the capacity of this vector is larger than its current
     * size, then the capacity is changed to equal the size by replacing
     * its internal data array, kept in the field {@code elementData},
     * with a smaller one. An application can use this operation to
     * minimize the storage of a vector.
     */
    // 缩减向量表的容量为刚好容纳当前所有的元素
    public synchronized void trimToSize() {
        modCount++;
        int oldCapacity = elementData.length;
        if(elementCount<oldCapacity) {
            elementData = Arrays.copyOf(elementData, elementCount);
        }
    }
    
    /**
     * Sets the size of this vector. If the new size is greater than the
     * current size, new {@code null} items are added to the end of
     * the vector. If the new size is less than the current size, all
     * components at index {@code newSize} and greater are discarded.
     *
     * @param newSize the new size of this vector
     *
     * @throws ArrayIndexOutOfBoundsException if the new size is negative
     */
    // 重新设置向量表的容量，如果新容量小于元素数量，则会移除超出新容量的元素
    public synchronized void setSize(int newSize) {
        modCount++;
        if(newSize>elementData.length) {
            grow(newSize);
        }
        final Object[] es = elementData;
        for(int to = elementCount, i = newSize; i<to; i++) {
            es[i] = null;
        }
        elementCount = newSize;
    }
    
    /**
     * Increases the capacity of this vector, if necessary, to ensure
     * that it can hold at least the number of components specified by
     * the minimum capacity argument.
     *
     * <p>If the current capacity of this vector is less than
     * {@code minCapacity}, then its capacity is increased by replacing its
     * internal data array, kept in the field {@code elementData}, with a
     * larger one.  The size of the new data array will be the old size plus
     * {@code capacityIncrement}, unless the value of
     * {@code capacityIncrement} is less than or equal to zero, in which case
     * the new capacity will be twice the old capacity; but if this new size
     * is still smaller than {@code minCapacity}, then the new capacity will
     * be {@code minCapacity}.
     *
     * @param minCapacity the desired minimum capacity
     */
    // 确保当前向量表至少拥有minCapacity的容量
    public synchronized void ensureCapacity(int minCapacity) {
        if(minCapacity>0) {
            modCount++;
            if(minCapacity>elementData.length) {
                grow(minCapacity);
            }
        }
    }
    
    /**
     * Copies the components of this vector into the specified array.
     * The item at index {@code k} in this vector is copied into
     * component {@code k} of {@code anArray}.
     *
     * @param anArray the array into which the components get copied
     *
     * @throws NullPointerException      if the given array is null
     * @throws IndexOutOfBoundsException if the specified array is not
     *                                   large enough to hold all the components of this vector
     * @throws ArrayStoreException       if a component of this vector is not of
     *                                   a runtime type that can be stored in the specified array
     * @see #toArray(Object[])
     */
    // 将当前向量表中的元素复制到指定的数组中
    public synchronized void copyInto(Object[] anArray) {
        System.arraycopy(elementData, 0, anArray, 0, elementCount);
    }
    
    /*▲ 杂项 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 序列化 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /** use serialVersionUID from JDK 1.0.2 for interoperability */
    private static final long serialVersionUID = -2767605614048989439L;
    
    
    /**
     * Loads a {@code Vector} instance from a stream
     * (that is, deserializes it).
     * This method performs checks to ensure the consistency
     * of the fields.
     *
     * @param in the stream
     *
     * @throws java.io.IOException    if an I/O error occurs
     * @throws ClassNotFoundException if the stream contains data
     *                                of a non-existing class
     */
    private void readObject(ObjectInputStream in) throws IOException, ClassNotFoundException {
        ObjectInputStream.GetField gfields = in.readFields();
        int count = gfields.get("elementCount", 0);
        Object[] data = (Object[]) gfields.get("elementData", null);
        if(count<0 || data == null || count>data.length) {
            throw new StreamCorruptedException("Inconsistent vector internals");
        }
        elementCount = count;
        elementData = data.clone();
    }
    
    /**
     * Saves the state of the {@code Vector} instance to a stream
     * (that is, serializes it).
     * This method performs synchronization to ensure the consistency
     * of the serialized data.
     *
     * @param s the stream
     *
     * @throws java.io.IOException if an I/O error occurs
     */
    private void writeObject(java.io.ObjectOutputStream s) throws java.io.IOException {
        final java.io.ObjectOutputStream.PutField fields = s.putFields();
        final Object[] data;
        synchronized(this) {
            fields.put("capacityIncrement", capacityIncrement);
            fields.put("elementCount", elementCount);
            data = elementData.clone();
        }
        fields.put("elementData", data);
        s.writeFields();
    }
    
    /*▲ 序列化 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /**
     * Compares the specified Object with this Vector for equality.  Returns
     * 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 <em>equal</em>.  (Two elements {@code e1} and
     * {@code e2} are <em>equal</em> 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.
     *
     * @param o the Object to be compared for equality with this Vector
     *
     * @return true if the specified Object is equal to this Vector
     */
    public synchronized boolean equals(Object o) {
        return super.equals(o);
    }
    
    /**
     * Returns the hash code value for this Vector.
     */
    public synchronized int hashCode() {
        return super.hashCode();
    }
    
    /**
     * Returns a clone of this vector. The copy will contain a
     * reference to a clone of the internal data array, not a reference
     * to the original internal data array of this {@code Vector} object.
     *
     * @return a clone of this vector
     */
    public synchronized Object clone() {
        try {
            @SuppressWarnings("unchecked")
            Vector<E> v = (Vector<E>) super.clone();
            v.elementData = Arrays.copyOf(elementData, elementCount);
            v.modCount = 0;
            return v;
        } catch(CloneNotSupportedException e) {
            // this shouldn't happen, since we are Cloneable
            throw new InternalError(e);
        }
    }
    
    /**
     * Returns a string representation of this Vector, containing
     * the String representation of each element.
     */
    public synchronized String toString() {
        return super.toString();
    }
    
    
    
    @SuppressWarnings("unchecked")
    static <E> E elementAt(Object[] es, int index) {
        return (E) es[index];
    }
    
    @SuppressWarnings("unchecked")
    E elementData(int index) {
        return (E) elementData[index];
    }
    
    void checkInvariants() {
        // assert elementCount >= 0;
        // assert elementCount == elementData.length || elementData[elementCount] == null;
    }
    
    private static int hugeCapacity(int minCapacity) {
        if(minCapacity<0) {
            // overflow
            throw new OutOfMemoryError();
        }
        return (minCapacity>MAX_ARRAY_SIZE) ? Integer.MAX_VALUE : MAX_ARRAY_SIZE;
    }
    
    private static long[] nBits(int n) {
        return new long[((n - 1) >> 6) + 1];
    }
    
    private static void setBit(long[] bits, int i) {
        bits[i >> 6] |= 1L << i;
    }
    
    private static boolean isClear(long[] bits, int i) {
        return (bits[i >> 6] & (1L << i)) == 0;
    }
    
    // 对当前向量表扩容
    private Object[] grow() {
        return grow(elementCount + 1);
    }
    
    /**
     * Increases the capacity to ensure that it can hold at least the
     * number of elements specified by the minimum capacity argument.
     *
     * @param minCapacity the desired minimum capacity
     *
     * @throws OutOfMemoryError if minCapacity is less than zero
     */
    // 对当前向量表扩容，minCapacity是申请的容量
    private Object[] grow(int minCapacity) {
        // 根据申请的容量，返回一个合适的新容量
        int newCapacity = newCapacity(minCapacity);
        return elementData = Arrays.copyOf(elementData, newCapacity);
    }
    
    /**
     * Returns a capacity at least as large as the given minimum capacity.
     * Will not return a capacity greater than MAX_ARRAY_SIZE unless
     * the given minimum capacity is greater than MAX_ARRAY_SIZE.
     *
     * @param minCapacity the desired minimum capacity
     *
     * @throws OutOfMemoryError if minCapacity is less than zero
     */
    // 根据申请的容量，返回一个合适的新容量
    private int newCapacity(int minCapacity) {
        // overflow-conscious code
        int oldCapacity = elementData.length;   // 旧容量
        int newCapacity = oldCapacity + ((capacityIncrement>0) ? capacityIncrement : oldCapacity);  // 预期新容量（默认增加一倍）
        
        // 如果预期新容量小于申请的容量
        if(newCapacity - minCapacity<=0) {
            if(minCapacity<0) {
                // overflow
                throw new OutOfMemoryError();
            }
            
            // 直接使用申请的容量
            return minCapacity;
        }
        
        // 在预期新容量大于申请的容量时，按新容量走
        return (newCapacity - MAX_ARRAY_SIZE<=0) ? newCapacity : hugeCapacity(minCapacity);
    }
    
    /**
     * This helper method split out from add(E) to keep method
     * bytecode size under 35 (the -XX:MaxInlineSize default value),
     * which helps when add(E) is called in a C1-compiled loop.
     */
    // 将元素e添加到elementData[s]处
    private void add(E e, Object[] elementData, int s) {
        // 如果容器已经满了，则需要扩容
        if(s == elementData.length) {
            elementData = grow();
        }
        elementData[s] = e;
        elementCount = s + 1;
    }
    
    // 批量移除满足过滤条件的元素，线程安全
    private synchronized boolean bulkRemove(Predicate<? super E> filter) {
        int expectedModCount = modCount;
        final Object[] es = elementData;
        final int end = elementCount;
        int i;
        // Optimize for initial run of survivors
        for(i = 0; i<end && !filter.test(elementAt(es, i)); i++)
            ;
        // Tolerate predicates that reentrantly access the collection for
        // read (but writers still get CME), so traverse once to find
        // elements to delete, a second pass to physically expunge.
        if(i<end) {
            final int beg = i;
            final long[] deathRow = nBits(end - beg);
            deathRow[0] = 1L;   // set bit 0
            for(i = beg + 1; i<end; i++) {
                if(filter.test(elementAt(es, i))) {
                    setBit(deathRow, i - beg);
                }
            }
            if(modCount != expectedModCount) {
                throw new ConcurrentModificationException();
            }
            modCount++;
            int w = beg;
            for(i = beg; i<end; i++) {
                if(isClear(deathRow, i - beg)) {
                    es[w++] = es[i];
                }
            }
            for(i = elementCount = w; i<end; i++) {
                es[i] = null;
            }
            return true;
        } else {
            if(modCount != expectedModCount) {
                throw new ConcurrentModificationException();
            }
            return false;
        }
    }
    
    /** Erases the gap from lo to hi, by sliding down following elements. */
    private void shiftTailOverGap(Object[] es, int lo, int hi) {
        System.arraycopy(es, hi, es, lo, elementCount - hi);
        for(int to = elementCount, i = (elementCount -= hi - lo); i<to; i++) {
            es[i] = null;
        }
    }
    
    
    
    
    
    
    /**
     * An optimized version of AbstractList.ListItr
     */
    final class ListItr extends Itr implements ListIterator<E> {
        ListItr(int index) {
            super();
            cursor = index;
        }
        
        public boolean hasPrevious() {
            return cursor != 0;
        }
        
        public int nextIndex() {
            return cursor;
        }
        
        public int previousIndex() {
            return cursor - 1;
        }
        
        public E previous() {
            synchronized(Vector.this) {
                checkForComodification();
                int i = cursor - 1;
                if(i<0) {
                    throw new NoSuchElementException();
                }
                cursor = i;
                return elementData(lastRet = i);
            }
        }
        
        public void set(E e) {
            if(lastRet == -1) {
                throw new IllegalStateException();
            }
            synchronized(Vector.this) {
                checkForComodification();
                Vector.this.set(lastRet, e);
            }
        }
        
        public void add(E e) {
            int i = cursor;
            synchronized(Vector.this) {
                checkForComodification();
                Vector.this.add(i, e);
                expectedModCount = modCount;
            }
            cursor = i + 1;
            lastRet = -1;
        }
    }
    
    /** Similar to ArrayList Spliterator */
    final class VectorSpliterator implements Spliterator<E> {
        private Object[] array;
        private int index; // current index, modified on advance/split
        private int fence; // -1 until used; then one past last index
        private int expectedModCount; // initialized when fence set
        
        /** Creates new spliterator covering the given range. */
        VectorSpliterator(Object[] array, int origin, int fence, int expectedModCount) {
            this.array = array;
            this.index = origin;
            this.fence = fence;
            this.expectedModCount = expectedModCount;
        }
        
        public Spliterator<E> trySplit() {
            int hi = getFence(), lo = index, mid = (lo + hi) >>> 1;
            return (lo >= mid) ? null : new VectorSpliterator(array, lo, index = mid, expectedModCount);
        }
        
        @SuppressWarnings("unchecked")
        public boolean tryAdvance(Consumer<? super E> action) {
            Objects.requireNonNull(action);
            int i;
            if(getFence()>(i = index)) {
                index = i + 1;
                action.accept((E) array[i]);
                if(modCount != expectedModCount) {
                    throw new ConcurrentModificationException();
                }
                return true;
            }
            return false;
        }
        
        @SuppressWarnings("unchecked")
        public void forEachRemaining(Consumer<? super E> action) {
            Objects.requireNonNull(action);
            final int hi = getFence();
            final Object[] a = array;
            int i;
            for(i = index, index = hi; i<hi; i++) {
                action.accept((E) a[i]);
            }
            if(modCount != expectedModCount) {
                throw new ConcurrentModificationException();
            }
        }
        
        public long estimateSize() {
            return getFence() - index;
        }
        
        public int characteristics() {
            return Spliterator.ORDERED | Spliterator.SIZED | Spliterator.SUBSIZED;
        }
        
        private int getFence() { // initialize on first use
            int hi;
            if((hi = fence)<0) {
                synchronized(Vector.this) {
                    array = elementData;
                    expectedModCount = modCount;
                    hi = fence = elementCount;
                }
            }
            return hi;
        }
    }
    
    /**
     * An optimized version of AbstractList.Itr
     */
    private class Itr implements Iterator<E> {
        int cursor;       // index of next element to return
        int lastRet = -1; // index of last element returned; -1 if no such
        int expectedModCount = modCount;
        
        public boolean hasNext() {
            // Racy but within spec, since modifications are checked
            // within or after synchronization in next/previous
            return cursor != elementCount;
        }
        
        public E next() {
            synchronized(Vector.this) {
                checkForComodification();
                int i = cursor;
                if(i >= elementCount) {
                    throw new NoSuchElementException();
                }
                cursor = i + 1;
                return elementData(lastRet = i);
            }
        }
        
        public void remove() {
            if(lastRet == -1) {
                throw new IllegalStateException();
            }
            synchronized(Vector.this) {
                checkForComodification();
                Vector.this.remove(lastRet);
                expectedModCount = modCount;
            }
            cursor = lastRet;
            lastRet = -1;
        }
        
        @Override
        public void forEachRemaining(Consumer<? super E> action) {
            Objects.requireNonNull(action);
            synchronized(Vector.this) {
                final int size = elementCount;
                int i = cursor;
                if(i >= size) {
                    return;
                }
                final Object[] es = elementData;
                if(i >= es.length) {
                    throw new ConcurrentModificationException();
                }
                while(i<size && modCount == expectedModCount) {
                    action.accept(elementAt(es, i++));
                }
                // update once at end of iteration to reduce heap write traffic
                cursor = i;
                lastRet = i - 1;
                checkForComodification();
            }
        }
        
        final void checkForComodification() {
            if(modCount != expectedModCount) {
                throw new ConcurrentModificationException();
            }
        }
    }
}