ArrayList实现原理

概述

ArrayList可以简单的看作是动态数组，相对于普通的数组它可以动态的增加容量或者减少容量。要注意的是ArrayList并不是线程安全的，因此一般建议在单线程中使用ArrayList。

实现原理

继承关系

public class ArrayList<E> extends AbstractList<E> implements List<E>, RandomAccess, Cloneable, java.io.Serializable

ArrayList继承AbstractList实现List, RandomAccess, Cloneable, java.io.Serializable接口

关键属性

    /**
     * The array buffer into which the elements of the ArrayList are stored.
     * The capacity of the ArrayList is the length of this array buffer. Any
     * empty ArrayList with elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA
     * will be expanded to DEFAULT_CAPACITY when the first element is added.
     */
    // 数据的数组
    transient Object[] elementData; // non-private to simplify nested class access

    /**
     * The size of the ArrayList (the number of elements it contains).
     *
     * @serial
     */
    // 实际数据的数量
    private int size;

底层使用数组保存所有元素
transient 如果用transient声明一个实例变量，当对象存储时，它的值不需要维持。换句话来说就是，用transient关键字标记的成员变量不参与序列化过程

构造方法

    /**
     * Shared empty array instance used for default sized empty instances. We
     * distinguish this from EMPTY_ELEMENTDATA to know how much to inflate when
     * first element is added.
     */
    private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};
    /**
     * Constructs an empty list with an initial capacity of ten.
     */
    public ArrayList() {
        this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
    }

默认情况下初始化空数组（长度为0的数组）

    /**
     * Shared empty array instance used for empty instances.
     */
    private static final Object[] EMPTY_ELEMENTDATA = {};
    /**
     * Constructs an empty list with the specified initial capacity.
     *
     * @param  initialCapacity  the initial capacity of the list
     * @throws IllegalArgumentException if the specified initial capacity
     *         is negative
     */
    public ArrayList(int initialCapacity) {
        if (initialCapacity > 0) {
            this.elementData = new Object[initialCapacity];
        } else if (initialCapacity == 0) {
            this.elementData = EMPTY_ELEMENTDATA;
        } else {
            throw new IllegalArgumentException("Illegal Capacity: "+
                                               initialCapacity);
        }
    }

指定数组的初始容量
当指定的初始容量大于0，初始化指定大小的数组
当指定的初始容量等于0，初始化空数组
当指定的初始容量小于0，抛出IllegalArgumentException异常

    /**
     * Constructs a list 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 list
     * @throws NullPointerException if the specified collection is null
     */
    public ArrayList(Collection<? extends E> c) {
        elementData = c.toArray();
        if ((size = elementData.length) != 0) {
            // c.toArray might (incorrectly) not return Object[] (see 6260652)
            if (elementData.getClass() != Object[].class)
                elementData = Arrays.copyOf(elementData, size, Object[].class);
        } else {
            // replace with empty array.
            this.elementData = EMPTY_ELEMENTDATA;
        }
    }

初始化指定集合的数组
当指定集合不为空即长度不为0，则复制该集合，否则初始化一个空数组

E get(int index) 获取index位置的元素

    // Positional Access Operations
    // 返回index下标的元素且强制转化为E（List<E>中的E）类型
    @SuppressWarnings("unchecked")
    E elementData(int index) {
        return (E) elementData[index];
    }

    /**
     * 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 {@inheritDoc}
     */
    public E get(int index) {
        // 检查index是否越界
        rangeCheck(index);
        // 返回index下标的元素
        return elementData(index);
    }

    /**
     * Checks if the given index is in range.  If not, throws an appropriate
     * runtime exception.  This method does *not* check if the index is
     * negative: It is always used immediately prior to an array access,
     * which throws an ArrayIndexOutOfBoundsException if index is negative.
     */
    private void rangeCheck(int index) {
        // 检查index是否大于等于size（数组的元素数量），因为数组下标从0开始计算，所以也不能等于元素数量
        // 这里没有检查index < 0的情况，因为index < 0时数组会自动抛出异常，所以并未检查index<0的情况
        if (index >= size)
            throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
    }

    /**
     * Constructs an IndexOutOfBoundsException detail message.
     * Of the many possible refactorings of the error handling code,
     * this "outlining" performs best with both server and client VMs.
     */
    private String outOfBoundsMsg(int index) {
        return "Index: "+index+", Size: "+size;
    }

首先判断index是否越界，这里并没有判断是否小于0，因为下标小于0时数组会抛出异常。越界则抛出IndexOutOfBoundsException异常，反之返回数组对应index位置的元素

E set(int index, E element) 设置（覆盖）index位置的元素

    /**
     * Replaces the element at the specified position in this list 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 IndexOutOfBoundsException {@inheritDoc}
     */
    public E set(int index, E element) {
        rangeCheck(index);

        E oldValue = elementData(index);
        elementData[index] = element;
        return oldValue;
    }

和get一样先判断index（下标）是否越界，不越界则先获取原来index位置上的元素，接着设置（覆盖）index位置上的元素，然后返回原来的元素，反之抛出IndexOutOfBoundsException异常

boolean add(E e) 添加一个元素到列表尾

    /**
     * Appends the specified element to the end of this list.
     *
     * @param e element to be appended to this list
     * @return <tt>true</tt> (as specified by {@link Collection#add})
     */
    public boolean add(E e) {
        // 检查当前容量是否还可以容纳一个元素，不够则扩容
        ensureCapacityInternal(size + 1);  // Increments modCount!!
        // 添加到数组末尾
        // 这个语句可以分解为
        // elementData[size] = e;
        // size += 1;
        elementData[size++] = e;
        return true;
    }

    /**
     * Default initial capacity.
     */
    private static final int DEFAULT_CAPACITY = 10;  // 默认容量为10

    // 如果数据等于默认数据，返回默认容量和minCapacity（所需容量最小值）的最大值，反之返回所需容量最小值
    private static int calculateCapacity(Object[] elementData, int minCapacity) {
        if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
            return Math.max(DEFAULT_CAPACITY, minCapacity);
        }
        return minCapacity;
    }

    private void ensureCapacityInternal(int minCapacity) {
        ensureExplicitCapacity(calculateCapacity(elementData, minCapacity));
    }

    private void ensureExplicitCapacity(int minCapacity) {
        modCount++;  // 操作数+1

        // overflow-conscious code
        // 如果所需容量最小值大于实际数组的长度就扩大实际数组容量
        if (minCapacity - elementData.length > 0)
            grow(minCapacity);
    }

    /**
     * The maximum size of array to allocate.
     * 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;  // 数组最大容量为Integer最大值-8

    /**
     * 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
     */
    private void grow(int minCapacity) {
        // overflow-conscious code
        int oldCapacity = elementData.length;
        // 新的容量为旧的容量的1.5倍
        int newCapacity = oldCapacity + (oldCapacity >> 1);
        // 如果扩充容量后还是不够，则新的容量等于所需容量最小值（一般就是数组实际元素个数）
        if (newCapacity - minCapacity < 0)
            newCapacity = minCapacity;
        // 如果新的容量大于数组最大容量，再调用hugeCapacity计算新的容量
        if (newCapacity - MAX_ARRAY_SIZE > 0)
            newCapacity = hugeCapacity(minCapacity);
        // minCapacity is usually close to size, so this is a win:
        // 复制原来的数据到新的数组，数组容量为新的容量
        elementData = Arrays.copyOf(elementData, newCapacity);
    }

    private static int hugeCapacity(int minCapacity) {
        if (minCapacity < 0) // overflow
            throw new OutOfMemoryError();
        // 大于数组最大容量返回Integer最大值，反之返回数组最大容量
        return (minCapacity > MAX_ARRAY_SIZE) ?
                Integer.MAX_VALUE :
                MAX_ARRAY_SIZE;
    }

添加一个元素到列表尾，当列表容量不足时自动扩容（通常是扩容至原来的1.5倍），添加成功返回true

void add(int index, E element) 在index处放置元素

    /**
     * Inserts the specified element at the specified position in this
     * list. 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 IndexOutOfBoundsException {@inheritDoc}
     */
    public void add(int index, E element) {
        // 检查下标是否越界
        rangeCheckForAdd(index);
        // 检查当前容量是否还可以在容纳一个元素，不够则扩容
        ensureCapacityInternal(size + 1);  // Increments modCount!!
        // 将elementData从index开始后面的元素往后移一位
        System.arraycopy(elementData, index, elementData, index + 1,
                size - index);
        elementData[index] = element;
        size++;
    }

    /**
     * A version of rangeCheck used by add and addAll.
     */
    private void rangeCheckForAdd(int index) {
        // 当index等于size时相当于添加元素到列表尾
        if (index > size || index < 0)
            throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
    }

将elementData数组从index开始后面的元素往后移一位，接着在index处放置元素
模拟添加数据(lierabbit)到index=4过程如下

boolean addAll(Collection<? extends E> c) 添加一个集合里的所有元素到列表尾

    /**
     * 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.  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 list, and this
     * list is nonempty.)
     *
     * @param c collection containing elements to be added to this list
     * @return <tt>true</tt> if this list changed as a result of the call
     * @throws NullPointerException if the specified collection is null
     */
    public boolean addAll(Collection<? extends E> c) {
        Object[] a = c.toArray();
        int numNew = a.length;
        // 检查当前容量是否还可以在容纳a数组的元素，不够则扩容
        ensureCapacityInternal(size + numNew);  // Increments modCount
        // 将a数组里的元素添加到elementData末尾
        System.arraycopy(a, 0, elementData, size, numNew);
        size += numNew;
        // a数组不为空（长度不为0）时返回true，反之false
        return numNew != 0;
    }

将要添加的集合变为数组，然后将其复制到elementData数组末尾

boolean addAll(int index, Collection<? extends E> c) 添加一个集合里的所有元素到index位置

    /**
     * Inserts all of the elements in the specified collection into this
     * list, starting 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 list 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 collection containing elements to be added to this list
     * @return <tt>true</tt> if this list changed as a result of the call
     * @throws IndexOutOfBoundsException {@inheritDoc}
     * @throws NullPointerException if the specified collection is null
     */
    public boolean addAll(int index, Collection<? extends E> c) {
        // 检查下标是否越界
        rangeCheckForAdd(index);

        Object[] a = c.toArray();
        int numNew = a.length;
        // 检查当前容量是否还可以在容纳a数组的元素，不够则扩容
        ensureCapacityInternal(size + numNew);  // Increments modCount

        // 需要往后移动几个位置
        int numMoved = size - index;
        if (numMoved > 0)
            // 从index开始，往后的元素向后移动numMoved个位置
            System.arraycopy(elementData, index, elementData, index + numNew,
                    numMoved);
        // 将a数组里的所有元素复制到elementData从index到index + numNew -1的位置上
        System.arraycopy(a, 0, elementData, index, numNew);
        size += numNew;
        // a数组不为空（长度不为0）时返回true，反之false
        return numNew != 0;
    }

将要添加的集合变为数组，然后把elementData数组从index开始，往后的元素向后移动numMoved个位置，接着将要添加的数组里的所有元素复制到elementData从index到index + numNew -1的位置上

void trimToSize() 改变列表内部数组容量至列表实际元素数量

    /**
     * Trims the capacity of this <tt>ArrayList</tt> instance to be the
     * list's current size.  An application can use this operation to minimize
     * the storage of an <tt>ArrayList</tt> instance.
     */
    public void trimToSize() {
        modCount++;  // 操作数+1
        // 如果数组实际元素数量小于数组长度
        if (size < elementData.length) {
            // 如果数组实际元素数量等于0则数组被赋值为空数组，反之创建一个新的元素数量等于数组长度的数组
            elementData = (size == 0)
                    ? EMPTY_ELEMENTDATA
                    : Arrays.copyOf(elementData, size);
        }
    }

当数据稳定了之后可以使用这个方法来减少内存的使用

int indexOf(Object o) 查找o元素在列表第一次出现的位置

    /**
     * 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 <tt>i</tt> such that
     * <tt>(o==null&nbsp;?&nbsp;get(i)==null&nbsp;:&nbsp;o.equals(get(i)))</tt>,
     * or -1 if there is no such index.
     */
    public int indexOf(Object o) {
        //元素可以为null，如果为null返回null的下标
        if (o == null) {
            for (int i = 0; i < size; i++)
                if (elementData[i]==null)
                    return i;
        } else {
            for (int i = 0; i < size; i++)
                if (o.equals(elementData[i]))
                    return i;
        }
        // 没有找到对应的元素返回-1
        return -1;
    }

ArrayList中可以存放null元素，indexof是返回elementData数组中值相同的首个元素的下标，indexof中比较方法是equals而equals是比较元素的值，因此必须对null单独查找。如果未找到该元素则返回-1

E remove(int index) 删除index位置上的元素

    /**
     * Removes the element at the specified position in this list.
     * Shifts any subsequent elements to the left (subtracts one from their
     * indices).
     *
     * @param index the index of the element to be removed
     * @return the element that was removed from the list
     * @throws IndexOutOfBoundsException {@inheritDoc}
     */
    public E remove(int index) {
        // 检查下标是否越界
        rangeCheck(index);

        modCount++;  // 操作数+1
        E oldValue = elementData(index);  // 获取index位置上的元素

        int numMoved = size - index - 1;  // 需要往前移动几个位置
        if (numMoved > 0)
            // 从index + 1开始，往后的元素向前移动1个位置
            System.arraycopy(elementData, index+1, elementData, index,
                    numMoved);
        // 将数组末尾元素置空
        elementData[--size] = null; // clear to let GC do its work

        return oldValue;
    }

模拟删除index=4（值为lierabbit）过程如下

boolean remove(Object o) 删除o元素

    /**
     * Removes the first occurrence of the specified element from this list,
     * if it is present.  If the list does not contain the element, it is
     * unchanged.  More formally, removes the element with the lowest index
     * <tt>i</tt> such that
     * <tt>(o==null&nbsp;?&nbsp;get(i)==null&nbsp;:&nbsp;o.equals(get(i)))</tt>
     * (if such an element exists).  Returns <tt>true</tt> 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 <tt>true</tt> if this list contained the specified element
     */
    public boolean remove(Object o) {
        // 元素可以为null，分开搜索o
        if (o == null) {
            for (int index = 0; index < size; index++)
                if (elementData[index] == null) {
                    fastRemove(index);
                    return true;
                }
        } else {
            for (int index = 0; index < size; index++)
                if (o.equals(elementData[index])) {
                    fastRemove(index);
                    return true;
                }
        }
        // 没有找到返回false
        return false;
    }

    /*
     * Private remove method that skips bounds checking and does not
     * return the value removed.
     */
    // 由于已经找到元素，则元素必定存在，则index必定合理，所以不需要在检查index是否越界
    private void fastRemove(int index) {
        modCount++;
        int numMoved = size - index - 1;
        if (numMoved > 0)
            System.arraycopy(elementData, index+1, elementData, index,
                    numMoved);
        elementData[--size] = null; // clear to let GC do its work
    }

通过寻找o元素，可以获得其下标，再根据下标删除o元素

forEach(Consumer<? super E> action)遍历列表

    /**
     * The number of times this list has been <i>structurally modified</i>.
     * 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.
     *
     * <p>This field is used by the iterator and list iterator implementation
     * returned by the {@code iterator} and {@code listIterator} methods.
     * If the value of this field changes unexpectedly, the iterator (or list
     * iterator) will throw a {@code ConcurrentModificationException} in
     * response to the {@code next}, {@code remove}, {@code previous},
     * {@code set} or {@code add} operations.  This provides
     * <i>fail-fast</i> behavior, rather than non-deterministic behavior in
     * the face of concurrent modification during iteration.
     *
     * <p><b>Use of this field by subclasses is optional.</b> If a subclass
     * wishes to provide fail-fast iterators (and list iterators), then it
     * merely has to increment this field in its {@code add(int, E)} and
     * {@code remove(int)} methods (and any other methods that it overrides
     * that result in structural modifications to the list).  A single call to
     * {@code add(int, E)} or {@code remove(int)} must add no more than
     * one to this field, or the iterators (and list iterators) will throw
     * bogus {@code ConcurrentModificationExceptions}.  If an implementation
     * does not wish to provide fail-fast iterators, this field may be
     * ignored.
     */
    protected transient int modCount = 0;//操作数

    @Override
    public void forEach(Consumer<? super E> action) {
        // 确保不为空
        Objects.requireNonNull(action);
        final int expectedModCount = modCount;
        @SuppressWarnings("unchecked")
        final E[] elementData = (E[]) this.elementData;
        final int size = this.size;
        for (int i=0; modCount == expectedModCount && i < size; i++) {
            action.accept(elementData[i]);
        }
        if (modCount != expectedModCount) {
            throw new ConcurrentModificationException();
        }
    }

    /**
     * Checks that the specified object reference is not {@code null}. This
     * method is designed primarily for doing parameter validation in methods
     * and constructors, as demonstrated below:
     * <blockquote><pre>
     * public Foo(Bar bar) {
     *     this.bar = Objects.requireNonNull(bar);
     * }
     * </pre></blockquote>
     *
     * @param obj the object reference to check for nullity
     * @param <T> the type of the reference
     * @return {@code obj} if not {@code null}
     * @throws NullPointerException if {@code obj} is {@code null}
     */
    public static <T> T requireNonNull(T obj) {
        if (obj == null)
            throw new NullPointerException();
        return obj;
    }

这里可以看到modCount的用处，当modCount发生改变后，立刻抛出ConcurrentModificationException异常。通过之前的分析可以知道当列表内容被修改时modCount会增加。也就是说如果在遍历ArrayList的过程中有其他线程修改了ArrayList，那么将抛出ConcurrentModificationException异常

ArrayList小结

• ArrayList是List接口的一个可变大小的数组的实现
• ArrayList的内部是使用一个Object对象数组来存储元素的
• 初始化ArrayList的时候，可以指定初始化容量的大小，如果不指定，就会使用默认大小，为10
• 当添加一个新元素的时候，首先会检查容量是否足够添加这个元素，如果够就直接添加，如果不够就进行扩容，扩容为原数组容量的1.5倍
• 当在index处放置一个元素的时候，会将数组index处右边的元素全部右移
• 当在index处删除一个元素的时候，会将数组index处右边的元素全部左移

本文首发：https://lierabbit.cn/2018/01/...