ArrayList实现原理

摘要：要注意的是并不是线程安全的，因此一般建议在单线程中使用。实现原理继承关系继承实现接口关键属性数据的数组实际数据的数量底层使用数组保存所有元素如果用声明一个实例变量，当对象存储时，它的值不需要维持。

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。

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

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

    /**
     * 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

    /**
     * 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 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

    // Positional Access Operations
    // 返回index下标的元素且强制转化为E（List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

    /**
     * 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

    /**
     * Appends the specified element to the end of this list.
     *
     * @param e element to be appended to this list
     * @return true (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

    /**
     * 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

    /**
     * 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 true if this list changed as a result of the call
     * @throws NullPointerException if the specified collection is null
     */
    public boolean addAll(Collection 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

    /**
     * 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 true 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 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

    /**
     * Trims the capacity of this ArrayList instance to be the
     * list"s current size.  An application can use this operation to minimize
     * the storage of an ArrayList 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

    /**
     * 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 i such that
     * (o==null ? get(i)==null : o.equals(get(i))),
     * 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多带带查找。如果未找到该元素则返回-1

    /**
     * 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

    /**
     * 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
     * i such that
     * (o==null ? get(i)==null : o.equals(get(i)))
     * (if such an element exists).  Returns 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 true 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

    /**
     * The number of times this list has been structurally modified.
     * 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.
     *
     * 
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
     * fail-fast behavior, rather than non-deterministic behavior in
     * the face of concurrent modification during iteration.
     *
     * 
Use of this field by subclasses is optional. 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 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:
     * 
     * public Foo(Bar bar) {
     *     this.bar = Objects.requireNonNull(bar);
     * }
     * 
     *
     * @param obj the object reference to check for nullity
     * @param  the type of the reference
     * @return {@code obj} if not {@code null}
     * @throws NullPointerException if {@code obj} is {@code null}
     */
    public static  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/...