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前言

learn from collection framework design中提到,collection framework分为两部分,分别为CollectionMap,其中Collection又分为三类分别为ListSetQueue,本篇文章先来分析ArrayList的实现。

ArrayList继承关系

ArrayList 源码分析-LMLPHP
如上图所示,它实现了RandomAccess(可随机访问),Cloneable(可克隆),Serializable(支持序列化和反序列化)接口以及List接口,并且它还继承了List的抽象模板类AbstractList
其中,前三个接口都是marker interface,没有可以让实现类实现的方法。

下面直接来看ArrayList内部的一些实现机制。

内部实现

数据结构

其内部维护了一个Object类型的数组,即elementData成员变量,成员变量size记录list的大小。。

初始化

ArrayList的构造方法有如下三种重载,分别是:
第一种方式:根据初始容量初始化ArrayList。

/**
 * 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) { // 根据传入的初始的容量大小初始化List,其内部维护的是
		this.elementData = new Object[initialCapacity];
	} else if (initialCapacity == 0) {
		this.elementData = EMPTY_ELEMENTDATA; // 是一个长度为0的空数组,即{}
	} else { // 因数组长度不能小于0,故抛出异常
		throw new IllegalArgumentException("Illegal Capacity: "+
										   initialCapacity);
	}
}

第二种:使用默认大小,默认内部数组长度为0。

/**
 * Constructs an empty list with an initial capacity of ten.
 */
public ArrayList() {
    this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA; // DEFAULTCAPACITY_EMPTY_ELEMENTDATA默认为长度为0的空数组
}

第三种:根据传入的集合构建ArrayList

/**
 * 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(); // 注意,先构造一个新的数组,然后使用数组拷贝,将旧数据拷贝到新数组,这样效率并不高,并且还浪费内存
	if ((size = elementData.length) != 0) { // collection包含元素
		// c.toArray might (incorrectly) not return Object[] (see 6260652)
		if (elementData.getClass() != Object[].class)
			elementData = Arrays.copyOf(elementData, size, Object[].class);
	} else { // collection不包含元素,使用内部预定义的长度为0的数组。
		// replace with empty array.
		this.elementData = EMPTY_ELEMENTDATA;
	}
}

内部数组扩容机制

java.util.ArrayList#ensureCapacityInternal是专门用于扩容的私有方法,具体如下:

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

一共有两个步骤,分别为计算所需容量以及扩容两个步。

计算所需容量

calculateCapacity源码如下:

private static int calculateCapacity(Object[] elementData, int minCapacity) {
    if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
        return Math.max(DEFAULT_CAPACITY, minCapacity); // 如果刚开始是空数组,则第一次扩容,数组长度需扩容到 max(10,需要的最小容量)
    }
    return minCapacity;
}

扩容

private void ensureExplicitCapacity(int minCapacity) {
    modCount++; // 记录内部数组扩容次数

    // overflow-conscious code
    if (minCapacity - elementData.length > 0)
        grow(minCapacity);
}

这里为什么要用减法而不直接比较?
因为minCapacity这个是由原始的大小 + 需要插入的元素的个数得到的,在加法运算后可能会出现溢出,变为负数,变为负数了就不能继续扩容了。
grow具体如下:

/**
 * 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;
    int newCapacity = oldCapacity + (oldCapacity >> 1);
    if (newCapacity - minCapacity < 0)
        newCapacity = minCapacity;
    if (newCapacity - MAX_ARRAY_SIZE > 0) // 这里之所以用减法还是考虑到新的数组长度可能会溢出
        newCapacity = hugeCapacity(minCapacity);
    // minCapacity is usually close to size, so this is a win:
    elementData = Arrays.copyOf(elementData, newCapacity);
}

huge源码如下:

private static int hugeCapacity(int minCapacity) {
    if (minCapacity < 0) // overflow
        throw new OutOfMemoryError();
    return (minCapacity > MAX_ARRAY_SIZE) ?
        Integer.MAX_VALUE :
        MAX_ARRAY_SIZE;
}

扩容倍数是1.5,最大数组长度为 MAX_ARRAY_SIZE,即Integer.MAX_VALUE - 8,之所以要取这个值是因为,有的JVM在实现数组的时候,刚开始会保留一些header的信息,这些信息会占8个字节。在扩展数组时,长度一旦超过这个大小,会抛出OutOfMemoryError异常。
也就是说,如果当前数组不足以容纳新的元素,则需要1.5倍扩容,最终容量最大为Integer.MAX_VALUE - 8

缩容

/**
 * 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++;
    if (size < elementData.length) {
        elementData = (size == 0)
          ? EMPTY_ELEMENTDATA
          : Arrays.copyOf(elementData, size);
    }
}

缩容效率其实不高,因为又重新创建了一个长度等于size的数组,然后再逐一拷贝旧数组上元素到新的数组上。

单个元素插入

有两种方式,分别如下:
方式一,默认在结尾插入,如下:

/**
 * 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) {
    ensureCapacityInternal(size + 1);  // Increments modCount!!
    elementData[size++] = e;
    return true;
}

方式二,在指定位置插入元素,如下:

/**
 * 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) {
  rangeCheckForAdd(index); // 注意,检查下标的合法性,这个下标是跟ArrayList的长度比较的,不是跟内部数据的capacity比较的!

  ensureCapacityInternal(size + 1);  // Increments modCount!!
  // 把指定下标后(包括该下标)的数据整体后移一位
  System.arraycopy(elementData, index, elementData, index + 1,
                   size - index);
  elementData[index] = element;
  size++;
}

多个元素插入

也有两种方式。
方式一,在结尾插入,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 <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;
    ensureCapacityInternal(size + numNew);  // Increments modCount
    System.arraycopy(a, 0, elementData, size, numNew);
    size += numNew;
    return numNew != 0;
}

方式二,在指定位置插入,如下:

/**
 * 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) {
    rangeCheckForAdd(index);

    Object[] a = c.toArray();
    int numNew = a.length;
    ensureCapacityInternal(size + numNew);  // Increments modCount

    int numMoved = size - index; // 计算需要index后(包括index)空出的元素的个数
    if (numMoved > 0)
        System.arraycopy(elementData, index, elementData, index + numNew,
                         numMoved);

    System.arraycopy(a, 0, elementData, index, numNew);
    size += numNew;
    return numNew != 0;
}

移除单个元素

主要有两种方式,分别为:
方式一,移出指定下标对应位置的元素,如下:

/**
 * 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) {
    rangeCheck(index); // index 有效性校验,跟 内部元素个数 size 比较

    modCount++;
    E oldValue = elementData(index); // 获取指定下标下的元素

    int numMoved = size - index - 1; // 计算需要移动的元素的个数
    if (numMoved > 0) // 指定index后的所有元素统一向前一个索引距离
        System.arraycopy(elementData, index+1, elementData, index,
                         numMoved);
    elementData[--size] = null; // clear to let GC do its work 设置为null,允许gc回收不用的对象,并更新list的大小

    return oldValue;
}

方式二,移出左边第一个出现的指定元素

/**
 * 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) {
    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;
            }
    }
    return false;
}

其中,fastRemove方法如下:

/*
 * Private remove method that skips bounds checking and does not
 * return the value removed.
 */
private void fastRemove(int index) {
    modCount++; // 修改次数+1
    int numMoved = size - index - 1; // 计算需要向前移动的元素的个数
    if (numMoved > 0) // 如果需要移动,则将index后的元素统一向前移动一个元素大小位置,并把最后的元素的引用设为null,便于gc回收不再使用的对象,并更新list的大小。
        System.arraycopy(elementData, index+1, elementData, index,
                         numMoved);
    elementData[--size] = null; // clear to let GC do its work
}

移除多个元素

方式一,移除所有元素

/**
 * Removes all of the elements from this list.  The list will
 * be empty after this call returns.
 */
public void clear() {
    modCount++; // 修改次数 + 1

    // clear to let GC do its work
    for (int i = 0; i < size; i++) // 所有索引下标下的元素引用设置为null
        elementData[i] = null;

    size = 0; // 重置list的大小为0
}

方式二,移出指定范围内的元素,包括开始索引不包括结束索引

/**
 * 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.)
 *
 * @throws IndexOutOfBoundsException if {@code fromIndex} or
 *         {@code toIndex} is out of range
 *         ({@code fromIndex < 0 ||
 *          fromIndex >= size() ||
 *          toIndex > size() ||
 *          toIndex < fromIndex})
 */
protected void removeRange(int fromIndex, int toIndex) {
    modCount++; // 修改次数 + 1
    int numMoved = size - toIndex; // 计算需要移动的元素的个数
    System.arraycopy(elementData, toIndex, elementData, fromIndex,
                     numMoved);

    // clear to let GC do its work
    int newSize = size - (toIndex-fromIndex); // 计算list新的大小
    for (int i = newSize; i < size; i++) { // 从后往前依次清除指定位置上的元素
        elementData[i] = null;
    }
    size = newSize; // 更新list的大小
}

方式三,批量移出给定集合内的元素或不在给定集合内的元素

private boolean batchRemove(Collection<?> c, boolean complement) {
    final Object[] elementData = this.elementData;
    int r = 0, w = 0;
    boolean modified = false;
    try {
        for (; r < size; r++) // 从前向后遍历
            if (c.contains(elementData[r]) == complement)
                elementData[w++] = elementData[r];
    } finally {
        // Preserve behavioral compatibility with AbstractCollection,
        // even if c.contains() throws.
        if (r != size) { // 剩余的整体前移
            System.arraycopy(elementData, r,
                             elementData, w,
                             size - r);
            w += size - r;
        }
        if (w != size) { // 有元素被移除
            // clear to let GC do its work
            for (int i = w; i < size; i++) // 移除之后的设置为null
                elementData[i] = null;
            modCount += size - w; // 修改次数 + 移除的元素的个数
            size = w; // 修改list的大小
            modified = true; // 设置修改标志位为true
        }
    }
    return modified;
}

方式四,移除指定集合内的所有元素

public boolean removeAll(Collection<?> c) {
    Objects.requireNonNull(c);
    return batchRemove(c, false);
}

其内部调用的是方式三的方法,不做过多说明。
方式五,移除指定集合外的所有元素

public boolean retainAll(Collection<?> c) {
    Objects.requireNonNull(c);
    return batchRemove(c, true);
}

方式六,移除符合条件的所有数据

@Override
public boolean removeIf(Predicate<? super E> filter) {
    Objects.requireNonNull(filter);
    // figure out which elements are to be removed
    // any exception thrown from the filter predicate at this stage
    // will leave the collection unmodified
    int removeCount = 0;
    final BitSet removeSet = new BitSet(size);
    final int expectedModCount = modCount;
    final int size = this.size;
    for (int i=0; modCount == expectedModCount && i < size; i++) {
        @SuppressWarnings("unchecked")
        final E element = (E) elementData[i];
        if (filter.test(element)) {
            removeSet.set(i);
            removeCount++;
        }
    }
    if (modCount != expectedModCount) {
        throw new ConcurrentModificationException();
    }

    // shift surviving elements left over the spaces left by removed elements
    final boolean anyToRemove = removeCount > 0;
    if (anyToRemove) {
        final int newSize = size - removeCount;
        for (int i=0, j=0; (i < size) && (j < newSize); i++, j++) {
            i = removeSet.nextClearBit(i);
            elementData[j] = elementData[i];
        }
        for (int k=newSize; k < size; k++) {
            elementData[k] = null;  // Let gc do its work
        }
        this.size = newSize;
        if (modCount != expectedModCount) {
            throw new ConcurrentModificationException();
        }
        modCount++;
    }

    return anyToRemove;
}

对序列化的支持

/**
 * Save the state of the <tt>ArrayList</tt> instance to a stream (that
 * is, serialize it).
 *
 * @serialData The length of the array backing the <tt>ArrayList</tt>
 *             instance is emitted (int), followed by all of its elements
 *             (each an <tt>Object</tt>) in the proper order.
 */
private void writeObject(java.io.ObjectOutputStream s)
    throws java.io.IOException{
    // Write out element count, and any hidden stuff
    int expectedModCount = modCount;
    s.defaultWriteObject();

    // Write out size as capacity for behavioural compatibility with clone()
    s.writeInt(size);

    // Write out all elements in the proper order.
    for (int i=0; i<size; i++) {
        s.writeObject(elementData[i]);
    }

    if (modCount != expectedModCount) {
        throw new ConcurrentModificationException();
    }
}

注意,在序列化的时候,list大小不能修改,序列化的时候把list的大小size也保存下来了。

/**
 * Reconstitute the <tt>ArrayList</tt> instance from a stream (that is,
 * deserialize it).
 */
private void readObject(java.io.ObjectInputStream s)
    throws java.io.IOException, ClassNotFoundException {
    elementData = EMPTY_ELEMENTDATA;

    // Read in size, and any hidden stuff
    s.defaultReadObject();

    // Read in capacity
    s.readInt(); // ignored

    if (size > 0) {
        // be like clone(), allocate array based upon size not capacity
        int capacity = calculateCapacity(elementData, size);
        SharedSecrets.getJavaOISAccess().checkArray(s, Object[].class, capacity);
        ensureCapacityInternal(size);

        Object[] a = elementData;
        // Read in all elements in the proper order.
        for (int i=0; i<size; i++) {
            a[i] = s.readObject();
        }
    }
}

反序列化后,list的capacity和size是一样的。
测试代码如下:

package com.company;

import java.io.ByteArrayInputStream;
import java.io.ByteArrayOutputStream;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.lang.reflect.Field;
import java.util.ArrayList;
import java.util.List;

public class Main {
    public static void main(String[] args) throws Exception {
	// write your code here
        List<Integer> list = new ArrayList<>();
        for (int i = 0; i < 12; i++) {
            list.add(i);
        }
        System.out.println(list.size());
        System.out.println(list);
        ByteArrayOutputStream os = new ByteArrayOutputStream();
        ObjectOutputStream oos = new ObjectOutputStream(os);
        oos.writeObject(list);
        oos.flush();
        byte[] bytes = os.toByteArray();
        ObjectInputStream inputStream = new ObjectInputStream(new ByteArrayInputStream(bytes));
        List<Integer> o = (List<Integer>)inputStream.readObject();
        System.out.println(o.size());
        System.out.println(o);
        Field elementData1 = o.getClass().getDeclaredField("elementData");
        elementData1.setAccessible(true);
        Object[] elementData = (Object[]) elementData1.get(list);
        System.out.println(elementData.length);
        elementData = (Object[]) elementData1.get(o);
        System.out.println(elementData.length);
    }
}

克隆

/**
 * Returns a shallow copy of this <tt>ArrayList</tt> instance.  (The
 * elements themselves are not copied.)
 *
 * @return a clone of this <tt>ArrayList</tt> instance
 */
public Object clone() {
    try {
        ArrayList<?> v = (ArrayList<?>) super.clone(); // Object.clone是浅克隆,只克隆引用,内部数据需要重新拷贝一份
        v.elementData = Arrays.copyOf(elementData, size); // 重新建一个新的数组存放数据
        v.modCount = 0; // 修改内部数组长度变更次数
        return v;
    } catch (CloneNotSupportedException e) {
        // this shouldn't happen, since we are Cloneable
        throw new InternalError(e);
    }
}

测试代码

ArrayList<Integer> list = new ArrayList<>();
list.add(0);
List<Integer> clone = (List<Integer>) list.clone();
clone.set(0,1000);
System.out.println(list.get(0)); // 0
System.out.println(clone.get(0)); // 1000

替换/变换

替换,本质上就是一个变换,只不过这个是在原数组上修改旧值,原来的数据丢失了,如果还想要原来的数据,需要提前拷贝一份。

@Override
@SuppressWarnings("unchecked")
public void replaceAll(UnaryOperator<E> operator) {
    Objects.requireNonNull(operator);
    final int expectedModCount = modCount;
    final int size = this.size;
    for (int i=0; modCount == expectedModCount && i < size; i++) {
        elementData[i] = operator.apply((E) elementData[i]);
    }
    if (modCount != expectedModCount) {
        throw new ConcurrentModificationException();
    }
    modCount++;
}

排序

排序,其实现了通用的排序算法(调用Array.sort方法),排序比较规则交给用户来指定。

@Override
@SuppressWarnings("unchecked")
public void sort(Comparator<? super E> c) {
    final int expectedModCount = modCount;
    Arrays.sort((E[]) elementData, 0, size, c);
    if (modCount != expectedModCount) {
        throw new ConcurrentModificationException();
    }
    modCount++;
}

遍历

  • Itr实现了可以向后遍历remove操作的迭代器,由iterator方法返回。
  • ListItr实现了可以向前遍历向后遍历元素的添加删除修改的迭代器,由listIterator方法返回。

ArrayList的优势和缺点

优势

  • 顺序存储,随机存取,数据元素与位置相关联,因此查找效率高,索引遍历快,时间复杂度O(1)
  • 尾部插入与删除的速度速度快

缺点

  • 线程不安全
  • 非尾节点的插入和删除需要移除后续的元素,效率较低
  • 虽然支持扩缩容,但是,原数据需逐一拷贝,效率较低

总结

本篇文章,相对来说比较简单,归根结底,对ArrayList的各种操作都是对底层数组的操作,深刻理解数组这种非常简单的数据结构对理解ArrayList的各个操作有很大帮助。

05-12 13:37