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JDK源码那些事儿之常用的ArrayList

前面已经讲解集合中的HashMap并且也对其中使用的红黑树结构做了对应的说明，这次就来看下简单一些的另一个集合类，也是日常经常使用到的ArrayList，整体来说，算是比较好理解的集合了，一起来看下

前言

jdk版本：1.8

类定义

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

AbstractList

JDK源码那些事儿之常用的ArrayList

变量说明

private static final long serialVersionUID = 8683452581122892189L;

    /**
     * 默认的初始化容量
     * 这里和HashMap初始容量不同，默认10
     * 有些面试官可能问，虽然我感觉没必要记这玩意
     */
    private static final int DEFAULT_CAPACITY = 10;

    /**
     * 空集合，在构造函数中看说明
     */
    private static final Object[] EMPTY_ELEMENTDATA = {};

    /**
     * 默认容量大小的空集合，这里和上边一样，但是第一次添加的时候会自动扩容到默认容量，看构造函数的说明
     */
    private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};

    /**
     * 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.
     * 
     * 基于数组实现容量大小变化，上边注释也说了第一次添加元素时，将容量扩展到DEFAULT_CAPACITY
     * 更详细的接着往下看
     */
    transient Object[] elementData; // non-private to simplify nested class access

    /**
     * 数组长度，即arraylist的长度
     */
    private int size;
    
    /**
     * 最大数组长度限制
     */
    private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;

从上边变量定义也能看出来ArrayList本质上是基于Object[]实现，故方法上的操作都是基于数组来进行

构造方法

从构造方法中能看出：

如果不设置初始化容量或者初始化赋值集合则elementData赋值为空数组而不是默认容量为10的数组

/**
     * 无参构造方法，初始化为默认空数组
     */
    public ArrayList() {
        this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
    }
    public ArrayList(Collection<? extends E> c) {
        elementData = c.toArray();
        // 原集合不为空，则进行复制
        if ((size = elementData.length) != 0) {
            // c.toArray might (incorrectly) not return Object[] (see 6260652)
            /**
             * 官方bug
             * c.toArray() 返回类型取决于其实际类型
             * 查了下，应该是调用子类的toArray（重写）方法返回具体的类型
             * 自己多想下也明白了，父类保存了子类的数组对象，这里需要调整成Object[]
             * 不明白的自己Google下
             */ 
            if (elementData.getClass() != Object[].class)
                elementData = Arrays.copyOf(elementData, size, Object[].class);
        } else {
            // 原集合为空，elementData赋值为空数组
            this.elementData = EMPTY_ELEMENTDATA;
        }
    }
    /**
     * 初始化容量 代码比较简单
     */
    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);
        }
    }

重要方法

add

每次增加元素时会通过ensureCapacityInternal进行容量大小的验证，不满足则进行扩容操作，通过grow方法进行扩容操作，在允许的范围上扩容为原来的1.5倍

/**
     * 增加元素
     */
    public boolean add(E e) {
        ensureCapacityInternal(size + 1);  // Increments modCount!!
        elementData[size++] = e;
        return true;
    }
    /**
     * 确认容量
     */
    private void ensureCapacityInternal(int minCapacity) {
        ensureExplicitCapacity(calculateCapacity(elementData, minCapacity));
    }
    /**
     * 计算容量
     * elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA
     * 在这里进行了初始化判断
     * 最小容量为10
     */
    private static int calculateCapacity(Object[] elementData, int minCapacity) {
        if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
            return Math.max(DEFAULT_CAPACITY, minCapacity);
        }
        return minCapacity;
    }
    /**
     * 修改次数记录modCount，容量是否扩容判断
     */
    private void ensureExplicitCapacity(int minCapacity) {
        modCount++;

        // overflow-conscious code
        if (minCapacity - elementData.length > 0)
            grow(minCapacity);
    }
    /**
     * 扩容
     */
    private void grow(int minCapacity) {
        // overflow-conscious code
        int oldCapacity = elementData.length;
        // 右移操作扩容为原来的1.5倍（位移操作，自己试下就明白）
        int newCapacity = oldCapacity + (oldCapacity >> 1);
        // 比较最小值
        if (newCapacity - minCapacity < 0)
            newCapacity = minCapacity;
        // 比较最大值
        if (newCapacity - MAX_ARRAY_SIZE > 0)
            newCapacity = hugeCapacity(minCapacity);
        // minCapacity is usually close to size, so this is a win:
        elementData = Arrays.copyOf(elementData, newCapacity);
    }
    /**
     * 大容量值处理
     */
    private static int hugeCapacity(int minCapacity) {
        // 溢出抛出异常
        if (minCapacity < 0) // overflow
            throw new OutOfMemoryError();
        // 计算超出时取值判断
        return (minCapacity > MAX_ARRAY_SIZE) ?
            Integer.MAX_VALUE :
            MAX_ARRAY_SIZE;
    }
    /**
     * 将element插入index的位置
     */    
    public void add(int index, E element) {
        rangeCheckForAdd(index);
        ensureCapacityInternal(size + 1);  // Increments modCount!!
        // native方法实现拷贝
        System.arraycopy(elementData, index, elementData, index + 1,
                         size - index);
        elementData[index] = element;
        size++;
    }

addAll

/**
     * 先对集合容量进行检查，记录修改次数，调用arraycopy将旧数组元素拷贝到新数组元素中
     */ 
    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;
    }
    /**
     * 和上边不同之处在于将数组拷贝到新数组index位置，其后元素依次排序
     */    
    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;
        if (numMoved > 0)
            System.arraycopy(elementData, index, elementData, index + numNew,
                             numMoved);

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

clear

/**
     * 清空
     */ 
    public void clear() {
        modCount++;

        // clear to let GC do its work
        // 注释上也写明了原因，置空为了让GC工作，回收空间
        for (int i = 0; i < size; i++)
            elementData[i] = null;

        size = 0;
    }

contains

/**
     * 判断某个元素是否在集合中
     */ 
    public boolean contains(Object o) {
        return indexOf(o) >= 0;
    }
    /**
     * 返回元素在集合中的首个索引（从小到大）
     * 主要是判空区分
     */     
    public int indexOf(Object o) {
        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;
        }
        return -1;
    }

get

/**
     * 获取索引为index的元素，先检查索引值,再调用elementData方法
     */
    public E get(int index) {
        rangeCheck(index);

        return elementData(index);
    }

iterator

/**
     * 返回迭代器 内部类实现
     */
    public Iterator<E> iterator() {
        return new 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;

        Itr() {}

        public boolean hasNext() {
            return cursor != size;
        }
        /**
         * 获取索引为cursor的元素，并置cursor = cursor + 1，方便下次调用，lastRet记录当前返回的元素索引
         */
        @SuppressWarnings("unchecked")
        public E next() {
            checkForComodification();
            int i = cursor;
            if (i >= size)
                throw new NoSuchElementException();
            Object[] elementData = ArrayList.this.elementData;
            if (i >= elementData.length)
                throw new ConcurrentModificationException();
            cursor = i + 1;
            return (E) elementData[lastRet = i];
        }
        /**
         * 移除当前lastRet对应元素，cursor置为lastRet，修改次数修改
         */
        public void remove() {
            if (lastRet < 0)
                throw new IllegalStateException();
            checkForComodification();

            try {
                ArrayList.this.remove(lastRet);
                cursor = lastRet;
                lastRet = -1;
                expectedModCount = modCount;
            } catch (IndexOutOfBoundsException ex) {
                throw new ConcurrentModificationException();
            }
        }
        /**
         * jdk 1.8新增接口，调用accept接口对每个元素执行动作
         */
        @Override
        @SuppressWarnings("unchecked")
        public void forEachRemaining(Consumer<? super E> consumer) {
            Objects.requireNonNull(consumer);
            final int size = ArrayList.this.size;
            int i = cursor;
            if (i >= size) {
                return;
            }
            final Object[] elementData = ArrayList.this.elementData;
            if (i >= elementData.length) {
                throw new ConcurrentModificationException();
            }
            while (i != size && modCount == expectedModCount) {
                consumer.accept((E) elementData[i++]);
            }
            // update once at end of iteration to reduce heap write traffic
            cursor = i;
            lastRet = i - 1;
            checkForComodification();
        }
        /**
         * 检查
         */
        final void checkForComodification() {
            if (modCount != expectedModCount)
                throw new ConcurrentModificationException();
        }
    }

lastIndexOf

/**
     * 返回匹配对象的首个索引（从大到小）
     */
    public int lastIndexOf(Object o) {
        if (o == null) {
            for (int i = size-1; i >= 0; i--)
                if (elementData[i]==null)
                    return i;
        } else {
            for (int i = size-1; i >= 0; i--)
                if (o.equals(elementData[i]))
                    return i;
        }
        return -1;
    }

remove

/**
     * 删除索引为index的元素
     */
    public E remove(int index) {
        rangeCheck(index);
        //修改记录+1
        modCount++;
        E oldValue = elementData(index);

        int numMoved = size - index - 1;
        if (numMoved > 0)
            //使用arraycopy重新整理集合
            System.arraycopy(elementData, index+1, elementData, index,
                             numMoved);
        elementData[--size] = null; // clear to let GC do its work

        return oldValue;
    }
    /**
     * 根据给定的元素删除，这里看源码也能发现，只删除第一个匹配成功的元素即返回
     */
    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;
    }

removeAll

/**
     * 移除所有和参数集合相同的元素
     */
    public boolean removeAll(Collection<?> c) {
        Objects.requireNonNull(c);
        return batchRemove(c, false);
    }
    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++)
                //将保留的数据写回elementData
                if (c.contains(elementData[r]) == complement)
                    elementData[w++] = elementData[r];
        } finally {
            // Preserve behavioral compatibility with AbstractCollection,
            // even if c.contains() throws.
            if (r != size) {
                System.arraycopy(elementData, r,
                                 elementData, w,
                                 size - r);
                w += size - r;
            }
            if (w != size) {
                // clear to let GC do its work
                for (int i = w; i < size; i++)
                    //清理为空的数据
                    elementData[i] = null;
                modCount += size - w;
                size = w;
                modified = true;
            }
        }
        return modified;
    }

set

/**
     * 设置索引为index的值为element
     */
    public E set(int index, E element) {
        rangeCheck(index);

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

toArray

/**
     * 将list元素拷贝返回
     */
    public Object[] toArray() {
        return Arrays.copyOf(elementData, size);
    }
    @SuppressWarnings("unchecked")
    public <T> T[] toArray(T[] a) {
        if (a.length < size)
            // Make a new array of a's runtime type, but my contents:
            return (T[]) Arrays.copyOf(elementData, size, a.getClass());
        System.arraycopy(elementData, 0, a, 0, size);
        if (a.length > size)
            a[size] = null;
        return a;
    }

subList

/**
     * 获取子数组，内部类实现，子数组只是引用了原来的数组，因此改变子数组，相当于改变了原来的数组
     * 子数组不再详细说明，ArrayList类相似，只是多了几个成员变量，来限制范围
     * 源码部分自行查看
     */
    public List<E> subList(int fromIndex, int toIndex) {
        subListRangeCheck(fromIndex, toIndex, size);
        return new SubList(this, 0, fromIndex, toIndex);
    }