【Java集合类】ArrayList源码分析(jdk1.8)

ArrayList就是一个以动态数组形式实现的集合类。所以优势是随机访问元素,但是在list中间插入和移动元素时较慢。特别是插入效率。

由于要维护数组的size,即动态数组的实现就是扩容时将原数组的元素复制到更大数组中。

所以在此之前补充下复制数组元素函数,其中Arrays.copyOf()System.arraycopy()的原理介绍:

目录

  • 数据结构
    • 初始化
    • 扩容
  • 查找操作
  • 插入操作
  • 删除操作
  • 转化成数组
  • 迭代器实现

数据结构

底层基于数组实现容量大小动态变化。并且元素允许 null 的存在。

其源码如下:

/**
     * 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.
     */
    transient Object[] elementData; // non-private to simplify nested class access

    /**
     * The size of the ArrayList (the number of elements it contains).
     *
     * @serial
     */
    private int size;

初始化

有三种初始化方式(构造函数):

  • 无参构造函数:
public ArrayList() {
       this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
   }
  • 有参构造函数:指定初始容量
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);
        }
    }
  • 有参构造函数:指定 collection 列表
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;
        }
    }

我们发现当初始化后集合为空,数组则指向如下的空数组常量:

//①如果使用不带参数的构造函数,则就用默认容量10来进行开辟空间,并且elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA
private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};

//②如果使用带参数的构造函数,但是初始容量为0,则elementData = EMPTY_ELEMENTDATA
private static final Object[] EMPTY_ELEMENTDATA = {};

两者区别在于如何扩张数组(虽然都指向空数组,但是前者的容量为10,后者的容量为0),具体分析如下。

扩容

简单来说,就是将数组扩容到渴望的数组大小minCapacity,具体流程如下分析。

——>扩容流程:

  1. 首先从ensureCapacityInternal(int minCapacity)开始:
/**
     * @param   minCapacity   the desired minimum capacity
     */
private void ensureCapacityInternal(int minCapacity) {
        ensureExplicitCapacity(calculateCapacity(elementData, minCapacity));
    }
  1. 调用calculateCapacity(elementData, minCapacity):若数组是DEFAULTCAPACITY_EMPTY_ELEMENTDATA状态(无参初始化状态),则多比较下max(DEFAULT_CAPACITY, minCapacity),即从10开始扩容
private static int calculateCapacity(Object[] elementData, int minCapacity) {
        if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
            return Math.max(DEFAULT_CAPACITY, minCapacity);
        }
        return minCapacity;
    }
  1. 接着,ensureExplicitCapacity(int minCapacity):检测扩容容量minCapacity要大于elementData.length
private void ensureExplicitCapacity(int minCapacity) {
        modCount++;

        // overflow-conscious code
        if (minCapacity - elementData.length > 0)
            grow(minCapacity);
    }
  1. grow(int minCapacity):开始扩容(最关键的部分):
  • (1)每次从1.5*oldCapacity扩容;
  • (2)若minCapacity大于1.5*oldCapacity, 则选minCapacity
  • (3)若扩容超过MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8(为什么选MAX_ARRAY_SIZE?),则调用hugeCapacity(int minCapacity)选择,最大限制的容量minCapacity > MAX_ARRAY_SIZE) ?Integer.MAX_VALUE : MAX_ARRAY_SIZE;
  • (4)最后elementData = Arrays.copyOf(elementData, newCapacity);复制数组到扩容后的新数组
/**
     * 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)
            newCapacity = hugeCapacity(minCapacity);
        // minCapacity is usually close to size, so this is a win:
        elementData = Arrays.copyOf(elementData, newCapacity);
    }

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

查找操作

1、通过索引查找元素(随机访问):

public E get(int index) {
        rangeCheck(index);
        return elementData(index);
    }

2、直接查找元素:

public boolean contains(Object o) {
        return indexOf(o) >= 0;
    }

    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;
    }

    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;
    }

插入操作

1、插入单个元素:

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

    public void add(int index, E element) {
        rangeCheckForAdd(index);

        ensureCapacityInternal(size + 1);  // Increments modCount!!
        //后移
        System.arraycopy(elementData, index, elementData, index + 1,
                         size - index);
        elementData[index] = element;
        size++;
    }

2、插入集合:

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;
    }


    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;
    }

删除操作

1、通过索引删除元素:

public E remove(int index) {
        rangeCheck(index);

        modCount++;
        E oldValue = elementData(index);

        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

        return oldValue;
    }

2、删除指定元素:

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;
    }

    /*
     * Private remove method that skips bounds checking and does not
     * return the value removed.
     */
    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
    }

3、删除部分集合:

 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++)
                //判断元素是否需要保留
                if (c.contains(elementData[r]) == complement)
                    //保留元素到原数组,从w=0开始
                    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++)
                    elementData[i] = null;
                modCount += size - w;
                size = w;
                modified = true;
            }
        }
        return modified;
    }

转化成数组

// 返回ArrayList的Object数组 
 public Object[] toArray() {
        return Arrays.copyOf(elementData, size);
    }


 // 返回ArrayList元素组成的数组
    @SuppressWarnings("unchecked")
    public <T> T[] toArray(T[] a) {
         // 若数组a的大小 < ArrayList的元素个数;
         // 则新建一个T[]数组,数组大小是“ArrayList的元素个数”,并将“ArrayList”全部拷贝到新数组中
        if (a.length < size)
            // Make a new array of a's runtime type, but my contents:
            return (T[]) Arrays.copyOf(elementData, size, a.getClass());
        
        // 若数组a的大小 >= ArrayList的元素个数;  
        // 则将ArrayList的全部元素都拷贝到数组a中
        System.arraycopy(elementData, 0, a, 0, size);
        if (a.length > size)
            a[size] = null;
        return a;
    }

迭代器实现

首先了解什么是迭代器:

迭代器中fail-fast机制的实现:

迭代器接口:

package java.util;
public interface Iterator<E> {
    boolean hasNext();//判断是否存在下一个对象元素

    E next();//获取下一个元素

    void remove();//移除元素
}

迭代器通过接口+内部类的形式实现。

为什么使用内部类?对于集合的每次迭代,索引都不会一样。所以每次遍历都需要生成一个新的迭代器。

ArrayList中迭代器的实现:

	/**
     * 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
        //fail-fast机制
        int expectedModCount = modCount;

        Itr() {}

        public boolean hasNext() {
            return cursor != size;
        }

        @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];
        }

        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();
            }
        }

        @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();
        }
		//fail-fast机制
        final void checkForComodification() {
            if (modCount != expectedModCount)
                throw new ConcurrentModificationException();
        }
    }

参考:

    原文作者:java集合源码分析
    原文地址: https://blog.csdn.net/u013480581/article/details/82970307
    本文转自网络文章,转载此文章仅为分享知识,如有侵权,请联系博主进行删除。
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