【Java】ArrayList

ゝ一纸荒年。 2023-05-29 06:18 52阅读 0赞

一、ArrayList类图

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3ljYjE5MTQ4NDU0NTE_size_16_color_FFFFFF_t_70][]

二、ArrayList主要的成员变量

（1）private static final int DEFAULT\_CAPACITY = 10；默认容量。

（2）private static final Object\[\] EMPTY\_ELEMENTDATA = \{\};

Shared empty array instance used for empty instances.

（3）private static final Object\[\] DEFAULTCAPACITY\_EMPTY\_ELEMENTDATA = \{\};

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.

（4）transient Object\[\] 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.

（5）private static final int MAX\_ARRAY\_SIZE = Integer.MAX\_VALUE - 8;

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

三、构造函数

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);
            }
        }

调用带参数的构造函数，当initialCapacity>0时直接new Object\[initialCapacity\]的数组，如果initialCapacity=0，则赋值为EMPTY\_ELEMENTDATA

/**
         * Constructs an empty list with an initial capacity of ten.
         */
        public ArrayList() {
            this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
        }

调用无参构造函数时，设置this.elementData = DEFAULTCAPACITY\_EMPTY\_ELEMENTDATA，当加入一个元素第一次扩容时会扩容为DEFAULT\_CAPACITY

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;
            }
        }

// c.toArray might (incorrectly) not return Object\[\] (see 6260652)解释：[https://blog.csdn.net/xiaotuji\_/article/details/72598546][https_blog.csdn.net_xiaotuji_article_details_72598546]

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3ljYjE5MTQ4NDU0NTE_size_16_color_FFFFFF_t_70 1][]

三、add方法及扩容

1、add方法的代码如下：

/**
         * 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;
        }

扩容逻辑在ensureCapacityInternal（int minCapacity）里面。ensureCapacityInternal的代码逻辑如下：

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

calculateCapacity里面有涉及到DEFAULTCAPACITY\_EMPTY\_ELEMENTDATA的处理，代码如下：

private static int calculateCapacity(Object[] elementData, int minCapacity) {
            if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
                return Math.max(DEFAULT_CAPACITY, minCapacity);
            }
            return minCapacity;
        }

处理完DEFAULTCAPACITY\_EMPTY\_ELEMENTDATA之后接着执行，ensureExplicitCapacity，如下：

private void ensureExplicitCapacity(int minCapacity) {
            modCount++;
    
            // overflow-conscious code
            if (minCapacity - elementData.length > 0)
                grow(minCapacity);
        }

如果minCapacity > elementData.length，执行grow(minCapacity)进行扩容，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)
                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;
        }

仅记录个人学习情况，如有错误，请帮指正，谢谢！

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[https_blog.csdn.net_xiaotuji_article_details_72598546]: https://blog.csdn.net/xiaotuji_/article/details/72598546
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