源码分析【二】ArrayList数组

蔚落 2022-12-02 12:23 166阅读 0赞

### **源码分析系列** ###

**[源码分析【一】LinkedList链表结构][LinkedList]**

**[源码分析【二】ArrayList数组][ArrayList]**

**[源码分析【三】ArrayList与LinkedList的比较][ArrayList_LinkedList]**

--------------------

Java中List是一个必须要掌握的基础知识，List是一个接口，实现List接口的基础类有很多，其中最具有代表性的两个：ArrayList和LinkedList。

### 1、变量 ###

ArrayList是一个底层基于数组实现动态大小扩容的数据结构，快速访问、可复制、序列化的。继承自 AbstractList，实现了 List 接口。写代码时，我们经常用到这个数据结构来做大数据量的访问和（程序）缓存存储等。

/**
         * Default initial capacity.
         * 默认初始容量10
         */
        private static final int DEFAULT_CAPACITY = 10;
    
        /**
         * Shared empty array instance used for empty instances.
         * 共享空数组实例值
         */
        private static final Object[] 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.
         * 跟第一个空数组实例区分，当第一个元素添加进来时，数组会扩展多大
         */
        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.
         * 太难翻译了，transient用来表示一个域不是该对象序行化的一部分
         */
        transient Object[] elementData; // non-private to simplify nested class access
    
        /**
         * The size of the ArrayList (the number of elements it contains).
         * 数组的大小
         * @serial
         */
        private int size;

以上是arrayList的基本变量组成，private int size; 和 transient Object\[\] elementData; 则是ArrayList的成员变量。

### 2、构造函数 ###

**2.1 无参构造函数**

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

注释说初始给数组赋值空间大小时为10的list集合，当第一次添加元素时即给数组容量扩大了10倍。

**2.2 带初始容量大小的构造函数**

/**
         * 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);
            }
        }

当initialCapacity = 0时，则把EMPTY\_ELEMENTDATA赋值给 elementData；当initialCapacity大于0时，初始化为一个大小为initialCapacity的Object数组，并赋值给elementData。否则，抛出异常“非法的大小”；即初始容量大小大于等于0.

### 3、主要操作方法 ###

** 3.1 add（）添加方法**

往数组中添加一个元素e，数组容量大小增加一。

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

每次添加元素到集合中时都会先确认下集合容量大小，随后再增加1。

**3.2 remove()移除方法**

分2个，一个是移除元素在数组中的位置的方法。一个是移除元素的方法。

/**
         * 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);
    
            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;
        }
    
    /**
         * 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 ? get(i)==null : 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;
        }
    
        /*
         * 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
        }

可以很快速的理解：移除元素在数组中位置的方法是，把该元素在数组的中的下标位置对应的元素移除。先检查对应入参的下标位置是否超出数组的大小限制（数组溢出），然后把该元素移除，再把数组大小减一。

移除元素，当元素为空的时候，匹配数组中是否有空的元素，有则移除，数组大小减一。元素不为空的时候，循环数组元素，匹配元素后，移除数组，数组大小减一。

**3.3 isEmpty()和size()等常用函数方法**

/**
         * Returns the number of elements in this list.
         *
         * @return the number of elements in this list
         */
        public int size() {
            return size;
        }
    
        /**
         * Returns <tt>true</tt> if this list contains no elements.
         *
         * @return <tt>true</tt> if this list contains no elements
         */
        public boolean isEmpty() {
            return size == 0;
        }

在进行数组的大小比较和判断操作时用的一些基本方法。

ArrayList 底层基于数组实现容量大小动态可变。 扩容机制为首先扩容为原始容量的 1.5 倍。如果1.5倍太小的话，则将我们所需的容量大小赋值给 newCapacity，如果1.5倍太大或者我们需要的容量太大，那就直接拿 `newCapacity = (minCapacity > MAX_ARRAY_SIZE) ? Integer.MAX_VALUE : MAX_ARRAY_SIZE` 来扩容。

[LinkedList]: https://blog.csdn.net/xuri24/article/details/108110637
[ArrayList]: https://blog.csdn.net/xuri24/article/details/108310753
[ArrayList_LinkedList]: https://blog.csdn.net/xuri24/article/details/108425058