JDK8之ArrayList源码

ArrayList三个构造器

/**
     * Default initial capacity.
     */
    private static final int DEFAULT_CAPACITY = 10; //初始化容量大小

    /**
     * Shared empty array instance used for empty instances.
     */
    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.
     */
    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.
     */
    transient Object[] elementData; // non-private to simplify nested class access //保存数据的数组

    /**
     * The size of the ArrayList (the number of elements it contains).
     *
     * @serial
     */
    private int size;//ArrayList实际数据的数量

三个构造器主要目的是初始化Object[] 的长度

1、ArrayList()，默认构造数组长度为10空列表，但是DEFAULTCAPACITY_EMPTY_ELEMENTDATA长度我们并不知道，真正设置数组长度是在add()方法扩容时候设置

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

 2、new Array()，指定数组长度的列表，这个构造器的优点是如果我们知道数据的大小，初始化时指定长度则会减少数组扩容的次数，大大提高ArrayList的效率

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);
        }
    }

3、new ArrayList(Collection c)，这个构造器是可以把其它实现Collection接口的类当做参数，构造Object[] 

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;
        }
    }

ArrayList-add

1、add(E e) 

/**
     * 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!! //容量扩容
        elementData[size++] = e;
        return true;
    }

private void ensureCapacityInternal(int minCapacity) {
  if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
        minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);//DEFAULT_CAPACITY默认是10，Math.max 取最大值
    } 
        ensureExplicitCapacity(minCapacity); 
}

 private void ensureExplicitCapacity(int minCapacity) {
        modCount++;//modCount 数组修改次数 ，无论add remove 都会++
        // overflow-conscious code
        if (minCapacity - elementData.length > 0)
            grow(minCapacity);
    }