ArrayList源码

曾经终败给现在 2022-05-06 04:56 230阅读 0赞

1.构造函数

无参的构造函数！

该无参的构造函数，默认构造默认容量为10的空数组
    即：一个new ArrayList的时候，不管用不用都会占用10单位的空间
        /**
         * Constructs an empty list with an initial capacity of ten.
         */
        public ArrayList() {
            this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
        }

有参的构造函数

以集合为参数的构造函数
    
    
        /**
         * Constructs a list containing the elements of the specified
         * collection, in the order they are returned by the collection's
         * iterator.
         *
         * @param c the collection whose elements are to be placed into this list
         * @throws NullPointerException if the specified collection is null
         */
        public ArrayList(Collection<? extends E> c) {
            elementData = c.toArray();//将继承集合接口的类变成数组，
            if ((size = elementData.length) != 0) {//如果出现了上面的bug，那么久执行下面的操作
                // 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;
            }
        }

内部的bug处理机制：

// c.toArray might (incorrectly) not return Object[] (see 6260652)
    
    当同时传入父类数组接受子类数组的引用时，同时存入子类与父类对象时；此时获取数组的getClass则会报错
    Exception in thread "main" java.lang.ArrayStoreException:
    public class App 
    {
        public static void main( String[] args )
        {
    
            AppSon[] b = new AppSon[3];
            App[] apps = b;
            apps[0] = new AppSon();
            apps[1] = new App();
    
            Class<? extends App[]> aClass = apps.getClass();
            System.out.println(aClass);
    
        }
    }
    
    class AppSon extends App{}

trimToSize方法主要是通过校验size(实际大小，存有元素数量的大小) 与已开辟的空间做对比进行；然后减少将elementData多余的空间删除。【这个方法主要是节省了内存】

public void trimToSize() {
            modCount++;
            if (size < elementData.length) {
                elementData = (size == 0)
                  ? EMPTY_ELEMENTDATA
                  : Arrays.copyOf(elementData, size);
            }
        }

* @param      src      the source array.
         * @param      srcPos   starting position in the source array.
         * @param      dest     the destination array.
         * @param      destPos  starting position in the destination data.
         * @param      length   the number of array elements to be copied.   
     public static native void arraycopy(Object src,  int  srcPos,
                                            Object dest, int destPos,
                                            int length);

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