java 学习笔记--利用反射实现泛型数组的复制

引用自Java核心技术卷I

使用反射编写泛型数组代码:

public class CopyOfTest {

    public static void main(String[] args)
    {
        int[] a = { 1, 2, 3 };
        a = (int[]) goodCopyOf(a, 10);
        System.out.println(Arrays.toString(a));

        String[] b = { "Tom", "Dick", "Harry" };
        b = (String[]) goodCopyOf(b, 10);
        System.out.println(Arrays.toString(b));

        System.out.println("The following call will generate an exception.");
        b = (String[]) badCopyOf(b, 10);
    }

    /**
     * This method attempts to grow an array by allocating a new array and copying all elements.
     * @param a the array to grow
     * @param newLength the new length
     * @return a larger array that contains all elements of a. However, the returned array has
     * type Object[], not the same type as a
     */
    public static Object[] badCopyOf(Object[] a, int newLength) // not useful
    {
        Object[] newArray = new Object[newLength];
        System.arraycopy(a, 0, newArray, 0, Math.min(a.length, newLength));
        return newArray;
    }

    /**
     * This method grows an array by allocating a new array of the same type and
     * copying all elements.
     * @param a the array to grow. This can be an object array or a primitive
     * type array
     * @return a larger array that contains all elements of a.
     */
    public static Object goodCopyOf(Object a, int newLength)
    {
        //获得字节码文件
        Class cl = a.getClass();
        //如果不是数据,应该返回
        if (!cl.isArray()) return null;
        //获得字节码的数据类型,比如int,boolean(这种方法不会返回int[])
        Class componentType = cl.getComponentType();//获得组成类型
        //String name = cl.getTypeName();//获得字节码类型-->比如int[],int之类的
        //获得a数组的长度-->只有Array类型才可以使用
        int length = Array.getLength(a);
        //创建一个新的componentType类型数组对象
        Object newArray = Array.newInstance(componentType, newLength);
        //复制元素
        System.arraycopy(a, 0, newArray, 0, Math.min(length, newLength));
        return newArray;
    }
}

PS:之所以public static Object goodCopyOf(Object a, int newLength)方法中的第一个参数为Object而不是Object[]原因是:
1.如果定义为Object[],那么只能接受引用数据类型的数组(对象数组),对于int[]等基本数据类型的对象就符合要求了
2.定义为Object,整形数组int[]等基本类型数组可以转为Object(记住,万物皆对象)

        //创建一个String[]类型的数组
        String[] strings = new String[100];
        //创建一个Object类型的list
        ArrayList<Object> list = new ArrayList<>();
        //集合存进String[]数组,其实就是存了一个地址而已
        list.add(strings);