我怎样才能在自己内部放置一个Java数组？

Question

我正在尝试创建一个Java对象数组并将数组放在其第二个索引中（为了表示与数组的自相似分形），但是当我尝试访问theArray[1][1][0]时，我收到这个错误：

Main.java:11: error: array required, but Object found。

这是我到目前为止所尝试过的，而且我不确定它为什么不起作用：

import java.util.*;
import java.lang.*;

class Main
{
    public static void main (String[] args) throws java.lang.Exception
    {
        Object[] theArray = new Object[2];
        theArray[0] = "This array should contain itself at its second index.";
        theArray[1] = theArray; //Now I'm attempting to put the array into itself.
        System.out.println(theArray[1][1][0]) //Main.java:11: error: array required, but Object found
    }
}

实际上是否可以在自己内部放置一个Java数组，正如我试图在这里做的那样？

Answer 1

theArray[1]属于编译时类型Object（因为它来自对象的数组）。

您需要将其强制转换为Object[]才能将其用作数组。

---

您遇到的基本问题是，虽然包含自身的数组是完全有效的对象，但它不是有效的类型。

您可以任意深度嵌套数组类型 - Object[][][][][][][][][][][][][]是有效类型 但是，该类型的“底层”不能是数组。

您正在尝试创建一个本身数组的类型 使用泛型，这是可能的：

class Evil extends ArrayList<Evil> { }

Answer 2

由于您已将theArray声明为对象数组，因此您遇到了转换错误。因此，您无法承诺theArray[1]是Array的Java - 它可以是任何类型的Object。您需要打破访问权限以执行您想要的操作：

Object[] innerArray = (Object[]) theArray[1];
System.out.println(innerArray[0] == theArray[0]); // Always true since innerArray IS theArray
while (true) {
    // Careful! This loops forever!
    // set innerArray = innerArray[1] = theArray = theArray[1] = innerArray... 
    // all of these are the exact same object (but you have to tell Java their type every time)
    innerArray = (Object[]) innerArray[1]; 
    System.out.println(innerArray[0]);
}

Answer 3

您的代码等同于

Object arr = theArray[1];  // arr is an Object here, not an array 

但你可以做到

Object[] arr = (Object[] ) theArray[1];    // Now it is an array

Answer 4

使用ArrayLists可以很容易地做到这一点：

ArrayList list = new ArrayList();
list.add(list);

所以现在

System.out.println(list.get(0));

和

System.out.println(((ArrayList)list.get(0)).get(0)); //Casting because .get() returns an Object

两者都会输出相同的内容。

如果需要，您可以将其扩展到任意多个级别。

        System.out.println(((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)((ArrayList)list.get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0)).get(0));