[Ljava.lang.Object; cannot be cast to [Ljava.util.ArrayList;

java

Solution

You said that you're trying to build a list of ArrayLists. But... you're trying to use an array to do that... Why not just use another ArrayList? It's actually pretty easy:

private List<List<Integer>> listoflist = new ArrayList<ArrayList<Integer>>();

Here's an example of using it:

ArrayList<Integer> list1 = new ArrayList<Integer>();
list1.add(Integer.valueOf(3));
list1.add(Integer.valueOf(4));
ArrayList<Integer> list2 = new ArrayList<Integer>();
list2.add(Integer.valueOf(6));
list2.add(Integer.valueOf(7));
listoflist.add(list1);
listoflist.add(list2);

Saying `ArrayList<ArrayList<Integer>>` so many times is kinda weird, so in Java 7 the construction can just be `new ArrayList<>();` (it infers the type from the variable you're assigning it to).

Problem

In my java code, I try to build a list of arraylist, my code is as follows, ``` private ArrayList<Integer>[] listoflist; listoflist = (ArrayList<Integer>[]) new Object[875715]; ``` However, when I compile the code, the compiler keeps saying that [Ljava.lang.Object; cannot be cast to [Ljava.util.ArrayList; Can I ask why I can not cast Object[] to ArrayList[]?

Original source