java.lang.ClassCastException - Accessing Java array of Object arrays like 2 dimensional object array

Question

In order to troubleshoot the Object array created in C code with JNI, I have created the pure Java code with an array of Object arrays as follows. I would like to access this array in the manner like two dimensional Object array (Object[][]) using [][] operator. However the code crashes when casting the array to Object[][] with the following exception.

java.lang.ClassCastException: [Ljava.lang.Object; cannot be cast to [[Ljava.lang.Object

Object[] outerArray = new Object[3];
outerArray[0] = new Object[] {1,2,3,4,5};
outerArray[1] = new Object[] {10,20,30,40,50};
outerArray[2] = new Object[] {100,200,300,400,500};

Object o = ((Object[])outerArray[0])[0]; // (1) OK but awkward
Object[][] = (Object[][])outerArray;     // (2) Runtime error!!
o = outerArray[0][0];                    // (3) I want to do this

Can anyone help me?

"using [][] operator" -- It's not a operator! It's just instead of one bracket, you will use two e.g. int[][] is a two dimensional integer array. — Jude Niroshan
– Jude Niroshan, Commented May 6, 2015 at 6:36
sigh Another ask-and-run. @user: When you ask for help, stick around for a few minutes to answer questions from people trying to help you. — T.J. Crowder
– T.J. Crowder, Commented May 6, 2015 at 6:38

Alexandru Severin · Accepted Answer · 2015-05-06 06:38:44Z

2

When declaring an array with one dimension, then adding arrays inside each cell, you're building this:

[ [][][] ] [ [][][] ] [ [][][] ]

Which cannot be accessed using [][] since it has only 1 row and each cell has another row of objects, therefor it needs to be accessed with the "awkward cast"

If you want a column structure:

[] [] []
[] [] []
[] [] []

It should be created with this:

Object[][] outerArray = new Object[3][5];

edited May 6, 2015 at 6:38

answered May 6, 2015 at 6:33

Alexandru Severin

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Comments

Raffaele · Accepted Answer · 2015-05-06 06:47:53Z

When an array is created it always has a type. The purpose is to protect you from a class of programming errors, when the items stored in the array are later read and "used" as another type (*). The type of your array is Object[].class and you can't cast it to Object[][].class just because the objects inside the array are Object[]. You could put Integer's as well, and the system would not throw ArrayStoreException because Integer is a subtype of Object.

Either you create (and declare) your array as Object[][] or use the "awkward" cast.

(*) still this can happen and you don't know until runtime

public class ArraysQuirks {
    public static void main(String... args) {
        String[] strings = {"a", "b", "c"};
        Object[] objects = strings;
        objects[0] = 1; // ArrayStoreException here
    }
}

pearzl · Accepted Answer · 2015-05-06 06:55:05Z

0

 Object[][] = (Object[][])outerArray;     // (2) Runtime error!!

here is a Syntax Error.Object[][] means you will declare an multiple Array of Object.just change like this:

Object[][] temp= (Object[][])outerArray; //it works

answered May 6, 2015 at 6:55

pearzl

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