Java: System.arraycopy is not copying my arrays

Question

I have an array called "first" and another array called "second", both arrays are of type byte and size of 10 indexes.

I am copying the two arrays into one array called "third" of type byte too and of length 2*first.length as follow:

byte[] third= new byte[2*first.length];
for(int i = 0; i<first.length;i++){
    System.arraycopy(first[i], 0, third[i], 0, first.length);
    }   
for(int i = 0; i<second.length;i++){
    System.arraycopy(second[i], 0, third[i], first.length, first.length);
    }

but it is not copying and it throws an exception: ArrayStoreException

I read on the here that this exception is thrown when an element in the src array could not be stored into the dest array because of a type mismatch. but all my arrays are in bytes so there is no mismatch

what exactly is the problem ?

That loop is unnecessary if you use arraycopy. The other way around, arraycopy is not required if you use a loop because you're assigning the values yourself. — Fritz
– Fritz, Commented May 8, 2013 at 15:40

Community · Accepted Answer · 2020-06-20 09:12:55Z

10

You pass System.arraycopy the array, not the array element. By passing first[i] into arraycopy as the first argument, you're passing in a byte, which (because arraycopy is declared as accepting Object for the src argument) gets promoted to Byte. So you're getting ArrayStoreException for the first reason in the list in the documentation:

...if any of the following is true, an ArrayStoreException is thrown and the destination is not modified:

The src argument refers to an object that is not an array.

Here's how you use arraycopy to copy two byte[] arrays into a third:

// Declarations for `first` and `second` for clarity
byte[] first = new byte[10];
byte[] second = new byte[10];
// ...presumably fill in `first` and `second` here...

// Copy to `third`
byte[] third = new byte[first.length + second.length];
System.arraycopy(first,  0, third, 0, first.length);
System.arraycopy(second, 0, third, first.length, second.length);

edited Jun 20, 2020 at 9:12

CommunityBot

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answered May 8, 2013 at 15:37

T.J. Crowder

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1 Comment

Alison Over a year ago

thanks to you all .. but I decided to accept this answer as for the explanation of my mistake ..

user2362987 · Accepted Answer · 2013-05-08 15:37:53Z

2

System.arraycopy(first, 0, third, 0, first.length);
System.arraycopy(second, 0, third, first.length, second.length);

answered May 8, 2013 at 15:37

user2362987

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