Is it possible to change the type of an array in Java?

Question

Possible Duplicate:
byte array to Int Array

I have a large array of bytes, can I somehow interpret it as an array of ints, such that each entry is just four of the original bytes?

I mean something like this:

for (int i = 0; i < byteArray.length; i += 4) {
    intArray[i / 4] = byteArray[i] << 24 + byteArray[i+1] << 16 + byteArray[i+2] << 8 + byteArray[i+3];
}

but rather than copying the whole array byte by byte which takes forever because the array is huge, just read ints off the array that already exists.

You can surely read off bytes and put them in int variables and they'll automatically be up-converted to ints. The bytes in the array itself however will and must remain bytes. — Hovercraft Full Of Eels, Aug 10 '12 at 00:28
Oh yes, that works. Sorry about the duplicate, I read that question before but didn't know what a bytebuffer was an assumned it would involve duplicating the array. — user1559670, Aug 10 '12 at 00:41

score 4 · Accepted Answer · answered Aug 10 '12 at 00:40

Using this code:

IntBuffer intarray = ByteBuffer.wrap(bytearray).order(ByteOrder.BIG_ENDIAN).asIntBuffer();
 intarray.get(100);

will take very few extra heap space. Actually, it wraps byte array and creates a int array view. Significantly faster, in my PC for buffer size of INT_MAX takes few nanoseconds.

score 1 · Answer 2 · answered Aug 10 '12 at 00:36

Q: Is it possible to change the type of an array in Java?

A: In general, "no". You'd have to copy/convert item by item.

In this specific case: yes, certainly. Just use asIntBuffer(). For example:

import java.nio.*;
import java.nio.IntBuffer;
import java.nio.ByteBuffer;

public class MyClass {

  public static void main(String[] args) throws Exception {
    ByteBuffer byteBuffer = ByteBuffer.allocate(1024);
    int[] array = new int[] { 1, 2, 3, 4 };
    IntBuffer IntBuf = byteBuffer.asIntBuffer();
    IntBuf.put(array);
    ...

Note that these conversions (and related "flip", "wrap", ability to change byte order, etc) were introduced with java.nio.

PS: Bohemian is correct. The *original* array is unchanged. asIntBuffer() and friends just *copies* the array. Transforming as needed. Very, very efficiently. — paulsm4, Aug 10 '12 at 01:29

score 1 · Answer 3 · answered Aug 10 '12 at 01:03

1

No. The type of an array is invariant.

No if's, but's or maybe's.

answered Aug 10 '12 at 01:03

Bohemian

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You're absolutely correct :) I think the OP just wanted to know if there was an alternative to copying and converting the array element-by-element. And, in this case, there is. – paulsm4 Aug 10 '12 at 01:30

Is it possible to change the type of an array in Java?

3 Answers3