How can I replace a subarray of a byte array?
I need something similar with replace method for strings:
"I want pizza".replace("pizza", "replace method for byte arrays")
but it should work for byte[].
2 Answers
Assuming you want to replace an n-byte subarray with another n-byte subarray, you can use System.arraycopy.
For example:
System.arraycopy(theNewBytes, startPosition,
theArrayYouWantToUpdate, startPosition1, length);
If you want to replace n bytes with some different number of bytes, you would need to create a new array anyway:
- Create the new array with the desired length (prefix length + new portion length + suffix length)
- Copy the prefix from the existing array, using
arrayCopy - Copy the new portion
- Copy the suffix from the existing array.
2 Comments
webpersistence
I do not have the replaced subarray location (the startPosition).
anabi
Minor correction it's arraycopy (just in case a beginner uses this and has an issue with arrayCopy not being found).
It is possible to use the power of the almighty String to do this :)
static byte[] replace(byte[] src, byte[] find, byte[] replace) {
String replaced = cutBrackets(Arrays.toString(src))
.replace(cutBrackets(Arrays.toString(find)), cutBrackets(Arrays.toString(replace)));
return Arrays.stream(replaced.split(", "))
.map(Byte::valueOf)
.collect(toByteArray());
}
private static String cutBrackets(String s) {
return s.substring(1, s.length() - 1);
}
private static Collector<Byte, ?, byte[]> toByteArray() {
return Collector.of(ByteArrayOutputStream::new, ByteArrayOutputStream::write, (baos1, baos2) -> {
try {
baos2.writeTo(baos1);
return baos1;
} catch (IOException e) {
throw new UncheckedIOException(e);
}
}, ByteArrayOutputStream::toByteArray);
}
Using custom Collector.toByteArray from this answer
Test results:
byte[] arr = {10, 20, 30, 40, 50, 51, 52, 53, 54, 62, 63};
byte[] find = {10, 20, 30};
byte[] replace = {21, 22, 23, 31, 32, 33};
System.out.println(Arrays.toString(replace(arr, find, replace)));
Output:
[21, 22, 23, 31, 32, 33, 40, 50, 51, 52, 53, 54, 62, 63]
5 Comments
webpersistence
Why are you using Strings here? Will the conversion from bytes work?
Nowhere Man
I did it just for fun to check if this could be implemented. Yes, conversion of bytes array with
Arrays.toString() works fine
Andy Turner
This is a rather expensive way of doing it: not only is this converting to a string (taking a lot more memory than the original), it's also using regex, because
String.replace actually invokes replaceAll, after quoting its arguments. You can reasonably easily implement an indexOf method that finds the start position of a subarray in another array (fancy algorithms exist, but String.indexOf is actually naive); then use System.arrayCopy.Nowhere Man
@AndyTurner I was and am aware of all those overheads :) The point was to reuse as much of existing String functionality as possible. Sorry if it was not that funny.
Andy Turner
@AlexRudenko the point isn't necessarily that you need to be made aware of them, but readers of your answer do.
new String(bytes)and pray it will work, then use String'sreplace()with Strings built in the same way, then get back your bytes withstr.getBytes(). But with an image file, I think you will run into illegal byte sequences for pretty much any String encoding, so that's likely unsafe.