Append part of Java byte array to StringBuilder

Question

How do I append a portion of byte array to a StringBuilder object under Java? I have a segment of a function that reads from an InputStream into a byte array. I then want to append whatever I read into a StringBuilder object:

byte[] buffer = new byte[4096];
InputStream is;
//
//some setup code
//
while (is.available() > 0)
{
   int len = is.read(buffer);
   //I want to append buffer[0] to buffer[len] into StringBuilder at this point
 }

Adrianople

Answer 1

You should not use a StringBuilder for this, since this can cause encoding errors for variable-width encodings. You can use a java.io.ByteArrayOutputStream instead, and convert it to a string when all data has been read:

byte[] buffer = new byte[4096];
ByteArrayOutputStream out = new ByteArrayOutputStream();
InputStream is;
//
//some setup code
//
while (is.available() > 0) {
   int len = is.read(buffer);
   out.write(buffer, 0, len);
}
String result = out.toString("UTF-8"); // for instance

If the encoding is known not to contain multi-byte sequences (you are working with ASCII data, for instance), then using a StringBuilder will work.

Fidel

Answer 2

You could just create a String out of your buffer:

String s = new String(buffer, 0, len);

Then if you need to you can just append it to a StringBuilder.

Grimy

What encoding does your text use? Method above will work with ASCII but may fail on any multi-byte strings like UTF-8 or UTF-16 (you may read partial string from buffer and get only half of char definition at the end; also leaving invalid beginning for next portion) – Babirusa

String s = new String(buffer, 0, len, "UTF-8"); for other encodings than ASCII – Krilov

Did you check by any chance if this performs faster than ByteArrayOutputStream? I suppose it will. – Ideation

@Ideation has a good point, and it may also break the string in the middle of a code point. – Chrysotile

Answer 3

Something like below should do the trick for you.

byte[] buffer = new byte[3];
buffer[0] = 'a';
buffer[1] = 'b';
buffer[2] = 'c';
StringBuilder sb = new StringBuilder(new String(buffer,0,buffer.length-1));
System.out.println("buffer has:"+sb.toString()); //prints ab

Honniball