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Create StringBuilder from Byte []

Is there any way to create StringBuilderfrom byte[]?

I want to improve memory usage with StringBuilder, but first I have byte[], so I need to create Stringfrom byte[]and then create StringBuilderfrom, Stringand I don’t think this solution is optimal.

thank

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2 answers

Basically, your best option seems to be using CharsetDecoder directly.

Here's how:

byte[] srcBytes = getYourSrcBytes();

//Whatever charset your bytes are endoded in
Charset charset = Charset.forName("UTF-8");
CharsetDecoder decoder = charset.newDecoder();

//ByteBuffer.wrap simply wraps the byte array, it does not allocate new memory for it
ByteBuffer srcBuffer = ByteBuffer.wrap(srcBytes);
//Now, we decode our srcBuffer into a new CharBuffer (yes, new memory allocated here, no can do)
CharBuffer resBuffer = decoder.decode(srcBuffer);

//CharBuffer implements CharSequence interface, which StringBuilder fully support in it methods
StringBuilder yourStringBuilder = new StringBuilder(resBuffer);

ADDED:

After some tests, it seems that simple is new String(bytes)much faster, and it seems that there is no easy way to do it faster than this. Here is the test I performed:

import java.io.IOException;
import java.io.UnsupportedEncodingException;
import java.nio.ByteBuffer;
import java.nio.CharBuffer;
import java.nio.charset.CharacterCodingException;
import java.nio.charset.Charset;
import java.nio.charset.CharsetDecoder;
import java.text.ParseException;

public class ConsoleMain {
    public static void main(String[] args) throws IOException, ParseException {
        StringBuilder sb1 = new StringBuilder("abcdefghijklmnopqrstuvwxyz");
        for (int i=0;i<19;i++) {
            sb1.append(sb1);
        }
        System.out.println("Size of buffer: "+sb1.length());
        byte[] src = sb1.toString().getBytes("UTF-8");
        StringBuilder res;

        long startTime = System.currentTimeMillis();
        res = testStringConvert(src);
        System.out.println("Conversion using String time (msec): "+(System.currentTimeMillis()-startTime));
        if (!res.toString().equals(sb1.toString())) {
            System.err.println("Conversion error");
        }

        startTime = System.currentTimeMillis();
        res = testCBConvert(src);
        System.out.println("Conversion using CharBuffer time (msec): "+(System.currentTimeMillis()-startTime));
        if (!res.toString().equals(sb1.toString())) {
            System.err.println("Conversion error");
        }
    }

    private static StringBuilder testStringConvert(byte[] src) throws UnsupportedEncodingException {
        String s = new String(src, "UTF-8");
        StringBuilder b = new StringBuilder(s);
        return b;
    }

    private static StringBuilder testCBConvert(byte[] src) throws CharacterCodingException {
        Charset charset = Charset.forName("UTF-8");
        CharsetDecoder decoder = charset.newDecoder();
        ByteBuffer srcBuffer = ByteBuffer.wrap(src);
        CharBuffer resBuffer = decoder.decode(srcBuffer);
        StringBuilder b = new StringBuilder(resBuffer);
        return b;
    }
}

Results:

Size of buffer: 13631488
Conversion using String time (msec): 91
Conversion using CharBuffer time (msec): 252

( ) IDEONE: .

+12

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, String, - :

new StringBuilder(Charset.forName(charsetName).decode(ByteBuffer.wrap(inBytes)))

, . , , averageCharsPerByte , . , , , StringBuilder .

CharsetDecoder cd = Charset.forName(charsetName).newDecoder();
cd.onMalformedInput(CodingErrorAction.REPLACE);
cd.onUnmappableCharacter(CodingErrorAction.REPLACE);
int lengthEstimate = Math.ceil(cd.averageCharsPerByte()*inBytes.length) + 1;
ByteBuffer inBuf = ByteBuffer.wrap(inBytes);
CharBuffer outBuf = CharBuffer.allocate(lengthEstimate);
StringBuilder out = new StringBuilder(lengthEstimate);
CoderResult cr;
while (true) {
    cr = cd.decode(inBuf, outBuf, true);
    out.append(outBuf);
    outBuf.clear();
    if (cr.isUnderflow()) break;
    if (!cr.isOverflow()) cr.throwException();
}
cr = cd.flush(outBuf);
if (!cr.isUnderflow()) cr.throwException();
out.append(outBuf);

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+4

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