Converting char[] to byte[]

Question

I would like to convert a character array to a byte array in Java. What methods exists for making this conversion?

Answer 1

Convert without creating String object:

import java.nio.CharBuffer;
import java.nio.ByteBuffer;
import java.util.Arrays;

byte[] toBytes(char[] chars) {
  CharBuffer charBuffer = CharBuffer.wrap(chars);
  ByteBuffer byteBuffer = Charset.forName("UTF-8").encode(charBuffer);
  byte[] bytes = Arrays.copyOfRange(byteBuffer.array(),
            byteBuffer.position(), byteBuffer.limit());
  Arrays.fill(byteBuffer.array(), (byte) 0); // clear sensitive data
  return bytes;
}

Usage:

char[] chars = {'0', '1', '2', '3', '4', '5', '6', '7', '8', '9'};
byte[] bytes = toBytes(chars);
/* do something with chars/bytes */
Arrays.fill(chars, '\u0000'); // clear sensitive data
Arrays.fill(bytes, (byte) 0); // clear sensitive data

Solution is inspired from Swing recommendation to store passwords in char[]. (See Why is char[] preferred over String for passwords?)

Remember not to write sensitive data to logs and ensure that JVM won't hold any references to it.

This method is needed only for security concerns. If data is not sensitive it better to use String.getBytes instead.

---

Here is pseudo-code (actually, Scala code) how to do the same thing manually for UTF-8:

val xs: Array[Char] = "A ß € 嗨  ".toArray
val len = xs.length
val ys: Array[Byte] = new Array(3 * len) // worst case
var i = 0; var j = 0 // i for chars; j for bytes
while (i < len) { // fill ys with bytes
  val c = xs(i)
  if (c < 0x80) {
    ys(j) = c.toByte
    i = i + 1
    j = j + 1
  } else if (c < 0x800) {
    ys(j) = (0xc0 | (c >> 6)).toByte
    ys(j + 1) = (0x80 | (c & 0x3f)).toByte
    i = i + 1
    j = j + 2
  } else if (Character.isHighSurrogate(c)) {
    if (len - i < 2) throw new Exception("overflow")
    val d = xs(i + 1)
    val uc: Int = 
      if (Character.isLowSurrogate(d)) {
        Character.toCodePoint(c, d)
      } else {
        throw new Exception("malformed")
      }
    ys(j) = (0xf0 | ((uc >> 18))).toByte
    ys(j + 1) = (0x80 | ((uc >> 12) & 0x3f)).toByte
    ys(j + 2) = (0x80 | ((uc >>  6) & 0x3f)).toByte
    ys(j + 3) = (0x80 | (uc & 0x3f)).toByte
    i = i + 2 // 2 chars
    j = j + 4
  } else if (Character.isLowSurrogate(c)) {
    throw new Exception("malformed")
  } else {
    ys(j) = (0xe0 | (c >> 12)).toByte
    ys(j + 1) = (0x80 | ((c >> 6) & 0x3f)).toByte
    ys(j + 2) = (0x80 | (c & 0x3f)).toByte
    i = i + 1
    j = j + 3
  }
}
// check
println(new String(ys, 0, j, "UTF-8"))

This code looks similar to what is in JDK[2] and Protobuf[3].

Answer 2

char[] ch = ?
new String(ch).getBytes();

Or, to get non-default charset:

new String(ch).getBytes("UTF-8");

Update: Since Java 7:

new String(ch).getBytes(StandardCharsets.UTF_8);

Answer 3

Edit: Andrey's answer has been updated so the following no longer applies.

Andrey's answer (the highest voted at the time of writing) is slightly incorrect. I would have added this as comment but I am not reputable enough.

In Andrey's answer:

char[] chars = {'c', 'h', 'a', 'r', 's'}
byte[] bytes = Charset.forName("UTF-8").encode(CharBuffer.wrap(chars)).array();

the call to array() may not return the desired value, for example:

char[] c = "aaaaaaaaaa".toCharArray();
System.out.println(Arrays.toString(Charset.forName("UTF-8").encode(CharBuffer.wrap(c)).array()));

output:

[97, 97, 97, 97, 97, 97, 97, 97, 97, 97, 0]

As can be seen a zero byte has been added. To avoid this use the following:

char[] c = "aaaaaaaaaa".toCharArray();
ByteBuffer bb = Charset.forName("UTF-8").encode(CharBuffer.wrap(c));
byte[] b = new byte[bb.remaining()];
bb.get(b);
System.out.println(Arrays.toString(b));

output:

[97, 97, 97, 97, 97, 97, 97, 97, 97, 97]

As the answer also alluded to using passwords it might be worth blanking out the array that backs the ByteBuffer (accessed via the array() function):

ByteBuffer bb = Charset.forName("UTF-8").encode(CharBuffer.wrap(c));
byte[] b = new byte[bb.remaining()];
bb.get(b);
blankOutByteArray(bb.array());
System.out.println(Arrays.toString(b));

Answer 4

private static byte[] charArrayToByteArray(char[] c_array) {
        byte[] b_array = new byte[c_array.length];
        for(int i= 0; i < c_array.length; i++) {
            b_array[i] = (byte)(0xFF & (int)c_array[i]);
        }
        return b_array;
}

Answer 5

If you just want to convert the data container (the array) type itself, only regarding the data size and being agnostic to any encoding:

// original byte[]
byte[] pattern = null;
char[] arr = new char[pattern.length * 2];
ByteBuffer wrapper = ByteBuffer.wrap(pattern);
wrapper.position(0);
int i = 0;
while(wrapper.hasRemaining()) {
    char character = wrapper.remaining() < 2 ? ((char) (((int) wrapper.get()) << 8)) : wrapper.getChar();
    arr[i++] = character;
}

Answer 6

You could make a method:

public byte[] toBytes(char[] data) {
byte[] toRet = new byte[data.length];
for(int i = 0; i < toRet.length; i++) {
toRet[i] = (byte) data[i];
}
return toRet;
}

Hope this helps