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.

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The code above is correct but not effective. If you don't need performance but want security you can use it. If security also not a goal then do simply String.getBytes. Code above is not effective if you look down of implementation of encode in JDK. Besides you need to copy arrays and create buffers. Another way to convert is inline all code behind encode (example 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")) 

Excuse me for using Scala language. If you have problems with converting this code to Java I can rewrite it. What about performance always check on real data (with JMH for example). This code looks very similar to what you can see in JDK[2] and Protobuf[3].