Returns a byte array of at least length 1 : Bitwise Operators « Operators « Java Tutorial

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2. Operators
3. Bitwise Operators

import java.util.BitSet;

public class Main {
  public static void main(String[] argv) throws Exception {
    BitSet bitset = new BitSet();
    bitset.set(1);
    System.out.println(toByteArray(bitset));
  }

  public static byte[] toByteArray(BitSet bits) {
    byte[] bytes = new byte[bits.length() / 8 + 1];
    for (int i = 0; i < bits.length(); i++) {
      if (bits.get(i)) {
        bytes[bytes.length - i / 8 - 1] |= 1 << (i % 8);
      }
    }
    return bytes;
  }
}