### Question:
ByteUtils { @Nullable public static String byteArrayToHexString(byte[] raw) { if (raw == null) { return null; } final StringBuilder hex = new StringBuilder(2 * raw.length); for (final byte b : raw) { hex.append(HEXES.charAt((b & 0xF0) >> 4)).append(HEXES.charAt((b & 0x0F))); } return hex.toString(); }  @Nullable static String byteArrayToHexString(byte[] raw); static String bytesToBase16String(byte[] bytes); static String numberToBase16String(int hi, long lo); static String intToBase16String(long value);  }### Answer:
@Test void testHexString() { byte[] zeroArray = new byte[16]; assertThat(ByteUtils.byteArrayToHexString(zeroArray)).isEqualTo("00000000000000000000000000000000"); byte[] emptyArray = {}; assertThat(ByteUtils.byteArrayToHexString(emptyArray)).isEqualTo(""); byte[] zeroByte = {(byte) 0x00}; assertThat(ByteUtils.byteArrayToHexString(zeroByte)).isEqualTo("00"); byte[] fullByte = {(byte) 0xFF}; assertThat(ByteUtils.byteArrayToHexString(fullByte)).isEqualTo("FF"); byte[] leadingZero = {(byte) 0x0F}; assertThat(ByteUtils.byteArrayToHexString(leadingZero)).isEqualTo("0F"); byte[] longLeadingZero = {(byte) 0x00, (byte) 0x00, (byte) 0x00, (byte) 0x00, (byte) 0x00, (byte) 0x00, (byte) 0x11}; assertThat(ByteUtils.byteArrayToHexString(longLeadingZero)).isEqualTo("00000000000011"); byte[] trailingZero = {(byte) 0x11, (byte) 0x00}; assertThat(ByteUtils.byteArrayToHexString(trailingZero)).isEqualTo("1100"); byte[] longTrailingZero = new byte[16]; longTrailingZero[0] = (byte) 0xFF; assertThat(ByteUtils.byteArrayToHexString(longTrailingZero)).isEqualTo("FF000000000000000000000000000000"); byte[] basicArray = {(byte) 0xFF, (byte) 0xFF, (byte) 0xFF, (byte) 0x00, (byte) 0xFF, (byte) 0xF0, (byte) 0x0F, (byte) 0xFF}; assertThat(ByteUtils.byteArrayToHexString(basicArray)).isEqualTo("FFFFFF00FFF00FFF"); byte[] basicVariedArray = {(byte) 0x82, (byte) 0xF2, (byte) 0xAB, (byte) 0xA4, (byte) 0xDE, (byte) 0x15, (byte) 0x19, (byte) 0x11}; assertThat(ByteUtils.byteArrayToHexString(basicVariedArray)).isEqualTo("82F2ABA4DE151911"); }