Java code To convert byte to Hexadecimal

    byte[] bytes = {-1, 0, 1, 2, 3 };
    StringBuilder sb = new StringBuilder();
    for (byte b : bytes) {
        sb.append(String.format("%02X ", b));
    }
    System.out.println(sb.toString());
    // prints "FF 00 01 02 03 "

See also

• java.util.Formatter syntax
  • %[flags][width]conversion
    • Flag '0' – The result will be zero-padded
    • Width 2
    • Conversion 'X' – The result is formatted as a hexadecimal integer, uppercase

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Looking at the text of the question, it’s also possible that this is what is requested:

    String[] arr = {"-1", "0", "10", "20" };
    for (int i = 0; i < arr.length; i++) {
        arr[i] = String.format("%02x", Byte.parseByte(arr[i]));
    }
    System.out.println(java.util.Arrays.toString(arr));
    // prints "[ff, 00, 0a, 14]"

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Several answers here uses Integer.toHexString(int); this is doable, but with some caveats. Since the parameter is an int, a widening primitive conversion is performed to the byte argument, which involves sign extension.

    byte b = -1;
    System.out.println(Integer.toHexString(b));
    // prints "ffffffff"

The 8-bit byte, which is signed in Java, is sign-extended to a 32-bit int. To effectively undo this sign extension, one can mask the byte with 0xFF.

    byte b = -1;
    System.out.println(Integer.toHexString(b & 0xFF));
    // prints "ff"

Another issue with using toHexString is that it doesn’t pad with zeroes:

    byte b = 10;
    System.out.println(Integer.toHexString(b & 0xFF));
    // prints "a"

Both factors combined should make the String.format solution more preferrable.

References

• JLS 4.2.1 Integral Types and Values
  • For byte, from -128 to 127, inclusive
• JLS 5.1.2 Widening Primitive Conversion