Is there a quick function to encode/decode strings in Android using full percent encoding?

Question

I want to use percent value encoding/decoding for some string operations in my Android application. I don't want to use a URI encode/decode function pair because I want to encode every character, not just the ones that are encoded when encoding URI's for making web requests. Are there any built-in functions in the Android libraries or Java libraries that can do this?

-- roschler

Answer 1

This is pretty trivial, and doesn't need library functions:

public static String escapeString(String input) {
    String output = "";

    for (byte b : input.getBytes()) output += String.format("%%%02x", b);
    return output;
}

public static String unescapeString(String input) {
    String output = "";

    for (String hex: input.split("%")) if (!"".equals(hex)) output += (char)Integer.parseInt(hex, 16);
    return output;
}

public static String unescapeMultiByteString(String input, String charset) {
    ByteArrayOutputStream output = new ByteArrayOutputStream();
    String result = null;

    for (String hex: input.split("%")) if (!"".equals(hex)) output.write(Integer.parseInt(hex, 16));
    try { result = new String(output.toByteArray(), charset); }
    catch (Exception e) {}
    return result;
}

Answer 2

There's nothing built in to the API to do this directly, but it's pretty simple. It's better to use a specific character encoding (like UTF-8) to convert characters to bytes. This should do the trick for encoding:

static final String digits = "0123456789ABCDEF";

static void convert(String s, StringBuffer buf, String enc)
        throws UnsupportedEncodingException {
    byte[] bytes = s.getBytes(enc);
    for (int j = 0; j < bytes.length; j++) {
        buf.append('%');
        buf.append(digits.charAt((bytes[j] & 0xf0) >> 4));
        buf.append(digits.charAt(bytes[j] & 0xf));
    }
}

Oh yes, you asked for decoding as well:

static String decode(String s, String enc)
        throws UnsupportedEncodingException {
    StringBuffer result = new StringBuffer(s.length());
    ByteArrayOutputStream out = new ByteArrayOutputStream();
    for (int i = 0; i < s.length();) {
        char c = s.charAt(i);
        if (c == '%') {
            out.reset();
            do {
                if (i + 2 >= s.length()) {
                    throw new IllegalArgumentException(
                        "Incomplete trailing escape (%) pattern at " + i);
                }
                int d1 = Character.digit(s.charAt(i + 1), 16);
                int d2 = Character.digit(s.charAt(i + 2), 16);
                if (d1 == -1 || d2 == -1) {
                    throw new IllegalArgumentException(
                        "Illegal characters in escape (%) pattern at " + i
                        + ": " + s.substring(i, i+3));
                }
                out.write((byte) ((d1 << 4) + d2));
                i += 3;
            } while (i < s.length() && s.charAt(i) == '%');
            result.append(out.toString(enc));
            continue;
        } else {
            result.append(c);
        }
        i++;
    }
}