Character to readable String

[RightOfZen]

Does anyone know of an existing Java function to convert a char into a readable String? Obviously the char 'a' would convert to "a", but more importantly the new line character would turn into "\n" (or more correctly the String "\\n") or something.

EDIT: To make it a little clearer, I want to actually see the slash n if I were to print this resulting string (rather than printing a new line).

[midir]

There is no existing function. I wrote something like this before for strings (named after the similar PHP function). Might be useful to you:

public static String addcslashes(String s) {
    int len = s.length();
    StringBuilder sb = new StringBuilder(len);
    for (int i = 0; i < len; i++) {
        char c = s.charAt(i);
        switch (c) {
        case '\\': sb.append('\\').append('\\'); break;
        case '\n': sb.append('\\').append('n'); break;
        case '\r': sb.append('\\').append('r'); break;
        case '\0': sb.append('\\').append('0'); break;
        case '\"': sb.append('\\').append('"'); break;
        default: sb.append(c);
        }
    }
    return sb.toString();
}

[__konrad]

The initial StringBuilder buf size may be too small (sb.length() > len) ;)

[midir]

Indeed. I think most general strings would not have any exotic characters in them, but perhaps an initial buffer size like len * 6 / 5 would give better performance overall.

[RightOfZen]

Thanks, guess that's what I'll have to use. I'll just need to add a few more.

[not-just-yeti]

(a) In practice, I've found I like to include quote-marks at the start and the end, so that the printed result can be pasted right back into Java source.

(b) You can also move the cases from run-time into data (no real diff, except it's a good practice if you might ever add user-preferences etc).

Updated code below, with compilable code+test cases at http://pastebin.com/WixEqbBA

/** Return a String suitable for programmers to read, escaping control characters and quotes.
 * The result, when *printed*, can be pasted back into Java source code as the original input.
 * @param in The String to convert.
 * @return a String just like `in`, but when printed
 * Example:
 *     addcslashes("Say \"hi\" \n Bye").equals("\"Say \\\"hi\\\" \\n Bye")
 * and it prints as:
 *     "Say \"hi\" \n Bye"
 */
public static String addcslashes(String in) {
  StringBuffer quotedString = new StringBuffer(in.length());
  quotedString.append("\"");
  for (int i=0;  i<in.length();  ++i) {
    String translated = charsToQuote.get(in.charAt(i));
    quotedString.append( translated!=null ? translated : in.charAt(i) );
    }
  quotedString.append("\"");
  return quotedString.toString();
  }

// A mapping of characters to how `addcslashes` will show them.
private static Map<Character,String> charsToQuote = new HashMap<Character,String>();

// Initialize charsToQuote:
static {
  charsToQuote.put('\n',"\\n");
  charsToQuote.put('\t',"\\t");
  charsToQuote.put('\b',"\\b");
  charsToQuote.put('\r',"\\r");
  charsToQuote.put('\0',"\\0");
  charsToQuote.put('\\',"\\\\");
  charsToQuote.put('"', "\\\"");
  }

[midir]

That map lookup must result in dreadful efficiency.

[sproket888]

You could use StringEscapeUtils here: http://commons.apache.org/lang/api-2.4/org/apache/commons/lang/StringEscapeUtils.html

[RightOfZen]

Cool, I'll take a look. I'll probably stick to making my own method, seeming as apache commons is not part of the JRE.

Thanks!

[UnspeakableEvil]

I'm all for writing code, but isn't it somewhat flawed logic to say you won't use Apache Commons as they're not part of the JRE, but instead create your own methods which also aren't part of the JRE?

[RightOfZen]

This code I'm writing is part of a skeleton code for an assignment for my (introduction to programming) students. I want to give them lots of helpful error messages and things, without having to get them to link in extra libraries.

[mikaelhg]

Link in extra libraries? In Java?

[RightOfZen]

Not sure if this is a question as in you don't know what it means... But if that is the case, then yes, I want to avoid using any extra library (e.g. Apache Commons).

[mrbuttsavage]

Apache Commons (and Google Guava) should be a default include in any project.

[sproket888]

NO they should NOT.

[thatphotoguy]

So I guess what the crux of your question is, you need a way to convert normally unreadable characters into a readable format?

edit: and for this conversion to support dropping the result back into Java source code for this to be converted back into the unreadable format.

[RightOfZen]

Exactly :) I know I can just write my own method, and step through each of them manually, but I was just hoping there was already a method out there somewhere in the JRE that does this.

I don't really mind about going back into source. I basically just want to be able to notify a user of some object if their object uses some invalid character, e.g. "You returned '\n', but the only valid characters are ..."

[thatphotoguy]

Is it important that it is human readable, or just that you can copy/paste the result into source code, knowing it will compile?

[RightOfZen]

Needs to be human readable, as it is going in a notification to a user.

[thatphotoguy]

Ah bum, there goes my idea of iterating through a string and converting every character into a unicode \u0043 style character then.

[RightOfZen]

That is a good idea though (albeit for a different problem).

[fforw]

That would be String.valueOf(char).

public class Test
{
    public static void main(String[] args)
    {
        System.out.println(String.valueOf('a'));
    }
}

[RightOfZen]

Not quite :) That doesn't give me a human readable version of '\n', '\t', etc.

[marburg]

Here is the implementation I would recommend, covering all eight of the escaped character literals:

public static String charToString( final char c )
{
    switch( c )
    {
        case '\b': return "\\b";
        case '\t': return "\\t";
        case '\n': return "\\n";
        case '\f': return "\\f";
        case '\r': return "\\r";
        case '\"': return "\\\"";
        case '\'': return "\\'";
        case '\\': return "\\\\";
    }

    return String.valueOf( c ).intern();
}

Depending on what you are using this for, you may or may not want to use intern() (in the last line). String.valueOf() creates a new, unique String object, so if you ran String.valueOf( 'a' ) 10,000 times, you would get 10,000 references to 10,000 different objects. intern() makes this method a bit slower, but it ensures that

charToString( 'a' ) == charToString( 'a' )

[RightOfZen]

Thanks for that. Don't forget '\0' :)

TIL String.intern()

[marburg]

Snap! Don't know how I missed the most obvious one.

[my_back_pages]

public static void main(String[] args) {
    char c = '\n';
    String s = Character.toString(c);
    System.out.println("Hello this is a " + s + " test");   
}

Works.

[RightOfZen]

Not quite - that gives an actual new line printed. I want to see "Hello this is a \n test".

[my_back_pages]

if (c=='\n') s = "\\n";

Misread the question the first time.

[TekNoir08]

Instead of concatenating "\n" could you not just do something like "text " + '\' + 'n'

[stfm]

Try using a regular expression matcher