Over a million developers have joined DZone.

Hidden Code

DZone's Guide to

Hidden Code

· Java Zone ·
Free Resource

Download Microservices for Java Developers: A hands-on introduction to frameworks and containers. Brought to you in partnership with Red Hat.

Sometime ago I came across the issue of invisible characters in Strings.  These can really cause confusion because they are invisible.
    String a = "Hello\u200e";
    String b = "Hello\u200f";
    System.out.println('\'' + a + "' and '" + b + "' are length "
                     + a.length() + " and " + b.length() 
                     + ", equals() is " + a.equals(b));

'Hello‎' and 'Hello‏' are length 6 and 6, equals() is false

Invisible identifiers

Imagine my reaction to discovering you can use invisible characters in identifiers in Java :P.  These characters cannot appear at the start of identifiers in Java but can be anywhere else.

 System.out.println("String _\u200e = \"Hello \";");
    System.out.println("String _\u200f = \"World\";");
    System.out.println("String _\u200e\u200f = \" !!\";");

String _‎ = "Hello ";
String _‏ = "World";
String _‎‏ = " !!";

which when run prints

Hello World !!

So we have three identifiers which all appear the same because they have different invisible characters in their names !!

Amazingly this code compiles, runs and prints all the characters which can be in an identifier but not start them. The code contains \u202e which really messes with the display of the code
for (char c‮h = 0; c‮h < Character.MAX_VALUE; c‮h++)
        if (Character.isJavaIdentifierPart(c‮h) && !Character.isJavaIdentifierStart(c‮h))
            System.out.printf("%04x <%s>%n", (int) c‮h, "" + c‮h);

Download Building Reactive Microservices in Java: Asynchronous and Event-Based Application Design. Brought to you in partnership with Red Hat


Published at DZone with permission of

Opinions expressed by DZone contributors are their own.

{{ parent.title || parent.header.title}}

{{ parent.tldr }}

{{ parent.urlSource.name }}