When defining strings in programming languages, they’re usually delimited by ” and “, such as “This is a string” and “Hello World”. The immediate question is what do you do when the string itself should contain a “? “Hello “World”” is hard to read and practically impossible to parse for the compiler (which tries to make sense out of everything you’ve written). To solve this (and similiar issues) people started using escape characters, special characters that tell the parser that it should pay attention to the following character(s) (some escape sequences may contain more than one character after the escape character).
Usually the escape character is \, and rewriting our example above we’ll end up with “Hello \”World\””. The parser sees the \, telling it that it should parse the next characters in a special mode and then inserts the ” into the string itself instead of using it as a delimiter. In Java, C, PHP, Python and several other languages there are also special versions of the escape sequences that does something else than just insert the character following the escape character.
\n – Inserts a new line.
\t – Inserts a tab character.
\xNN – Inserts a byte with the byte value provided (\x13, \xFF, etc).
A list of the different escape sequences that PHP supports can be found in the PHP manual.
Anyways, the issue is that Java found an escape sequence that it doesn’t know how to handle. Attempting to define a string such as “! # \ % &” will trigger this message, as it sees the escape character \, and then attempts to parse the following byte – which is a space (” “). The escape sequence “\ ” is not a valid escape sequence in the Java language specification, and the parser (or NetBeans or Eclipse) is trying to tell you this is probably not what you want.
The correct way to define the string above would be to escape the escape character (now we’re getting meta): “! # \\ % &”. This would define a string with just a single backlash in it.