Here is some code that I found on the Internet:
class M{public static void main(String[]a){System.out.print(new char[]
{'H','e','l','l','o',' ','W','o','r','l','d','!'});}}
This code prints Hello World! onto the screen; you can see it run here. I can clearly see public static void main written, but it is backwards. How does this code work? How does this even compile?
Edit: I tried this code in IntellIJ, and it works fine. However, for some reason it doesn't work in notepad++, along with cmd. I still haven't found a solution to that, so if anyone does, comment down below.
There are invisible characters here that alter how the code is displayed. In Intellij these can be found by copy-pasting the code into an empty string (""), which replaces them with Unicode escapes, removing their effects and revealing the order the compiler sees.
Here is the output of that copy-paste:
"class M\u202E{public static void main(String[]a\u202D){System.out.print(new char[]\n"+
"{'H','e','l','l','o',' ','W','o','r','l','d','!'});}} "
The source code characters are stored in this order, and the compiler treats them as being in this order, but they're displayed differently.
Note the \u202E character, which is a right-to-left override, starting a block where all characters are forced to be displayed right-to-left, and the \u202D, which is a left-to-right override, starting a nested block where all characters are forced into left-to-right order, overriding the first override.
Ergo, when it displays the original code, class M is displayed normally, but the \u202E reverses the display order of everything from there to the \u202D, which reverses everything again. (Formally, everything from the \u202D to the line terminator gets reversed twice, once due to the \u202D and once with the rest of the text reversed due to the \u202E, which is why this text shows up in the middle of the line instead of the end.) The next line's directionality is handled independently of the first's due to the line terminator, so {'H','e','l','l','o',' ','W','o','r','l','d','!'});}} is displayed normally.
For the full (extremely complex, dozens of pages long) Unicode bidirectional algorithm, see Unicode Standard Annex #9.