In Java's case it is mandatory to have class Rextester (note no access modifier) as an entry point for your code.
There is simillar requirement in C#, but Java folks seem to miss this much more frequently.
If your code waits for input from stdin (keyboard) you have to supply it all at once in the input text-area (which shows up after you click '+'
sign at the bottom). The whole input should be supplied before running code - at runtime it will be piped to the stdin stream of the process.
Codewall - put there code snippets you want to find later easily. For example, google for 'rebuild indexes rextester' to get Sql Server's script to rebuild all indexes.
Personal code walls - put anything you like there. Nice and encouraged usage would be to put tutorials there as in C# tutorial.