The public type Square must be defined in its own file

This error message means that you have defined a public class called "Square" in a file where other classes are also defined. To resolve this error, move the definition of the "Square" class into its own file, separate from the other classes.

Here's an example of what the file structure might look like:

- Square.java (contains the definition of the public class "Square")
- OtherClass1.java (contains the definition of another class)
- OtherClass2.java (contains the definition of another class)

By separating the definition of the "Square" class into its own file, you can ensure that it is properly encapsulated and can be easily reused in other parts of your code.