Write a Java program that displays all 4-character strings in lexicographic order (also
called dictionary order) on the screen. It must display a string of 4 characters per line.
Only the characters a, b, c, d are allowed within character strings, but you can reuse the
same character more than once within the same string. Within the same string, the
character "b" must always be immediately followed by the character "a". The same string
cannot contain both the character "d" and the character "a". Your program should end with
the number of character strings displayed on the screen. Your program receives nothing
as input. Your program code must not include pre-calculated strings: it must generate
them at runtime. Your program should not contain more than 200 short lines of code.