How can I create an editor for a custom code using java code

I'm reading methods from a text file and then execute them in my java program. For this I'm using java reflection. I want to know how can I create an editor for these text files so that I can have syntax coloring and code completion (and maybe later on compiling) because today these files are hard to read.

Do you have any idea where should I start looking?

An example of a lines in the file

a := 5;

b := 6;

sum := add(a, b);