local variables referenced from a lambda expression must be final or effectively final

Question

I have a JavaFX 8 program (for JavaFXPorts cross platfrom) pretty much framed to do what I want but came up one step short. The program reads a text file, counts the lines

Answer 1

The error you encountered means that every variable that you access inside a lambda expressions body has to be final or effectively final. For the difference, see this answer here: Difference between final and effectively final

The problem in your code is the following variable

String readln2 = null;

The variable gets declared and assigned later on, the compiler can not detect if it gets assigned once or multiple times, so it is not effectively final.

The easiest way to solve this is to use a wrapper object, in this case a StringProperty instead of a String. This wrapper gets assigned only once and thus is effectively final:

StringProperty readln2 = new SimpleStringProperty();
readln2.set(in.readLine());
button.setOnAction(e -> l.setText(readln2.get()));

I shortened the code to show only the relevant parts..