java - 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 to establish a random range, picks a random number from that range and reads that line in for display.

The error is: local variables referenced from a lambda expression must be final or effectively final
        button.setOnAction(e -> l.setText(readln2));

I am a bit new to java but is seems whether I use Lambda or not to have the next random line display in Label l, my button.setOnAction(e -> l.setText(readln2)); line is expecting a static value.

Any ideas how I can tweak what I have to simply make the next value of the var readln2 display each time I press the button on the screen?

Thanks in advance and here is my code:

String readln2 = null;
in = new BufferedReader(new FileReader("/temp/mantra.txt"));
long linecnt = in.lines().count();
int linenum = rand1.nextInt((int) (linecnt - Low)) + Low;
try {
    //open a bufferedReader to file 
    in = new BufferedReader(new FileReader("/temp/mantra.txt"));

    while (linenum > 0) {
        //read the next line until the specific line is found
        readln2 = in.readLine();
        linenum--;
    }

    in.close();
} catch (IOException e) {
    System.out.println("There was a problem:" + e);
}

Button button = new Button("Click the Button");
button.setOnAction(e -> l.setText(readln2));
//  error: local variables referenced from a lambda expression must be final or effectively final

Answer 1

You can just copy the value of readln2 into a final variable:

    final String labelText = readln2 ;
    Button button = new Button("Click the Button");
    button.setOnAction(e -> l.setText(labelText));

If you want to grab a new random line each time, you can either cache the lines of interest and select a random one in the event handler:

Button button = new Button("Click the button");
Label l = new Label();
try {
    List<String> lines = Files.lines(Paths.get("/temp/mantra.txt"))
        .skip(low)
        .limit(high - low)
        .collect(Collectors.toList());
    Random rng = new Random();
    button.setOnAction(evt -> l.setText(lines.get(rng.nextInt(lines.size()))));
} catch (IOException exc) {
    exc.printStackTrace();
}
// ...

Or you could just re-read the file in the event handler. The first technique is (much) faster but could consume a lot of memory; the second doesn't store any of the file contents in memory but reads a file each time the button is pressed, which could make the UI unresponsive.

The error you got basically tells you what was wrong: the only local variables you can access from inside a lambda expression are either final (declared final, which means they must be assigned a value exactly once) or "effectively final" (which basically means you could make them final without any other changes to the code).

Your code fails to compile because readln2 is assigned a value multiple times (inside a loop), so it cannot be declared final. Thus you can't access it in a lambda expression. In the code above, the only variables accessed in the lambda are l, lines, and rng, which are all "effectively final` as they are assigned a value exactly once. (You can declare them final and the code would still compile.)