从 lambda 表达式引用的局部变量必须是最终的或有效的最终变量

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.)

Answer 2

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..

Answer 3

I regularly pass outer object into interface implementation in this way: 1. Create some object holder, 2. Set this object holder with some desired state, 3. Change inner variables in the object holder, 4. Get those variables and use them.

Here is one example from Vaadin :

Object holder : 
    public class ObjectHolder<T> {
    private T obj;
    public ObjectHolder(T obj) {
        this.obj = obj;
    }
    public T get() {
        return obj;
    }
    public void set(T obj) {
        this.obj = obj;
    }
}

I want to pass button captions externally defined like this :

String[] bCaption = new String[]{"Start", "Stop", "Restart", "Status"};
String[] commOpt = bCaption;

Next, I have a for loop, and want to create buttons dynamically, and pass values like this :

for (Integer i = 0; i < bCaption.length; i++) {
    ObjectHolder<Integer> indeks = new ObjectHolder<>(i);
    b[i] = new Button(bCaption[i], 
        (Button.ClickEvent e) -> {
            remoteCommand.execute(
                cred, 
                adresaServera, 
                 comm + " " + commOpt[indeks.get()].toLowerCase()
             );
         }
        );

        b[i].setWidth(70, Unit.PIXELS);
        commandHL.addComponent(b[i]);
        commandHL.setComponentAlignment(b[i], Alignment.MIDDLE_CENTER);
  }

Hope this helps..