从内部 class 访问的变量需要声明为 final

Question

Hello I am trying to do pretty much the exact same thing as the person who asked this question: Variable is accessed from within inner class needs to be declared final

So I'm going to continue to use their code as a reference in this question:

 File directory = new File(prefs.getString("path",null));
 File[] files = directory.listFiles();
        for (File file :files){
            if(file.isDirectory()) {
                buttons.add(new Button(this));
                Button button = buttons.get(buttons.size() - 1);
                String fileName = file.getName();

                button.setText(fileName);
                button.setOnClickListener(new View.OnClickListener() {
                    @Override
                    public void onClick(View v) {
                        Intent intent = new Intent(currentActivity, EOChoiceActivity.class);
                        intent.putExtra("fileExtension",fileName);
                        startActivity(intent);
                    }
                });
                layout.addView(button);
            }
        }

I'm asking the same question as the original author also: why is it that only the last fileName selected is sent for any button? I understand that this has to do with the rules of how an inner class accesses local variables, as cited in JLS 8.1.3, but I still just can't understand the reasoning.

The only thing I can think of that makes sense of this is that the inner class is using a reference of fileName, and so when the fileName value changes on each iteration, and eventually is solidified in it's value on the last iteration, the "final" reference would be to the value of the last element in the files array and this reference would be present for each button.

Yeah I'm just really confused here... I can't understand why the different values for fileName wouldn't apply to each button, but instead only the last fileName value is applied to each button.

Answer 1

I do not understand the question, neither the original . I do not work with android, but with Java. So I did the following test:

JPanel buttons = new JPanel();
File directory = new File(".");
File[] files = directory.listFiles();
for (File file :files) {
    if (file.isDirectory()) {
        JButton button = new JButton();
        buttons.add(button);
        final String fileName = file.getName();
        button.setText(fileName);
        button.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent ev) {
                System.out.println(fileName);
            }
        });
    }
}
JOptionPane.showMessageDialog(null, buttons);

and all is working as expected that is, when pressing any button, I get its text sent to the standard output.

Regarding the original question, there should be no difference if the variable is declared final or not.
Maybe the author got confused since compiler of older versions (before Java 8) would generate an error if such variable was not declared final ; with Java 8 or newer, the variable must only be effectively final (document in JLS 8.1.3).

Sure, if fileName was declared as a class field (not a variable), then the last value set would be the one sent for all buttons (the value of fileName when the button is pressed).

If fileName is a variable, then it stops existing when the execution of the block it is in is terminated - so its value must be stored in the created instance(s) of the inner class - it would be confusing if the variable was assigned a new value after the creation of that instance - must the saved value be actualized? Or should the old value be used? I believe that is one of the reason for the requirement of being final. See Jons explanation here - surely better than mine.