如何在Java中正确实现FileChooser

Question

So I'm having problems implementing the JFileChooser, Im making a very simple program that needs to grab an CSV file. I was hoping to implement a very simple file chooser that would then check to make sure the extension was a CSV.

Here is my code for my run class where the user would be choosing the file.

import java.io.File;
import javax.swing.JFileChooser;
import javax.swing.JFrame;


public class Run 
{   
    JFrame frame = new JFrame();
    JFileChooser fileChooser = new JFileChooser();
    fileChooser.showOpenDialog(frame);
    File selFile = fileChooser.getSelectedFile();
    Reader reader = new Reader(selFile);
}

And here is the error I'm getting in eclipse BEFORE I compile.

Syntax error on tokens, misplaced construct.

The error occurs on the period between fileChooser and showOpenDialog.

The second error is on frame that which says.

Syntax error on token "frame", VariableDecleratorID expected after this token.

As of now I'm not entirely sure what I'm doing wrong.

Thank you for any help.

Answer 1

All your lines of code within the class are declarations of instance variables and their initialization except

fileChooser.showOpenDialog(frame);

This is a invocation of a method on an object, as such it cannot be a member of a class.

Class members (simplified) are attributes and methods (extended version: it can also include static variables, static methods, inner classes, static blocks, ....)

The reasoning behind it is simple: in an OO environment nothing happens if you don't trigger a method invocation and nothing should happen.

So a first step for you is to put the code inside a method, this might be the constructor or a plain method.

Second step is that you should call this method. If it is in the constructor then it is executed when (and every time) a new instance of your class is created. Otherwise you need to instantie the class and then call the method.

Third step is that all the code needs to be initiated when the program launches, you launch a java program by instructing the JVM to "run" a specific class, this class has then have to have a public static void main(String[] args) method, thats the entry point of every java application.

If you just want to test something, put all the 5 lines of code inside a main method...

so

import java.io.File;
import javax.swing.JFileChooser;
import javax.swing.JFrame;


public class Run {   
    public static void main(String[] args) {
        JFrame frame = new JFrame();
        JFileChooser fileChooser = new JFileChooser();
        fileChooser.showOpenDialog(frame);
        File selFile = fileChooser.getSelectedFile();
        Reader reader = new Reader(selFile);
    }
}

Answer 2

The statement

fileChooser.showOpenDialog(frame);

is a non-declarative statement so needs to be in a code block such as a method. The application has no main method so that can be simply be used not only as the surrounding code block for the statement but also as the entry point for the application.

public class Run {

    public static void main(String[] args) {
        SwingUtilities.invokeLater(new Runnable() {

            @Override
            public void run() {
                JFrame frame = new JFrame();
                JFileChooser fileChooser = new JFileChooser();
                fileChooser.showOpenDialog(frame);
                File selFile = fileChooser.getSelectedFile();
                // use selFile...
            }
        });
    }
}