Thread: Imitating a file system in Java and absolutely stumped by NullPointerException

I'm writing a program that vaguely imitates a file system in java and I'm having trouble with my methods and my test class

So this is my main class.

I wrote code to:
1)Open a File
2) Copy a File
3) write output to another file
4)Get Path information
5)Get File Size
6) Read Lines in the file (if it's a text file)


For Copying, writing output, getting path/file and reading lines i keep getting

java.lang.NULLPOINTEREXCEPTION


For the first one Im able to select a file, why isn't is still reference the file I selected?

public class BasicFile {
 
    File f;
 
   public BasicFile() {
 
        JFileChooser choose = new JFileChooser(".");
 
        int status = choose.showOpenDialog(null);
 
        try {
 
            if (status != JFileChooser.APPROVE_OPTION) {
                throw new IOException();
            }
 
            f = choose.getSelectedFile();
 
            if (!f.exists()) {
                throw new FileNotFoundException();
            }
 
        } catch (FileNotFoundException e) {
 
            display(e.toString(), "File not found ....");
 
        } catch (IOException e) {
 
            display(e.toString(), "Approve option was not selected");
 
        }
 
    }
 
 
 
    void display(String msg, String s) {
 
        JOptionPane.showMessageDialog(null, msg, s, JOptionPane.ERROR_MESSAGE);
 
    }
 
   public void copyFile(File sourceFile, File destFile) throws IOException
    {
        if(!destFile.exists()) {
        destFile.createNewFile();
    }
 
    FileChannel source = null;
    FileChannel destination = null;
 
    try {
        source = new FileInputStream(sourceFile).getChannel();
        destination = new FileOutputStream(destFile).getChannel();
        destination.transferFrom(source, 0, source.size());
    }
    finally {
        if(source != null) {
            source.close();
        }
        if(destination != null) {
            destination.close();
        }
    }
    }
 
    void writeOutput() throws IOException
    {
        BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
 
        String lineFromInput = in.readLine();
 
        PrintWriter out = new PrintWriter(new FileWriter("output.txt"));
 
        out.println(lineFromInput);
 
        out.close();
 
 
    }
 
    void getAttributes()
    {
        File file = null;
        this.f = file;
        String dirPath = file.getAbsoluteFile().getParentFile().getAbsolutePath();
 
        System.out.println("The absolute path is  " + dirPath);
 
        File folder = new File(file.getAbsolutePath());
        File[] listOfFiles = folder.listFiles();
 
        for (int i = 0; i < listOfFiles.length; i++) {
            if (listOfFiles[i].isFile()) {
                System.out.println("File " + listOfFiles[i].getName());
            } else if (listOfFiles[i].isDirectory()) {
                System.out.println("Directory " + listOfFiles[i].getName());
            }
        }
 
 
 
    }
 
    void getFileSize()
    {
        File file = null;
        this.f = file;
 
        if(file.exists()){
 
			double bytes = file.length();
			double kilobytes = (bytes / 1024);
			double megabytes = (kilobytes / 1024);
			double gigabytes = (megabytes / 1024);
			double terabytes = (gigabytes / 1024);
			double petabytes = (terabytes / 1024);
			double exabytes = (petabytes / 1024);
			double zettabytes = (exabytes / 1024);
			double yottabytes = (zettabytes / 1024);
 
			System.out.println("bytes : " + bytes);
			System.out.println("kilobytes : " + kilobytes);
			System.out.println("megabytes : " + megabytes);
			System.out.println("gigabytes : " + gigabytes);
			System.out.println("terabytes : " + terabytes);
			System.out.println("petabytes : " + petabytes);
			System.out.println("exabytes : " + exabytes);
			System.out.println("zettabytes : " + zettabytes);
			System.out.println("yottabytes : " + yottabytes);
		}else{
			 System.out.println("File does not exists!");
		}
    }
 
    void fileLineReader() throws FileNotFoundException, IOException {
        File file = null;
        this.f = file;
 
        LineNumberReader lnr = new LineNumberReader(new FileReader(file));
 
        lnr.skip(Long.MAX_VALUE);
 
 
        System.out.println(lnr.getLineNumber() + 1); 
 
        lnr.close();
    }

Test Class

class TestFile {
   static File f;
 
    public static void main(String[] arg) throws IOException
    {
 
        boolean done = false;
 
        BasicFile f = null;
        BasicFile();
 
 
        String menu = "enter option \n1. Open File \n2 Copy File \n3 Write Output \n4 Get Path Information \n5 get File Size \n6 Read Lines in File";
        System.out.println(menu);
        while(!done)
        {
            Scanner sc = new Scanner(System.in);
            int k = sc.nextInt();
 
            String s = JOptionPane.showInputDialog(menu);
            try
            {
                int i = Integer.parseInt(s);
                switch(i)
                {
                    case 1:
                        f = new BasicFile();
 
                        break;
                    case 2:
                        System.out.println("What is your source file?");
                        File source = new File(sc.next());
                        System.out.println("What is your destination file?");
                        File dest = new File(sc.next());
                        f.copyFile(source,dest);
                        break;
                    case 3:
                        f.writeOutput();
                        break;
                    case 4:
                        f.getAttributes();
                        break;
                    case 5:
                        f.getFileSize();
                        break;
                    case 6:
                        f.fileLineReader();
                        break;
 
 
 
                }
            } catch(NumberFormatException | NullPointerException e)
            {
                display(e.toString(), "Error");
            }
        }
 
    }
    static void display(String s, String err)
    {
        JOptionPane.showMessageDialog(null, s, err, JOptionPane.ERROR_MESSAGE);
    }
 
    public static void BasicFile() {
       JFileChooser choose = new JFileChooser(".");
 
        int status = choose.showOpenDialog(null);
 
        try {
 
            if (status != JFileChooser.APPROVE_OPTION) {
                throw new IOException();
            }
 
            f = choose.getSelectedFile();
 
            if (!f.exists()) {
                throw new FileNotFoundException();
            }
 
        } catch (FileNotFoundException e) {
 
            display(e.toString(), "File not found ....");
 
        } catch (IOException e) {
 
            display(e.toString(), "Approve option was not selected");
 
        }
    }
 
}

any help?

Originally Posted by NormR

Please copy the full text of the error message and paste it here. It has important info about the error.

The line number where the exception happened is shown in the error message. Look at that line and find the variable with the null value. Print all the variables on that line to see which one is null. Then backtrack in the code to find out why that variable does not have a valid value.

For copy files:
java.lang.NullPointerException
at File3.TestFile.main(BasicFile.java:222)

For write output

For get path information:
java.lang.NullPointerException
at File3.TestFile.main(BasicFile.java:228)

For file size:
java.lang.NullPointerException
at File3.TestFile.main(BasicFile.java:231)

For read lines:

java.lang.NullPointerException
at File3.TestFile.main(BasicFile.java:234)