How do I write String arrays to a text file in java using Printwriter?

Question

I need help writing some contents of array objects (following are two example of arrays I'm using) to a text file using Printwriter. Any ideas? I'm a beginner, so the simpler the better, thanks!

Astronauts[0][0] = new Passengers(-1, "", 1, 0, 0, "", "", 0, "", "", "", "", ""); 

Astronauts[0][1] = new Passengers(0, "Pilot", 2424, 14, 0, "Bruce", "Banner", 0, "678-884-6325", "Mom", "678-884-6323","","");

Astronauts[0][2] = new Passengers(0, "Pilot", 1248, 3, 0, "Sally", "Forth", 0, "678-921-1135", "Hannah", "678-921-1130","","");


 Astronauts[1][0] = new Passengers(-1, "", 2, 0, 0, "", "", 0, "", "", "", "", "");

Astronauts[1][1] = new Passengers(0, "Pilot", 1022, 55, 0, "Buz", "Aldrin", 0, "404-014-4553", "June", "404-014-4555","","");

Astronauts[1][2] = new Passengers(0, "Pilot", 2813, 8, 0, "Alice", "Dyer", 0, "678-884-6325", "Mom", "678-884-6323","","");

EvenLisle · Accepted Answer · 2015-04-20 14:52:55Z

I'm not sure if I'm catching your problem correctly, because writing the contents of an array to a file is pretty straight-forward:

String[] arr = {"a", "b", "c"};
try {
    PrintWriter pw = new PrintWriter(new FileWriter("output.txt"));
    pw.println(Arrays.toString(arr));
    pw.flush();
    System.out.println("Finished");
} catch (IOException e) {
    e.printStackTrace();
}

[EDIT]

I realise I may not have addressed your entire problem. If you are wondering how to write the desired traits of the objects contained in an array, you can override the toString() method of your custom class:

class A {
    public static void main(String[] args) {
        B[] bs = {new B("a", "b"),
                  new B("c", "d"),
                  new B("e", "f"),
                  new B("g", "h")};

        try {
            PrintWriter pw = new PrintWriter(new FileWriter("output.txt"));
            for (B b : bs) {
                pw.println(b);
            }
            pw.flush();
        } catch (IOException e) {
            e.printStackTrace();
        }
        System.out.println("Finished");
    }
}

class B {
    private String prop1;
    private String prop2;

    public B (String prop1, String prop2) {
        this.prop1 = prop1;
        this.prop2 = prop2;
    }

    @Override
    public String toString() {
        return this.prop1 + " " + this.prop2;
    }
}

ktorn · Accepted Answer · 2015-04-21 05:30:55Z

0

Since your array contents are all of the same class Passengers, if you're not happy with the default Array.toString format I'd create a toString() method in Passengers that returns your desired string representation.

Then:

try {
       PrintWriter printWriter = new PrintWriter(new FileWriter("output.txt"));

       for(Passengers[] passengers: Astronauts) {
           for(Passengers passenger: passengers) {
                printWriter.println(passenger);
           }
       }

       printWriter.close();  // no need to flush, since close() does it anyway.

   } catch (IOException e) {
       e.printStackTrace();
   }

Note: As others have mentioned, I'd rename the Astronauts variable to be lowercase astronauts. I'd also rename the Passengers class to Passenger.

Edit: Using the above code the output file should appear in the working directory where you are running the program from. Alternatively you can supply a full file path such as C:/Users/Me/directory/output.txt but in that case you need to make sure the directory path already exists.

Yet another alternative is to modify the code to automatically create the path for you:

    File file = new File ("C:/Users/Me/directory/output.txt");
    file.getParentFile().mkdirs();
    PrintWriter printWriter = new PrintWriter(file);

edited Apr 21, 2015 at 5:30

answered Apr 20, 2015 at 15:04

ktorn

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2 Comments

Mr. T Over a year ago

ok, so I used this code but adapted it to the whole file and I'm still not seeing a text file. Shouldn't one pop up on its own?

ktorn Over a year ago

Yes the above code should create the file for you. I've edited the answer with some explanation in this regard. Hope it helps.

Arjun Lajpal · Accepted Answer · 2015-04-20 14:57:31Z

Here is how you might want to do it.

String twoDArray[][] = {{"one","two"},{"one","two"},{"one","two"}};
    String filePath = "C:/Users/arjun.lajpal/Desktop/dummyFile.txt";
    PrintWriter writer = null;
    try {
        writer = new PrintWriter(filePath);
    } catch (FileNotFoundException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }

    writer.printf("%20s%20s","Astraunauts","Passengers");
    writer.println();
    for(int i=0;i<twoDArray.length;i++){
        for(int j=0;j<twoDArray[i].length;j++)
            writer.printf("%20s",twoDArray[i][j]);

        writer.println();

        }
    writer.flush();
    writer.close();

This not only will make your file but also write to the file in a nice tabular format without any need to override toString() method.

Albert · Accepted Answer · 2015-04-20 16:31:34Z

0

PrintWriter admits a String, so you can override toString() method in Astronauts class, and then iterate over 1-D or 2-D dimenssional array:

By the way, variable names should start with lowercase character.

1-D

for (int i = 0; i < astronauts.length; i++) { 
    pw.print(Arrays.toString(astronauts[i]); 
}

2-D

for (int i = 0; i < astronauts.length; i++) {
    for (int j = 0; j < astronauts[i].length; i++) {
        pw.print(astronauts[i][j]); 
    } 
}

Don't forget to flush() and close() PrintWriter

edited Apr 20, 2015 at 16:31

answered Apr 20, 2015 at 14:50

Albert

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