Something about array and file processing

Question

I am a java beginner... Now I am writing a program which, when you input a number k, it shows the name of the kth image in the folder "Image". But now when I input any number, the result would always be null.

I need some help;(

Here is my code:

public class Main {
    //create array, total length is number of images in file "Image"
    static int length = (new File("/Users/Sam/Desktop/Image").listFiles().length)-1;
    static String[] myArray = new String[length];   

    public static void listFilesForFolder(final File folder) {
    //put every file name into the array    
        for (final File fileEntry : folder.listFiles()) {
         int i = 0;
            myArray[i] = fileEntry.getName();           
        i++;
        }
    }

    public static void main(String[] args){

        final File folder = new File("/Users/Sam/Desktop/Image");
        listFilesForFolder(folder);

    //input a number x
        Scanner k = new Scanner(System.in);
        System.out.println("Choose a frame: ");

        int a=k.nextInt();
    //show the xth image's name
        System.out.println(myArray[a]);     
        k.close();      
    }

}

Try doing some debugging, Print out the value of length and the array returned by folder.listFiles(). You said: "it shows the name of the kth image in the folder "Image". But now ... " What did you change to make the code fail now after it used to work? — NormR
– NormR, Commented Oct 26, 2015 at 0:04
myArray = folder.listFiles()? Then simply use myArray[a].getName() instead? — MadProgrammer
– MadProgrammer, Commented Oct 26, 2015 at 0:05

Andy Turner · Accepted Answer · 2015-10-26 00:13:38Z

2

I'm assuming that you intend i to be incremented so that myArray gets filled with the names:

public static void listFilesForFolder(final File folder) {
//put every file name into the array    
    int i = 0;  // Declare outside the loop.
    for (final File fileEntry : folder.listFiles()) {
        myArray[i] = fileEntry.getName();           
    i++;
    }
}

Mind you, this will give an ArrayIndexOutOfBoundsException, because you've initialized myArray to have one element too few:

static int length = (new File("/Users/Sam/Desktop/Image").listFiles().length)-1;

You don't really want to use an array for this, since arrays have a fixed size. A List (e.g. ArrayList) would be a much better fit, and you wouldn't need to worry about indices and lengths:

static List<String> myList = new ArrayList<>();

public static void listFilesForFolder(final File folder) {
//put every file name into the list
    for (final File fileEntry : folder.listFiles()) {
        myList.add(fileEntry.getName());           
    }
}

edited Oct 26, 2015 at 0:13

answered Oct 26, 2015 at 0:02

Andy Turner

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1 Comment

SamW Over a year ago

Although I have never used Lists before, but after I went through some documentations this actually works better than array! Thank you very much xP

Tsung-Ting Kuo · Accepted Answer · 2015-10-26 00:19:53Z

1

Please try the following code (I have tested it). There are two main points:

Move the int i = 0; before the for loop, to avoid setting i to 0 in every loop.

Remove the "-1" while declaring length, which is not correct and will create ArrayIndexOutOfBoundsException.

public class Main {
    // create array, total length is number of images in file "Image"
    static int length = (new File("/Users/Sam/Desktop/Image").listFiles().length);
    static String[] myArray = new String[length];

    public static void listFilesForFolder(final File folder) {
        // put every file name into the array
        int i = 0;
        for (final File fileEntry : folder.listFiles()) {
            myArray[i] = fileEntry.getName();
            i++;
        }
    }

    public static void main(String[] args) {

        final File folder = new File("/Users/Sam/Desktop/Image");
        listFilesForFolder(folder);

        // input a number x
        Scanner k = new Scanner(System.in);
        System.out.println("Choose a frame: ");

        int a = k.nextInt();
        // show the xth image's name
        System.out.println(myArray[a]);
        k.close();
    }
}

edited Oct 26, 2015 at 0:19

answered Oct 26, 2015 at 0:05

Tsung-Ting Kuo

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5 Comments

Pinkie Swirl Over a year ago

Please add an explanation why it works (and what the differences are). Otherwise the answer might be useless for learning or new users that have a similar problem.

Tsung-Ting Kuo Over a year ago

@PinkieSwirl Thanks for reminding! I have added the explanations!

Pinkie Swirl Over a year ago

No problem :) Now myArray[a] might still cause an ArrayIndexOutOfBoundsException since a is not check to be >= 0 and < myArray.length (but maybe that is ok if correct input can be assumed) and the formatting is a bit off.

Tsung-Ting Kuo Over a year ago

@PinkieSwirl Yes you are absolutely right! I adjusted the format, and maybe SamW could try to add some code to check the array size? :-)

SamW Over a year ago

Thanks a lot! This really helps me ;)

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