Making multiple objects in a for-loop

Question

I have code in Java that should make multiple objects within a for-loop, then add each object into an array. However, the code just copies the same (which is the last in the for-loop) object into each index i in the array once the loop ends.

How can I correct this to have each separate object in its correct index in the array?

I can post code if necessary, but it would be nice if someone could give me an example of how this would work.

for (int i=0; i<file.listFiles().length; i++) {
        if (fileList[i].isFile() && !fileList[i].isHidden() && fileList[i].getName().substring(fileList[i].getName().length()-4).equalsIgnoreCase(".mp3") && !fileList[i].equals(null)) {
            try {

                songs.add(new Song(fileList[i]));


                //System.out.println(songs[i].getTitle());
                //playlistInfo[i] = fileList[i].getName();


            } catch (IOException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            } catch (TagException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
        }
    }

Thanks a ton!

Here is where I print some object info to the console.

System.out.println(getSong(1).getTitle());
System.out.println(getSong(4).getTitle());

Answer 1

Should be as simple as

for (int ii = 0; ii < yourArray.length; ii++) {
    yourArray[ii] = new YourObject();
}

We can't tell you what the problem is without seeing your code.

Answer 2

for (int i=1; i<file.listFiles().length-1; i++) { 
        if (fileList[i].isFile() && !fileList[i].isHidden() && fileList[i].getName().substring(fileList[i].getName().length()-4).equalsIgnoreCase(".mp3")) {
            try {

                Song s = new Song(fileList[i]);
                songs[i] = s;   

            } catch (IOException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            } catch (TagException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
            // Make counter to determine size of playlistInfo array
            // Use Comparator to sort
        }
    }

Answer 3

Your code

int i=1; i<file.listFiles().length-1

Should probably be

int i=0; i<file.listFiles().length

You are starting from the second element in your array (since the first one is zero) and ending one early because the '<' and the '-1' each make you stop one short of .length, for a total of 2 short (and since the array is zero based, you only want to stop one short).

Answer 4

You will have empty items on your songs array if your if conditions don't evaluate to true. I would recommend using an ArrayList class and add the items to there as you find them:

List<Song> songs = new ArrayList<Song>();
for (int i=1; i<file.listFiles().length-1; i++) {
    if (fileList[i].isFile() && !fileList[i].isHidden() && fileList[i].getName().substring(fileList[i].getName().length()-4).equalsIgnoreCase(".mp3")) {
        songs.add(new Song(fileList[i]));
    }
}

I would also recommend that you:

• Use the method File.listFiles to get the files that match a given criteria; You have a bunch of pre-defined filters in the package org.apache.commons.io.filefilter in the Commons IO project;
• Check the file name length before making your comparison or using the FilenameUtils.getExtension method in the Commons IO library to get the file extension (you may think it is overkill, but...);
• Using lists instead of arrays;
• I don't think your instantiation of the Song class is prone to generate exceptions. A try...catch block in a higher level could be more appropriate. And your treatment to the exception is completely wrong, if you won't do more to it than simply printing its stack trace, it is better not to catch it.

About your problem, I am not sure how all the generated objects would be the same, except if your fileList array contains the same element in all positions.

Answer 5

The code that you are using to create Song objects and assign then to songs is basically correct. Your problem is elsewhere:

• The for statement is almost certainly incorrect, the index of the first element of an array is 0 and the index of the last element is array.length - 1. You are iterating from 1 to array.length - 2.

• If you get a NullPointerException in the loop it is most likely because either fileList is null or it contains a null element.

• If the fileList array contains directories, hidden files or files that don't match, then the songs array will have a null at that the corresponding position. THAT could be the cause of NullPointerExceptions ... if you don't check before printing the array entries.

And I'd note that this code:

System.out.println(getSong(1).getTitle());
System.out.println(getSong(4).getTitle());

will not tell you that the Song objects are the same or different. Rather it will just tell you if the two Song objects have titles with the same characters.

This will tell you if the Song objects are different:

if (getSong(1) != getSong(4)) {
    System.err.println("They are different");
}

Answer 6

I've corrected the issue. A few of my class variables were static, which was causing the problem of creating new objects in each index in my arraylist. I apologize for any troubles this has caused anyone.

Thanks for everyone's help! :]