How to create for loop to add elements in an array that requires urls?

Question

I'm stuck on how to put the URLs into an array. I loop through fileNames and concatenate paths, but then I'm stuck on how to loop through the urls array and add what I created with the concatenation. How do I do that?

public class DataManager {

    private String[] fileNames;
    private URL[] urls;
    private ArrayList<String> gameData;
    Scanner s; 

    public DataManager(String[] fileNames, String path) throws IOException {
        this.fileNames = fileNames; 
        this.urls = new URL[fileNames.length];
        for (String file: fileNames) {
            String url = path + file;
            for (int i = 0; i < urls.length; i++) {  
                urls[i] = url;
            }
        }
    }

    public static void main(String[] args) throws IOException {
        String[] fileNames = { "50.csv", "100.csv", "500.csv" }; 
        String path = "http://people.uncw.edu/tompkinsj/331/ch07/";
        DataManager foo = new DataManager(fileNames, path);
    }

    // getters and other methods
}

ppeterka · Accepted Answer · 2016-09-15 07:41:26Z

When such a problem arises, and you have been diving deep into something, and spend too much time with one issue: always take a step back, take a break, do something else, and come back later... It pays off.

Your code is almost right

for (String file: fileNames){                   //logically OK
    String url = path + file;                 
    for (int i = 0; i < urls.length; i++) {     //things start going wrong
        urls[i] = url;                          //this line has a red underline
    }
}

You have 2 problems: a logical one, and a minor issue.

The minor problem:

This line is incorrect:

urls[i] = url;

The array urls is of type URL - which is not a supertype of String, so this line is most likely red in your IDE. To correct it, we need to create an URL instance from the String, and we are lucky: there is a constructor exactly for that: new URL(url). However, this can throw a MalformedUrlException if there is a problem which needs to be caught. For the sake of simpleness, let's just rethrow it as an IllegalStateException

try {
    urls[i] = new URL(url);
} catch (MalformedUrlException e) {
    throw new IllegalStateException("Wrong URL: " + url);
}

The logical problem:

While going through the filenames, on each iteration you also iterate over the whole urls[] array I think you don't want that: each filename will yield one URL - so it should be enough to traverse the filename array once. Also, by setting the value on each iteration - you would overwrite all previous values, and when all the loops complete, all you would be left with is an array filled with the last URL processed. I think the problem you try to solve with the inner loop is that you don't know which index to put the new element into, because you didn't keep track which filename entry you were processing. This is a scenario, where a traditional for loop is more handy than just iterating over the array:

for (int i = 0; i < fileNames.length; i++){
    String file = fileNames[i];
    String url = path + file;                 
    try {
        urls[i] = new URL(url);
    } catch (MalformedUrlException e) {
        throw new IllegalStateException("Wrong URL: " + url);
    }
}

Of course, this can be improved to be a bit shorter:

for (int i = 0; i < fileNames.length; i++){
    try {
        urls[i] = new URL(path + fileNames[i]);
    } catch (MalformedUrlException e) {
        throw new IllegalStateException("Wrong URL: " + path + fileNames[i]);
    }
}

... and of course the exception would need an appropriate handling, but that requires more in-depth knowledge about what the code is trying to do, e.g. is it considered an irrecoverable error if the URL is incorrect, etc.

ppeterka · Accepted Answer · 2016-09-15 08:52:41Z

1

Here's a stream version, which can make the steps clearer:

urls = Arrays.stream(fileNames)
    .map(f -> path + f)   // build full path from path + filename
    .map(this::createUrl) // create URL from full path
    .toArray(URL[]::new); // create and populate an array

private URL createUrl(String url) {
    try {
       return new URL(url);
    } catch (MalformedUrlException e) {
       throw new IllegalArgumentException(url);
    }
}

Streams don't allow exploding methods. You can use massive ugly lambdas, but the helper method makes the stream code much neater.

This will throw an IllegalArgumentException if the full path is not s valid URL; if that is not going to happen you don't need to do anything more. If it could happen and you want to take special action, wrap the stream code line in a try catch.

_{Disclaimer: Code may not compile or work as it was thumbed in on my phone (but there's a reasonable chance it will work)}

edited Sep 15, 2016 at 8:52

ppeterka

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answered Sep 14, 2016 at 20:24

Bohemian♦

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3 Comments

ppeterka Over a year ago

Nope... URL::new throws MalformedUrlException. This would not even compile. This is exactly why I didn't bother going with a Streams solution. This also exactly why I'm frustrated by Streams - this would be soooo elegant - but not possible.

Bohemian Over a year ago

@ppeterka Ah. I just thumbed that in. Nevertheless I have thumbed in a corrected version that should compile.

ppeterka Over a year ago

I feel your pain, coding on a touch screen is a disaster... The drawback is that if MalformedURLException is not considered a fatal problem, it would be quite a task to solve. But that might just be a question of structuring the surrounding code to be lambda and Streams friendly...

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How to create for loop to add elements in an array that requires urls?

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The minor problem:

The logical problem:

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The minor problem:

The logical problem:

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