Populate a string array from list array in Java

Question

This is my first foray into Java, so forgive me if this is simple, but I am having difficulty iterating through a list and populating a string array with the list items.

Specifically, I am using Jsoup to parse a url and extract certain elements:

Document doc = Jsoup.connect(content).get();
Elements threads = doc.getElementsByClass("page-link");
for (Element src : threads){
    String title = src.text();
    String href = src.attr("href");
        THREADS[0] = title;
}

THREADS is a string array:

static final String[] THREADS = new String[] {};

I don't seem to be able to iterate through the Elements array and populate THREADS with the title values. Leaving the THREADS[0] index in the above example, successfully pushes the final title value into the string[], as expected. But using a for(i=0;i<25;i++) type of loop around the THREADS[i] = title; statement causes a force close in the Android application.

Any tips would be awesome.

Matt Ball · Accepted Answer · 2011-03-11 18:37:21Z

3

You've created a zero-element array, and arrays are not resizable. Is there any particular reason that you're not just using a List<String>?

static final List<String> THREADS = new ArrayList<String>();

// ...

for (Element src : threads){
    String title = src.text();
    String href = src.attr("href");
        THREADS.add(title);
}

answered Mar 11, 2011 at 18:37

Matt Ball

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4 Comments

extraneon Over a year ago

possibly finishing with THREADS.toArray(new String[0]) to make an array at the end. If you really need an array.

aperture Over a year ago

Thanks, this works. However, every time I call "static final List<String> THREADS = new ArrayList<String>();" it's not re-initializing as an empty list, and so THREADS.add() pushes into the previously created List. I know this is probably better served as a different question, but my unfamiliarity with Java leaves me wondering if there is a simple method to ensure that THREADS is a initialized as an empty list every time it is called?

Matt Ball Over a year ago

@aperture: every time what is called? Do you understand what static does?

aperture Over a year ago

The Jsoup.connect().get(); is called from a java class which receives its URL variable from an onclick event in another class. So, ideally, every time the onclick event is fired, THREADS gets populated with the new Elements. What it's doing now is appending the new Elements on to the end of the previously called set. I am reading some documentation on static and final and what each implies and looking for what to change to achieve the desired result -- apparently, simply removing "static" was enough. Thanks

Johan Sjöberg · Accepted Answer · 2011-03-11 18:42:51Z

2

You have to initialize your array:

static final String[] THREADS = new String[25];

Sure you don't actually mean for(i=0;i>25;i++) but desire for(i=0;i<25;i++), see the differences in < >.

Or even better, as @Matt Ball suggested use a List<String>.

edited Mar 11, 2011 at 18:42

answered Mar 11, 2011 at 18:37

Johan Sjöberg

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Comments

pcdavid · Accepted Answer · 2011-03-11 18:52:23Z

The size of Java arrays is fixed on creation: they do not grow automatically when you add elements in them.

If you know beforehand the number of elements you will put into it, you can create the array with that size. In your case, something like:

String[] THREADS = new String[threads.size()];
int i = 0;
for (Element src : threads) {
  String title = src.text();
  String href = src.attr("href");
  THREADS[i++] = title;
}

Using arrays is not very idiomatic in Java however, as they are not very flexible (they have other issues with typing). It is more common to use one of the many collection types from java.util. The List interface and its ArrayList implementation provide the "growable array of elements" that you want:

List<String> THREADS = new ArrayList<String>();
for (Element src : threads) {
  String title = src.text();
  String href = src.attr("href");
  THREADS.add(title);
}

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