Java将Arraylist拆分为较小的ArrayList

Question

I have an ArrayList with a size of 258

Now i wish to split this into three different ArrayLists for this i have created the following code:

    Integer start = (int) Math.floor((sitesToSearch.size() / 3));
    Integer middle = (int) Math.floor((sitesToSearch.size() / 2));
    Integer end = (int) Math.floor((sitesToSearch.size() / 1));

    ArrayList<String> crawl_list1 = (ArrayList<String>)tmp.subList(0, start);
    ArrayList<String> crawl_list2 = (ArrayList<String>)tmp.subList(start+1, middle);
    ArrayList<String> crawl_list3 = (ArrayList<String>)tmp.subList(middle+1, end);

Sadly this throws the following error:

Exception in thread "main" java.lang.ClassCastException: java.util.ArrayList$SubList cannot be cast to java.util.ArrayList

So how can i devide it into three smaller ArrayList

tmp declaration:

public ArrayList<String> getExternalLinks(ArrayList<String> rootDomains){
ArrayList<String> result = new ArrayList<String>();

Document doc = null;
/*
 * Check if root is valid
 * Find search the site for internal links
 */
for (String root : rootDomains) {
    if (!(root == null) || !root.isEmpty() ||!root.contains("#")) {
        try {
            doc = Jsoup.connect(root)
                    .userAgent("Mozilla")
                    .get();
            result.addAll(findExternalLinks(findInternalLinks(doc,root),root)); 
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }
}
System.out.println(result.size());
return result;

}

Answer 1

Simply use List<String> as the type of crawl_list1 .

That's just an application of the general rule of "code against the interface" .

There's really no good reason to require the return value of subList to be an ArrayList (and it doesn't make sense as subList returns a view onto the original ArrayList ).

If you absolutely need ArrayList objects, then you need to copy the content into new ArrayList objects:

ArrayList<String> crawl_list1 = new ArrayList<String>(tmp.subList(0, start));
ArrayList<String> crawl_list2 = new ArrayList<String>(tmp.subList(start+1, middle));
ArrayList<String> crawl_list3 = new ArrayList<String>(tmp.subList(middle+1, end));

Answer 2

Look at the javadoc for ArrayList.subList() . It doesn't return an ArrayList . It returns a List . There's no reason to cast this list to an ArrayList. It's a list, and that's all you need to know:

List<String> crawl_list1 = tmp.subList(0, start);
List<String> crawl_list2 = tmp.subList(start+1, middle);
List<String> crawl_list3 = tmp.subList(middle+1, end);

Also, you should check your indices, because the end index passed to subList is exclusive.

Answer 3

.subList(fromIndex, toIndex) is inclusive for fromIndex and exclusive for toIndex. With the current code middle and start will never populate an ArrayList.

The below two are assumptions of the logic:

.subList(start, middle)

.subList(middle, end+1)