import java.util.Arrays;
import java.util.Collections;
import java.util.HashSet;
import java.util.LinkedList;
import java.util.List;
import java.util.regex.Pattern;

public class CollectionExample {
    public static void main(String[] args) {
        new CollectionExample().run();
    }

    private void run() {
        List<String> sortedNewsgroupNames = getGroupNames();
        for (String name: sortedNewsgroupNames) {
            System.out.println("All: " + name);
        }

        List<String> filterdGroupNames = filter(new String[] {"de.alt.foo"}, "", getGroupNames());
        for (String name: filterdGroupNames) {
            System.out.println("Filtered: " + name);
        }
    }

    private List<String> getGroupNames() {
        String[] loadedNewsgroupNames = loadGroupNames();
        List<String> sortedNewsgroupNames = new LinkedList<String>(new HashSet<String>(Arrays.asList(loadedNewsgroupNames)));
        Collections.sort(sortedNewsgroupNames);
        return sortedNewsgroupNames;
    }

    private String[] loadGroupNames() {
        return new String[] {"xyz", "de.alt.foo", "de.alt.bar", "comp.alt.science"};
    }

    private List<String> filter(final String[] selectedGroups, final String filterInput, final List<String> sortedGroupNames) {

        StringBuilder selectedGroupsSB = new StringBuilder("^(");
        for (String group: selectedGroups) {
            selectedGroupsSB.append(group).append('|');
        }
        Pattern selectedGroupsPattern = null;
        if (0 < selectedGroups.length) {
            selectedGroupsSB.deleteCharAt(selectedGroupsSB.length() - 1);
            selectedGroupsSB.append(")$");
            selectedGroupsPattern = Pattern.compile(selectedGroupsSB.toString());
        }
        Pattern filterInputPattern = null;
        if (null != filterInput && !filterInput.isEmpty()) {
            filterInputPattern = Pattern.compile(filterInput);
        }

        List<String> visibleGroupNames = new LinkedList<String>();
        for (String group: sortedGroupNames) {
            if (null != selectedGroupsPattern) {
                if (!selectedGroupsPattern.matcher(group).matches()) {
                    continue;
                }
            }
            if (null != filterInputPattern) {
                if (!filterInputPattern.matcher(group).find()) {
                    continue;
                }
            }
            visibleGroupNames.add(group);
        }

        return visibleGroupNames;
    }
}