Question

I am trying to write a quick search that searches a List<String>
Instead of looping through the list and manually checking, I want to do this using binarySearch, but I am not sure how to do it.

Old way:

for(String s : list) {
  if(s.startsWith("contact.")
     return true;
}

Instead I would like something like this:

Collections.sort(list);
Collections.binarySearch(list, FindContactComparator());

Can someone help me write this Comparator?
Is there any better way of doing this instead of using binarySearch?

Answer 1

This should work:

        Comparator<String> startsWithComparator = new Comparator<String>() {
            public int compare(String currentItem, String key) {
                if(currentItem.startsWith(key)) {
                    return 0;
                }
                return currentItem.compareTo(key);
            }
        };

int index = Collections.binarySearch(items, "contact.", startsWithComparator);

However sorting and then binary searching is less efficient than the single pass iteration.

Addendum:

Though the above answer helps you, here is another way (inspired from Scala, Google Collections) :

List<String> items = Arrays.asList("one", "two", "three", "four", "five", "six");
int index = find(items, startsWithPredicate("th"));
System.out.println(index);


public static Predicate<String> startsWithPredicate(final String key) {
    return new Predicate<String>(){
        @Override
        public boolean apply(String item) {
            return item.startsWith(key); 
        }
    };
}

public static <T> int find(Collection<T> items, Predicate<T> predicate) {
    int index = 0;
    for(T item: items) {
        if(predicate.apply(item)) {
            return index;
        }
        index++;
    }
    return -1;
}

interface Predicate<T> {
    boolean apply(T item);
}

Here the thing is the find() method is not tied with your ‘matching’ logic; it just finds an element that satisfies the predicate. So you could pass on a different implementation of predicate, for ex. which can check ‘endsWith’ to find() method and it would return the found item which ends with a particular string. Further the find() method works for any type of collection; all it needs is a predicate which transforms an element of collection element type to a boolean. This multiple lines of code around a simple logic also show the Java’s lack of support for first class functions.