How to correctly initialize a Comparator?

Question

I need to write a static method in a class MinTester that computes the "smallest" string from an ArrayList collection using a comparator object:

public static String min(ArrayList<String> list, Comparator<String> comp)

I cannot use the Collections class to compute the minimum.

Here is what I have so far.

public class MinTester 
{
    public static String min(ArrayList<String> list, Comparator<String> comp)
    {
        String shortest = list.get(0);

        for(String str : list) {
            if ( comp.compare(str, shortest) < 0) {
                shortest = str;
            }
        }
        return shortest;
    }
}

I am not getting any errors here from the method, So I try to test it in Main with this. I get this error when trying to pass comp: Variable comp may not have been initialized

public static void main(String[] args)
{
    // TODO code application logic here

    MinTester s = new MinTester();
    Comparator<String> comp;
    ArrayList<String> list = new ArrayList<>();

    list.add("a");
    list.add("ab");
    list.add("abc");
    list.add("abcd");

    String a = s.min(list,comp);//Error: Variable comp may not have been initialized

    System.out.println(a);
}

Heres where I run into my problem.

I try

Comparator<String> comp = new Comparator<>();//Error:Comparator is abstract, cannot be instantiated
Comparator<String> comp = new MinTester();//Error: MinTester cannot be converted to Comparator<String>

Can anyone tell me the proper way to handle this Comparator? Im not sure if Im just trying to initialize it incorrectly, or if I'm missing something in my MinTester class.

Answer 1

You should write a class that implements Comparator<String> for this. A quick approach using anonymous class:

String a = s.min(list, new Comparator<String>() {
    @Override
    public int compare(String s1, String s2) {
        return s1.compareTo(s2);
    }
});

Since you need to compare based on String length, just change the comparison logic in the compare method:

String a = s.min(list, new Comparator<String>() {
    @Override
    public int compare(String s1, String s2) {
        return (s1.length() > s2.length()) ? 1 : (s1.length() < s2.length()) ? -1 : 0;
    }
});

If you happen to use Java 7, then use Integer#compare:

String a = s.min(list, new Comparator<String>() {
    @Override
    public int compare(String s1, String s2) {
        return Integer.compare(s1.length(), s2.length());
    }
});

If you use Java 8, you can use a lambda expression:

String a = s.min(list, (s1, s2) -> Integer.compare(s1.length(), s2.length()));