Code Review - PE.SE

ColorFinder

public class ColorFinder {

    public static void main(String[] args) {
        Set<String> colorSet = new HashSet<String>();

        colorSet.add("red");
        colorSet.add("blue");
        colorSet.add("green");

        ColorFinder colorFinder = new ColorFinder(colorSet);

        System.out.println(colorFinder.findColor("xxred"));
        System.out.println(colorFinder.findColor("greenxx"));
        System.out.println(colorFinder.findColor("xxbluexx"));
    }

    private Set<String> colorSet;

    public ColorFinder(Set<String> colorSet) {
        Set<String> lowerCaseSet = new HashSet<String>(colorSet.size());
        for (String color : colorSet) {
            lowerCaseSet.add(color.toLowerCase());
        }

        this.colorSet = lowerCaseSet;
    }

    public String findColor(String input) {
        if (input == null) {
            throw new IllegalArgumentException("Input can't be null");
        }

        String lowerCaseInput = input.toLowerCase();

        for (String color : colorSet) {
            if (lowerCaseInput.startsWith(color)) {
                int start = lowerCaseInput.indexOf(color);
                int end = start + color.length();
                return lowerCaseInput.substring(start, end);
            }
        }

        return "";
    }

}