BasicConverter.java

import com.mojang.api.profiles.HttpProfileRepository;
import com.mojang.api.profiles.Profile;
import com.mojang.api.profiles.ProfileCriteria;

public class BasicConverter {

    private static final HttpProfileRepository profileRepository = new HttpProfileRepository();
    private static final String AGENT = "minecraft";

    public static String getUUID(String username) {
        Profile[] profiles = profileRepository.findProfilesByCriteria(new ProfileCriteria(username, AGENT));

        if (profiles.length == 1) {
            return profiles[0].getId();
        } else {
            return null;
        }
    }
}