With this MojangRepository you can get a UUID from a Username and other way.

MojangRepository.java

package de.codebucket.accounttool;

import java.util.UUID;

import de.codebucket.utils.NameFetcher;
import de.codebucket.utils.UUIDFetcher;

public class MojangRepository
{    
    public static String getUsername(UUID uuid) throws Exception
	{
		return NameFetcher.getRequest(uuid);
	}
	
	public static UUID getUniqueId(String username) throws Exception
	{
		return UUIDFetcher.getRequest(username);
	}
}