This snippet is a part from my MojangRepository.

UUIDFetcher.java

package de.codebucket.utils;

import java.io.BufferedReader;
import java.io.DataOutputStream;
import java.io.IOException;
import java.io.InputStreamReader;
import java.net.HttpURLConnection;
import java.net.Proxy;
import java.net.URL;
import java.util.UUID;

import org.json.simple.JSONObject;
import org.json.simple.parser.JSONParser;

import com.google.gson.Gson;


public class UUIDFetcher 
{
	private static final double MAX_REQUEST = 100;
	private static final String REQUEST_URL = "http://mcapi.sweetcode.de/api/v2/?uuid&request=";
	private static final String PROFILE_URL = "https://api.mojang.com/profiles/page/";
	
	public static UUID getRequest(String username)
	{
		try 
		{
			JSONParser jsonParser = new JSONParser();
			HttpURLConnection connection = (HttpURLConnection) new URL(REQUEST_URL + username).openConnection();
			JSONObject response = (JSONObject) jsonParser.parse(new InputStreamReader(connection.getInputStream()));	
			
			String name = (String) response.get("username");
			if (name.length() == 0) 
			{
				throw new IllegalArgumentException("A UUID for Username '" + username + "' was not found in the Mojang database! Is the account not premium?");
			}
			
			return fromString((String) response.get("uuid"));
		}
		catch (Exception e)
		{
			return callMojang(username);
		}
	}
	
	private static UUID callMojang(String username)
	{
		try 
		{
			Gson gson = new Gson();
			ProfileData data = new ProfileData(username);
			String uuid = null;
			for (int i = 1; i < MAX_REQUEST; i++) 
			{
				PlayerProfile[] result = post(new URL(PROFILE_URL + i), Proxy.NO_PROXY, gson.toJson(data).getBytes());
				if (result.length == 0) 
				{
					break;
				}
				uuid = result[0].getId();
			}
			return fromString(uuid);
		} 
		catch (Exception e) 
		{
			throw new IllegalArgumentException("A UUID for Username '" + username + "' was not found in the Mojang database! Is the account not premium?");
		}
	}
	
	private static UUID fromString(String uuid)
	{
		return UUID.fromString(uuid.substring(0, 8) + "-" + uuid.substring(8, 12) + "-"
				+ uuid.substring(12, 16) + "-" + uuid.substring(16, 20) + "-" + uuid.substring(20, 32));
	}
	
	private static PlayerProfile[] post(URL url, Proxy proxy, byte[] bytes) throws IOException
	{
        HttpURLConnection connection = (HttpURLConnection) url.openConnection(proxy);
        connection.setRequestMethod("POST");
        connection.setRequestProperty("Content-Type", "application/json");
        connection.setDoInput(true);
        connection.setDoOutput(true);

        DataOutputStream out = new DataOutputStream(connection.getOutputStream());
        out.write(bytes);
        out.flush();
        out.close();

        BufferedReader reader = new BufferedReader(new InputStreamReader(connection.getInputStream()));
        StringBuffer response = new StringBuffer();
        String line;
        while ((line = reader.readLine()) != null)
        {
                response.append(line);
                response.append('\r');
        }
        reader.close();
  
        return new Gson().fromJson(response.toString(), SearchResult.class).getProfiles();
	}
	
	private static class PlayerProfile 
	{
		private String id;

		public String getId() 
		{
			return id;
		}
	}

	private static class SearchResult
	{
		private PlayerProfile[] profiles;

		public PlayerProfile[] getProfiles() 
		{
			return profiles;
		}
	}

	@SuppressWarnings("unused")
	private static class ProfileData 
	{
		private String name;
		private String agent = "minecraft";

		public ProfileData(String name) 
		{
			this.name = name;
		}
	}
}

Would be great if you could update the old API endpoint with the new one (http://mcapi.de/user/<uuid|username>. :)