Since Litecoin is a fork of bitcoin you can use JavaBitcoindRpcClient to connect with litecoin client and execute API calls.

link:https://github.com/Polve/JavaBitcoindRpcClient

To connect with client, you can use below code :

String user = "RPC_USER_NAME";
String password = "RPC_PASSWORD";
String host = "127.0.0.1";
String port = "PORT";

try {
    URL url = new URL("http://" + user + ':' + password + "@" + host + ":" + port + "/");

    BitcoinJSONRPCClient bitcoinClient = new BitcoinJSONRPCClient(url);
    System.out.println(bitcoinClient.getInfo());
} catch (MalformedURLException e) {
    e.printStackTrace();
}