RpcClient client = new RpcClient(Cluster.TESTNET);

PublicKey fromPublicKey = new PublicKey("QqCCvshxtqMAL2CVALqiJB7uEeE5mjSPsseQdDzsRUo");
PublicKey toPublickKey = new PublicKey("GrDMoeqMLFjeXQ24H56S1RLgT4R76jsuWCd6SvXyGPQ5");
int lamports = 3000;

Account signer = new Account(secret_key);

Transaction transaction = new Transaction();
transaction.addInstruction(SystemProgram.transfer(fromPublicKey, toPublickKey, lamports));

String signature = client.getApi().sendTransaction(transaction, signer);

RpcClient client = new RpcClient(Cluster.TESTNET);

long balance = client.getApi().getBalance(new PublicKey("QqCCvshxtqMAL2CVALqiJB7uEeE5mjSPsseQdDzsRUo"));