public String[] invalidTransaction (String[] vTransaction) { List<Transaction> list = new ArrayList<>(); List<Transaction> result = new ArrayList<>(); for (int i = 0; i < vTransaction.length; i++) { String[] split = vTransaction[i].split(","); Transaction transaction = new Transaction(split[0], Integer.parseInt(split[1]), Integer.parseInt(split[2]), split[3]); list.add(transaction); } Map<String, List<Transaction>> collect = list.stream().collect(groupingBy(Transaction::getName));