example.java

private Page<GroupedCustomerTransaction> groupCustomerTransactionRecordByMonthOfYear(Page<CustomerTransactionDTO> customerTransactionPage) {
        List<CustomerTransactionDTO> customerTransactions = customerTransactionPage.getContent();
        Map<String, List<CustomerTransactionDTO>> store = new HashMap<>();
        DateTimeFormatter dateTimeFormatter = DateTimeFormatter.ofPattern("MMMM-yyyy");

        customerTransactions.stream().forEach(customerTransaction -> {
            String createdYearMonth = dateTimeFormatter.format(customerTransaction.getCreatedDate());
            if(store.containsKey(createdYearMonth)){
                store.get(createdYearMonth).add(customerTransaction);
            }else{
                List<CustomerTransactionDTO> list = new ArrayList<CustomerTransactionDTO>(){};
                list.add(customerTransaction);
                store.put(createdYearMonth, list);
            }
        });

        List<GroupedCustomerTransaction> groupedCustomerTransactions =  buildCustomerTransactionGroup(store);
        return new PageImpl<>(groupedCustomerTransactions, customerTransactionPage.getPageable(), customerTransactionPage.getTotalElements());
    }

    private List<GroupedCustomerTransaction> buildCustomerTransactionGroup(Map<String, List<CustomerTransactionDTO>> store) {

        return store.keySet().stream().map(key ->
                GroupedCustomerTransaction.builder()
                        .date(key)
                        .customerTransactions(store.get(key))
                        .build()).collect(Collectors.toList());
    }