java 收集列表忽略

Question

I am trying to write lambda in java that filter list by month and add data in the current month to the new list but when I try to collect the data I get an error collect is ignored.

public String getMonthlyExpensesNew() {
        Functions functions = new Functions();
        List<ShoppingMgnt> monthlyData = new ArrayList<>();
        try {
            monthlyData = getRecordsAsList();

            monthlyData.stream().filter(date -> functions.checkForCurrentMonth(date.getPurchaseDate())).collect(Collectors.toList());

        }catch (SQLException sqlException){
            System.err.println("Error in getMonthlyExpensesNew");
        }

        return String.valueOf(monthlyData);
    }


public boolean checkForCurrentMonth(String givenDate){
        LocalDate currentDate = LocalDate.now();
        LocalDate monthToCheck = LocalDate.parse(givenDate);
        return currentDate.getMonth().equals(monthToCheck.getMonth());
    }

Answer 1

Your initial code:

monthlyData.stream()
   .filter(date -> functions.checkForCurrentMonth(date.getPurchaseDate()))
   .collect(Collectors.toList());

Within this line the collect operation returns a List . You should store this List into your monthlyData reference to be returned later. So you should write like this:

monthlyData = monthlyData.stream()
    .filter(date -> functions.checkForCurrentMonth(date.getPurchaseDate()))
    .collect(Collectors.toList());

Answer 2

your final function will be as follow:

public String getMonthlyExpensesNew() {
    Functions functions = new Functions();
    List<ShoppingMgnt> monthlyData = new ArrayList<>();
    try {
        monthlyData = getRecordsAsList();
        //put the returned list in the same defined list
        monthlyData = monthlyData.stream()
        .filter(date -> functions.checkForCurrentMonth(date.getPurchaseDate()))
        .collect(Collectors.toList());

    }catch (SQLException sqlException){
        System.err.println("Error in getMonthlyExpensesNew");
    }
    //the return with updated list  
    return String.valueOf(monthlyData);
}