Getting Incompatible types error while trying to map a list

Martin :

I have a FeeAccount list that I would like to fill. I want to use .stream.map() to get it done. What I've managed to do is to make a method that would map my list and return it. I've written this code using some other examples I have found online. My problem is that somehow it returns a list that is incompatible with List.

I am getting an error: Incompatible types. Required List but 'map' was inferred to Stream: no instance(s) of type variable(s) R exist so that Stream conforms to List

As I understand the problem is with the part where I use collect(Collectors.toList()). But I am not sure. I don't even clearly understand what the error message means.

Maybe someone can explain what am I doing wrong? Is it with the .stream.map()? Because I never used it before. Or maybe the problem is somewhere else.

Method(List<contract> contractList){
 List<FeeAccount> feeAccounts = new ArrayList<>();

    feeAccounts = contractList
            .stream()
            .map(contract -> {

                List<Fee> monthlyFees=...;

                return monthlyFees.stream()
                        .map(monthlyFee -> {
                            FeeAccount account = new FeeAccount();
                            account.setFeeCode(monthlyFee.getFeeCode());
                            account.setDebtorAccount(contract.getDebtorAccount());
                            return account;
                        }).collect(Collectors.toList());
            });}

Eran :

You have two nested map operations. The outer transforms a contract to a List<FeeAccount>, and the inner transforms a Fee to a FeeAccount.

Hence, your pipeline results in a Stream<List<FeeAccount>> without a terminal operation.

If you add a .collect(Collectors.toList()) in the end, you'll get a List<List<FeeAccount>>.

If you want to merge all those inner lists into a single output list, you should use flatMap.

To obtain a flat List:

List<FeeAccount> feeAccounts = 
    contractList.stream()
                .flatMap(contract -> {
                    List<Fee> monthlyFees=...;
                    return monthlyFees.stream()
                                      .map(monthlyFee -> {
                                          FeeAccount account = new FeeAccount();
                                          account.setFeeCode(monthlyFee.getFeeCode());
                                          account.setDebtorAccount(contract.getDebtorAccount());
                                          return account;
                                      });
                })
                .collect(Collectors.toList();