Java 8 Stream Filter returning an empty list always [duplicate]

Question

I want to filter all the transfers for which the signedOn field is null. When I run the first piece of code it's clear which ones are null and which ones are not and they're logged correctly, but when I run the stream filter it returns an empty list and I can't seem to find what's the problem with it... signedOn is a Date field.

This works and logs all wether the entries are null or not:

    for (Transfer transfer : route.getTransferCollection()) {
        if (transfer.getSignedOn() == null) {
            logInfo("This transfer is null");
        } else if (transfer.getSignedOn() != null) {
            logInfo("This transfer is not null");
        }
    }

This returns an empty list:

    return route.getTransferCollection()
            .stream()
            .filter(transfer -> transfer.getSignedOn() == null)
            .collect(Collectors.toList());
}

Answer 1

You can log your statements inside the filter itself

 return route.getTransferCollection()
        .stream()
        .filter(transfer -> {
                boolean test = transfer.getSignedOn() == null;
                if(test){
                   // log wathever
                } else {
                   // log diff
                }
                return test
               })
        .collect(Collectors.toList());

You can use peek to log inside streams btw, but using it in your case would require to test against null twice.

You can also use a different collector may be to get the ones that are null and the ones that are not (without filter):

.collect(Collectors.partitioningBy(transfer -> transfer.getSignedOn() == null))