Java lambda expression filter issue

Question

I need to remove some elements from a list inside an hashmap. Old way works. New way won't work, seems do not remove requested elements.

Logging in Old way I never obtain "find after!", as expected. With New way I obtains some "find after!" :(

Old way:

workingMarketCards.forEach((k, v) -> {
    for (FixedPriceInsertion fpi : v) {
        if (fpi.getMasterId() == oc.getMasterCardId()) {
            System.out.println("found before!");
        }
    }
    ArrayList<FixedPriceInsertion> toBeRemoved = new ArrayList<>();
    for (FixedPriceInsertion fpi : v) {
        if (fpi.getMasterId() == (long) (oc.getMasterCardId())) {
            toBeRemoved.add(fpi);
        }
    }
   
    for (FixedPriceInsertion fpi : toBeRemoved) {
        v.remove(fpi);
    }
    for (FixedPriceInsertion fpi : v) {
        if (fpi.getMasterId() == oc.getMasterCardId()) {
            System.out.println("found after!");
        }
    }
});

New way:

workingMarketCards.forEach((k, v) -> {
    for (FixedPriceInsertion fpi : v) {
        if (fpi.getMasterId() == oc.getMasterCardId()) {
            System.out.println("found before!");
        }
    }
    v.stream().filter(s -> s.getMasterId() == (long) (oc.getMasterCardId()));
    for (FixedPriceInsertion fpi : v) {
        if (fpi.getMasterId() == oc.getMasterCardId()) {
            System.out.println("found after!");
        }
    }
});

Why? What wrong in my code?

.filter() does not remove items from the list, it "returns a stream consisting of the elements of this stream that match the given predicate". look at .removeIf() — MarcoLucidi
– MarcoLucidi, Commented Sep 10, 2020 at 17:26
also use .equals() instead of == when comparing objects. — Bohemian
– Bohemian ♦, Commented Sep 10, 2020 at 17:27

Bohemian · Accepted Answer · 2020-09-10 17:29:24Z

3

You want removeIf():

v.removeIf(s -> s.getMasterId() == (long) (oc.getMasterCardId()));

v.stream().filter() does nothing to v; it only affects the stream of elements generated from v.

It's not clear what type getMasterId() returns, but caution is advised using == for comparing things other than primitives; always use .equals() when comparing Objects.

answered Sep 10, 2020 at 17:29

Bohemian♦

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1 Comment

Xilo Over a year ago

.equals() is a good point, this is a sample code to understood java lambda expressions not a production code. Thanks.

Lukas Horak · Accepted Answer · 2020-09-10 17:30:28Z

1

The filter doesn't modify the values in the variable s. It only send the filtered stream to the next chain methods and since you don't have anything after that it won't do anything.

What you need to do is to use the filter to remove the values and the collect them.

ArrayList<FixedPriceInsertion> v2 = v.stream()
                        .filter(s -> s.getMasterId() != (long) (oc.getMasterCardId()))
                        .collect(Collectors.toList());

answered Sep 10, 2020 at 17:30

Lukas Horak

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