How to conditionally modify a Map in Java 8 stream API?

Question

I am trying to modify a Map's keys based on conditional logic and struggling. I'm new to Java 8 streams API. Let's say I have a map like this:

Map<String, String> map = new HashMap<>();

map.put("PLACEHOLDER", "some_data1");
map.put("Google", "some_data2");
map.put("Facebook", "some_data3");
map.put("Microsoft", "some_data4");

When I would like to do is find the references of PLACEHOLDER and conditionally change that key to something else based on a boolean condition. I feel like it should be something like the below, but this doesn't even compile of course.

boolean condition = foo();

map = map.entrySet().stream().filter(entry -> "PLACEHOLDER".equals(entry.getKey()))
        .map(key -> {
            if (condition) {
                return "Apple";
            } else {
                return "Netflix";
            }
        }).collect(Collectors.toMap(e -> e.getKey(), Map.Entry::getValue));

I found this question which kind of makes me think maybe I can't do this with Java 8 stream APIs. Hopefully someone better at this than me knows how to do this. Ideone link if you want to play with it.

Can you please clarify more, do you want to change the key or the value? — Youcef LAIDANI, Jul 21 '18 at 20:02
Could you please edit your question in that case - "_... and conditionally change it's value to something else_" is what was causing confusion. Normally that would mean "change the value associated with that key". — Boris the Spider, Jul 21 '18 at 20:06
Let us know if you solved your problem. If you did, please [accept](https://stackoverflow.com/help/someone-answers) the most helpful answer, or add your own answer explaining how you solved it. — Tomasz Linkowski, Jul 28 '18 at 11:47

Boris the Spider · Answer 1 · 2018-07-21T19:58:02.910

You've filtered out all elements that aren't PLACEHOLDER. You need to add that filter logic to your map operation:

final Map<String, String> output = input.entrySet().stream()
        .map(e -> {
            if (!e.getKey().equals("PLACEHOLDER")) {
                return e;
            }
            if (condition) {
                return new AbstractMap.SimpleImmutableEntry<>("Apple", e.getValue());
            }
            return new AbstractMap.SimpleImmutableEntry<>("Netflix", e.getValue());
        }).collect(toMap(Map.Entry::getKey, Map.Entry::getValue));

But as you are guaranteed to only have a single instance of PLACEHOLDER in the Map, you can just do

String placeholderData = input.remove("PLACEHOLDER");
if (placeholderData != null) {
    input.put(condition ? "Apple" : "Netflix", placeholderData);
}

score 5 · Answer 2 · answered Jul 21 '18 at 19:58

If you really want to do it using Streams, you just need to move the conditional logic to the collection phase, like that:

boolean condition = true;
map.entrySet().stream().collect(Collectors.toMap(
        entry -> mapKey(entry.getKey(), condition), Map.Entry::getValue
));

where:

private static String mapKey(String key, boolean condition) {
    if (!"PLACEHOLDER".equals(key)) {
        return key;
    }
    if (condition) {
        return "Apple";
    } else {
        return "Netflix";
    }
}

However, the second part of Boris the Spider's answer using Map.remove and Map.put seems the best way to go.

I think your approach of doing the magic in the key mapping function is neater than my `map` based approach. — Boris the Spider, Jul 21 '18 at 20:09

How to conditionally modify a Map in Java 8 stream API?

2 Answers2