Java 8: Change the values of an EntrySet stream

Question

I have the following setup:

Map<Instant, String> items;
...
String renderTags(String text) {
    // Renders markup tags in a string to human readable form
}
...
<?> getItems() {
    // Here is where I need help
}

My problems is, the strings that are the values of the items map are marked up with tags. I want getItems() to return all the items, but with the strings parsed using the renderTags(String) method. Something like:

// Doesn't work
items.entrySet().stream().map(e -> e.setValue(renderTags(e.getValue())));

What is the most effective way to do this?

Solution 1

If you want a Map as result:

Map<Instant, String> getItems() {
    return items.entrySet()
            .stream()
            .collect(Collectors.toMap(
                    Map.Entry::getKey,
                    e -> renderTags(e.getValue())));
}

Solution 2

If you want to modify an existing map instead of generating the new one (as in your example), there's no need to use the stream at all. Use Map.replaceAll:

items.replaceAll((k, v) -> renderTags(v));
return items;

If you want to keep the original map unchanged, consult other answers.

Solution 3

You can try it this way with Collectors.toMap():

Map<Instant, String> getItems() {
    return items.entrySet().stream()
                .collect(Collectors.toMap(
                            Map.Entry::getKey,
                            entry -> renderTags(entry.getValue())
                         ));
}

By the way, if the name says simply "get", you shouldn't generally transform it in there. One expects a getter to be simple and not costy at all.

Solution 4

An alternative could be

Map<Instant, String> getItems() {
return items.entrySet().stream()
           .peek(entry -> entry.setValue(renderTags(entry.getKey())))
           .collect(Collectors.toMap(Map.Entry::getKey,e -> e.getValue()));
}

Useful if you have to perform updates on the stream before collect call.

View more solutions

Comments

[Vlasec]

Oh well, you were almost a full minute faster :/

[Ruckus T-Boom]

Can either of you tell me why I'd get a Cannot resolve method 'getKey' on the line Map.Entry::getKey?

[Didier L]

Are you sure that your Map import is really java.util.Map?

[Ruckus T-Boom]

@DidierL I just checked to make sure and it is.

[Ruckus T-Boom]

Sorry, I figured it out. It was a typo in the method return type, that was somehow showing up as the error I mentioned. Fixed that and it works great :)

[Ruckus T-Boom]

Not what I need here (the new map is just for display purposes), but I didn't know that, so I'll file it away for later. Thanks!

[Ruckus T-Boom]

What are the advantages/disadvantages of doing it this way versus the way shown in the accepted answer?

[Max]

it depends on context (not the one of the question)... I havn't yet used the peek method but I keep in mind it exists.

[Ruckus T-Boom]

Sounds good. I use it a lot for debugging (just shooting the result to System.err), but I've never thought to use it like this.

[Max]

Me too :) And I havn't been in the case of a sequence of changes to apply to a collection or map.

[Vlasec]

Yep, to the victor go the spoils.

[Vlasec]

What does exactly happen when you modify an entry? I think it changes the map and the action called in peek should be non-interfering. It is good for logs, but if I got it, it shouldn't be used this way, as far as I know.

[Max]

You have to think your map with references of objects. Updating the value of an entry doesn't affect the map itself (nor the result of hashcode and equals mechanism used on keys for storing elements).

[Max]

And this remark is valid for collections in general. When iterating on a collection, you can update the elements inside. Not the collection itself.

[Nicholas DiPiazza]

yeah this is the solution i needed. if you don't want a new map, use this.

[рüффп]

+1 for giving a solution to alter the original map. Exactly what I was looking for. Note that with the accepted answer it can also work but the original Map must be merged with the altered keys like oMap.putAll(alteredMap). Method peek prevent to do this.

[user1803551]

Don't perform operations in peek, they are not guaranteed to execute. It's only for debugging purposes.

[Max]

Yes, definitely, I update my answer.

[user1803551]

That doesn't help at all. peek should not perform any operation with side effects. It's meant to be used to print intermediate results.

[Max]

baeldung.com/java-streams-peek-api. the last paragraph describes clearly the same use case in a non debugging context: "we want to alter the inner state of an element". Even if I try to avoid this situation, I conceive it can occurs. That's why I may have used at least once in my carrer.