Replace String With hashmap value using java 8 stream

Question

I have String and HashMap like below codes:

Map<String, String> map = new HashMap<>();
    map.put("ABC", "123");
    String test = "helloABC";
    map.forEach((key, value) -> {
        test = test.replaceAll(key, value);
    });

and I try to replace the string with the HashMap values, but this doesn't work because test is final and cannot be reassigned in the body of forEach.

So are there any solutions to replace String with HashMap using Java 8 Stream API?

`forEach()` is not synchronized. Your code does not compile because you cannot have a side effect on `message` from inside your lambda expression. Even if you could, all you'd get is `message` containing the last replacement on `test` for the last map entry in the map. Which is probably not what you want. Also because "last map entry" is non-deterministic for `HashMap`. — Ralf, Mar 23 '16 at 08:03
Ok Thanks, I got it. but what should I do to get all contents of test that already replaced? — Chhea Vanrin, Mar 23 '16 at 08:07

Alex Salauyou · Accepted Answer · 2016-03-23T10:07:33.897

As this cannot be made using only forEach() (message must be effectively final), workaround could be to create a final container (e. g. List) which stores a single String that is re-written:

final List<String> msg = Arrays.asList("helloABC");
map.forEach((key, value) -> msg.set(0, msg.get(0).replace(key, value)));
String test = msg.get(0);

Note that I changed replaceAll() to replace() because former works with regex, but judging by your code seems you need replacement by string itself (don't worry, despite of confusing name it also replaces all occurrences).

If you want exactly Stream API, you may use reduce() operation:

String test = map.entrySet()
                 .stream()
                 .reduce("helloABC", 
                         (s, e) -> s.replace(e.getKey(), e.getValue()), 
                         (s1, s2) -> null);

But take into account, that such reduction will work properly only in serial (not parallel) stream, where combiner function is never called (thus may be any).

I already tried it, but its seem so tricky. and how if we have many elements in Map? — Chhea Vanrin, Mar 23 '16 at 08:18
@ChheaVanrin `0` refers to `List`, not to a `Map`. `forEach()` will process as much map entries as you have. — Alex Salauyou, Mar 23 '16 at 08:20
Sorry I didn't see it clearly :) . but it seem not so logic. I just want to work with completely java 8 stream. Thanks anyways :) — Chhea Vanrin, Mar 23 '16 at 08:27

score 4 · Answer 2 · answered Mar 23 '16 at 09:05

This sort of a problem is not well-suited for Streams API. The current incarnation of Streams is mainly directed at tasks that can be made parallel. Perhaps support for this type of operation will be added in the future (see https://bugs.openjdk.java.net/browse/JDK-8133680).

One streams-based approach that you might find interesting is to reduce on functions rather than on the string:

Function<String, String> combined = map.entrySet().stream()
    .reduce(
        Function.identity(),
        (f, e) -> s -> f.apply(s).replaceAll(e.getKey(), e.getValue()),
        Function::andThen
    );

String result = combined.apply(test);

Seems that no chance to see `foldLeft` in Java-9. Unfortunately. — Tagir Valeev, Mar 23 '16 at 10:21
@TagirValeev I saw your efforts on core-lib mailing list a few months ago. Thank you for trying. — Misha, Mar 23 '16 at 20:47

Replace String With hashmap value using java 8 stream

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