how to flatten a nested map in java8

NestedMapFlatten.java

public void testFlattenMap() {
    Map<String, Map<String, Long>> counters = new HashMap<>();

    Map<String, Long> nested = new HashMap<>();
    nested.put("inner1", 1L);
    nested.put("inner2", 2L);
    nested.put("inner3", 3L);

    counters.put("out1", nested);
    counters.put("out2", nested);
    counters.put("out3", nested);
    
    //change the counters map to Map<String,Long>
    // the keys are like: out1.inner1,out1.inner2 ...

    Stream<SimpleEntry<String, Long>> entryStream = counters.entrySet().stream()
        .flatMap(e -> e.getValue().entrySet().stream().flatMap(
            v -> Stream.of(new SimpleEntry<>(e.getKey() + "." + v.getKey(), v.getValue()))));

    Map<String, Long> collect = entryStream
        .collect(Collectors.toMap(Entry::getKey, Entry::getValue));

    collect.forEach((k, v) -> {
      System.out.println("k:" + k + ",v:" + v);
    });
  }

notes: read the java docs about Stream.map and Stream.flatMap carefully, the difference is here:

Stream.map 

Returns a stream consisting of the results of applying the given function to the elements of this stream.

Stream.flatMap
Returns a stream consisting of the results of replacing each element of this stream with the contents of a mapped stream produced by applying the provided mapping function to each element.

Steam.map does not replace the streams, it only do something on the elements of the stream.
Strem.flatMap replaces the elements of the stream with another stream, the new stream is produced by the function you provided

In a nutshell, Stream.map does not replace elements with streams, it works on the elements of the stream, Stream.flatMap replace elements with steams.