Java 8 个流 - 在 stream 操作期间调用“映射”而不保存到变量

Question

I could not find many answers to this, though I suppose its not possible, but I was curious - is there anyway in Java8 streams to "refer" or to "call" the HashMap that is created via the .collectors(Collectors.toMap(key -> "key", val -> "val") ? I know I can save this resultant reduction which creates the Map to Map<String, String> myMap variable, but I still had to perform operations on this resultant HashMap and I wanted to keep the stream going, so I wanted to do something like this:

myHashSet.stream().
.filter(i -> i != null)
.collectors(Collectors.toMap(key -> "key", val -> "val")
.forEach((k,v) -> {

   // HERE IS WHERE I WANT TO CALL THE MAP CREATED ABOVE
   if( MAP.contains("someRandomValue") {
    
    }


}));

I'm assuming you can't do this, but I was hoping there was some method or something so I don't have to "kill" the stream, ie save the Map to a variable, then proceed to stream it out again etc...

Answer 1

The collect operation in Stream is a terminal operation. So there is no Stream after collect . You would have to do something like below.

var myMap = myHashSet.stream().
.filter(i -> i != null)
.collectors(Collectors.toMap(key -> "key", val -> "val");

myMap.forEach((k,v) -> {

   // HERE IS WHERE I WANT TO CALL THE MAP CREATED ABOVE
   if( MAP.contains("someRandomValue") {
    
    }


}));

Below article describes about the available intermediate operations in Stream .

https://www.javacodegeeks.com/2020/04/java-8-stream-intermediate-operations-methods-examples.html

Answer 2

There is no reason that you need to assign the collected map to a local variable, you could simply follow on with use of forEach or a second stream() :

myHashSet.stream().
.filter(Objects::nonNull)
.collectors(Collectors.toMap(key -> "key", val -> "val").
.entrySet().stream()  // or .forEach()
// Add operations on the second stream of Map.Entry here...

However the above creates a second stream. You can handle as just one stream by mapping as Map.entry() and do other intermediate operations before collect or other stream termination:

var map = myHashSet.stream()
.filter(Objects::nonNull)
.map(s -> Map.entry(s, "val"+s))
// Add operations on the stream of Map.Entry here...
.collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue));  // or other termination

The Map.entry() call is JDK9+, for earlier JDK you can use new AbstractMap.SimpleImmutableEntry(key,value) instead.

Answer 3

I solved this with @shmosel's help using collectingAndThen . This was the closest to what I wanted to do.

myArrayList.stream().
.collect(Collectors.collectingAndThen(Collectors.toMap(key -> key.getName(), val -> val), map -> map.forEach((key, val) -> System.out.println(map.contains(key)));