how do I use java 8 streams groupingby to calculate average with filters

I need to calculate the average of a List of objects that I'm streaming.
The objects have:

ClassX.id
ClassX.name
ClassX.value
ClassX.startTime
ClassX.endTime

The objects must be grouped by ClassX.name and having the average calculated using ClassX.value.

Each object streamed represents either an start or end of a transaction.
Start transactions has ClassX.endTime == null.
End transactions has ClassX.startTime == null.
End transactions has ClassX.name == null

The value to be aggregated is within the start object, but it must be summed to the average only if the stream also process the corresponding end object of the transation.

Here's what I have so far(based on the suggestion of Andreas):

List<ClassX> classXList = ...

Map<String, Double> average = classXListStrings.stream()
        .map(ClassX::new) //convert to ClassX(the input list is actually String)
        .filter(x -> x.getName() != null) //avoid null entries for getName
		.collect(Collectors.groupingBy(ClassX::getName, Collectors.toList()))
		.entrySet().stream()
		// skip group if no end transaction exists
		.filter(e -> e.getValue().stream().anyMatch(x -> x.getStartTime() != null))
		.collect(Collectors.toMap(Entry::getKey,
				e -> e.getValue().stream()
						// only average values of start transactions
						.filter(x -> x.getEndTime() == null)
						.collect(Collectors.averagingDouble(ClassX::getValue))
		));

Is there a way to maybe store the objects streamed into a data structure and then aggregate the value only if the pair of objects begin/end transaction are streamed based on a filter?

It's hard to associate one object in a stream with another that appears later.

One solution is run through the list twice: First you find the end transactions, collecting them to a set. Then you process the list again, computing the averages.

List<ClassX> inputList = ...

Set<String> endSet = inputList.stream()
    .filter(o -> o.endTime != null)
    .map(o -> o.id)
    .collect(Collectors.toSet());

Map<String, Double> average = inputList.stream()
    .filter(o -> o.startTime != null && endSet.contains(o.id))
    .collect(Collectors.groupingBy(
            o -> o.name, 
            Collectors.averagingDouble(o -> o.value)));

You can do it like this:

List<ClassX> classXList = ...

Map<String, Double> average = classXList.stream()
		.collect(Collectors.groupingBy(ClassX::getName, Collectors.toList()))
		.entrySet().stream()
		// skip group if no end transaction exists
		.filter(e -> e.getValue().stream().anyMatch(x -> x.getStartTime() == null))
		.collect(Collectors.toMap(Entry::getKey,
				e -> e.getValue().stream()
						// only average values of start transactions
						.filter(x -> x.getEndTime() == null)
						.collect(Collectors.averagingDouble(ClassX::getValue))
		));