Core java interview Questions You'll Most Likely Be Asked (Second Edition)

What is a parallel stream? Explain with a code sample
how you can create a parallel stream.
Answer:

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221.
In addition to sequential streams, the Stream API also supports
parallel streams. Parallel streams operate concurrently on the
elements in a Stream and hence provide a performance
improvement. A new method called parallelStream() has
been added to the Collection interface. This can be used to
obtain a parallel stream corresponding to the underlying
collection. The following code demonstrates this:
List colours =
Arrays.asList(“Red”,”Green”,”Blue”);
Stream parallelStream =
colours.parallelStream();
This code first creates a List of String values called colours. It
then invokes the parallelStream() method on the colours
List. This returns a parallel stream corresponding to the List. Any
further Stream operations performed on this stream will be
executed concurrently on the elements in the Stream.
How can you convert a Stream back to a Collection?
Answer:
The Stream interface has a method called collect(). This can
be used to convert a Stream back to a Collection. The following
code demonstrates this:
Stream numbers = Stream.of(7,3,9,6,1);
List filteredList = numbers.filter(num–> num >
5).collect(Collectors.toList());
This code first creates a List of Integers. It then invokes the
filter() method to obtain the numbers that are greater than 5.
After that, it invokes the collect() method. The collect()

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222.
223.
method accepts as parameter a Collector. The
Collectors.toList() method is invoked which returns a
Collector that converts the input Stream to a List. So, the
filteredList is a List that has only the numbers that are
greater than 5 from the input List. There are other methods in the
Collectors class like Collectors.toSet,
Collectors.toMap, Collectors.toCollection which can
be used to convert the Stream to other types of Collections.