Java 8 Stream API Interview Questions

Notes on Stream :

• The main goal of the Stream API is to make collection easier, also many times it helps in better performance.
• A stream is not a data structure to store any elements it transfers from a data structure like an array through a pipeline of operations.
• It is functional in nature, does not modify the source.
• The elements of a stream are visited only once like an iterator, to revisit we need to generate a new stream.
• To create Stream we can use stream() method of Stream class in java.
• 3 most used Stream methods are filter(), map(), reduce().

Parallel Stream java 8:

If we have large number of data to handle we can use parallelStream (.parallelStream() in place of .stream()). Parallel stream() works on all the cores instead of 1 core in the serial stream.
Operations on Stream :
There are 2 types of operation in Stream :
Intermediate Operation :
  These operations return a stream, hence we can use other methods in the stream to get another stream as a result.
 In this way, it is possible to use a chain of methods.
Example  -> filter(), map(), sorted().
Terminal Operations:
These operations return a values of a certain type.
Example  For each (),count(), collect().

Find all even number squares of an array and put it in a set using Stream API

Here is an example of same.

public class Test {
 
public static void main(String... args) { 
Integer [] arr = {1,2,3,4,5};
Set set =  new HashSet<>();
set = Arrays.asList(arr) 
 .stream().filter(x ->( x%2==0)) //Filter Even Numbers
 .map(x->Math.pow(x, 2)) //Square the numbers
 .peek(System.out::println) //print without termination
 .collect(Collectors.toSet()); // Collect to a Set
 
}
}

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