Stream API in Java

https://www.geeksforgeeks.org/stream-in-java/
http://tutorials.jenkov.com/java-collections/streams.html

Introduced in Java 8, the Stream API is used to process collections of objects.
A stream is a sequence of objects that supports various methods which can be pipelined to produce the desired result.

The features of Java stream are:

  • Takes input from Collections, Arrays or I/O channels.
  • Streams doesn’t chance the original data structure, only provided a result.
  • Intermediate Operations returns a Stream as a result and can be pipelined..
  • Terminal Operations mark the end of the Stream and return the result.

Intermediate Operations

  1. filter: filter method is used to select elements as per the Predicate passed as argument.
List names = Arrays.asList("Reflection", "Collection", "Stream");
List result = names.stream().filter(s->s.startsWith("S")).collect( Collectors.toList() );
  1. map: map method returns a stream consisting of the results of applying the given function to the stream elements.
List numbers = Arrays.asList(1,2,3,4,5);
List squares = numbers.stream().map(x->x*x).collect( Collectors.toList() );
  1. sorted: sorted method is used to sort the stream
List names = Arrays.asList("Reflection", "Collection", "Stream");
List result = names.stream().sorted().collect(Collectors.toList());

Terminal Operations

  1. collect: The collect method is used to return the result of the intermediate operations performed on the stream,

  2. forEach: The forEach method is used to iterate through every element on the stream.

List numbers = Arrays.asList(1,2,3,4,5);
numbers.stream().map( x->x*x ).forEach( y-> System.out.println(y));

  1. reduce: The reduce method is used to reduce the elements of a stream to a single value. The reduce method takes two arguments:

    • initial value: 0 here.
    • BynaryOperator, (a, b): to combine two elements and generate a new value.
List numbers = Arrays.asList(1,2,3,4,5);
int even = numbers.stream().filter( x->x%2==0 ).reduce( 0, (a, b) ->  a+b );