Stream API was introduced in Java 8 to facilitate functional programming in Java. A Stream API is targeted towards processing of collections of objects in functional way. By definition, a Stream is a Java component which can do internal iteration of its elements.

A Stream interface has terminal as well as non-terminal methods. Non-terminal methods are such operations which adds a listener to a stream. When a terminal method of stream is invoked, then internal iteration of stream elements get started and listener(s) attached to stream are getting called for each element and result is collected by the terminal method.

Such non-terminal methods are called Intermediate methods. The Intermediate method can only be invoked by called a terminal method. Following are some of the important Intermediate methods of Stream interface.

• filter − Filters out non-required elements from a stream based on given criteria. This method accepts a predicate and apply it on each element. If predicate function return true, element is included in returned stream.

• map − Maps each element of a stream to another item based on given criteria. This method accepts a function and apply it on each element. For example, to convert each String element in a stream to upper-case String element.

• flatMap − This method can be used to maps each element of a stream to multiple items based on given criteria. This method is used when a complex object needs to be broken into simple objects. For example, to convert list of sentences to list of words.

• distinct − Returns a stream of unique elements if duplicates are present.

• limit − Returns a stream of limited elements where limit is specified by passing a number to limit method.

Example – Intermediate Methods

import java.util.Arrays;
import java.util.List;
import java.util.stream.Stream;

public class FunctionTester {    
   public static void main(String[] args) {
      List<String> stringList = 
         Arrays.asList("One", "Two", "Three", "Four", "Five", "One");       

      System.out.println("Example - Filtern");
      //Filter strings whose length are greater than 3.
      Stream<String> longStrings = stringList
         .stream()
         .filter( s -> {return s.length() > 3; });

      //print strings
      longStrings.forEach(System.out::println);

      System.out.println("nExample - Mapn");
      //map strings to UPPER case and print
      stringList
         .stream()
         .map( s -> s.toUpperCase())
         .forEach(System.out::println);

      List<String> sentenceList 
         = Arrays.asList("I am Mahesh.", "I love Java 8 Streams.");     

      System.out.println("nExample - flatMapn");
      //map strings to UPPER case and print
      sentenceList
         .stream()
         .flatMap( s -> { return  (Stream<String>) 
            Arrays.asList(s.split(" ")).stream(); })
         .forEach(System.out::println);     

      System.out.println("nExample - distinctn");
      //map strings to UPPER case and print
      stringList
         .stream()
         .distinct()
         .forEach(System.out::println);     

      System.out.println("nExample - limitn");
      //map strings to UPPER case and print
      stringList
         .stream()
         .limit(2)
         .forEach(System.out::println);        
   }   
}

Output

Example - Filter

Three
Four
Five

Example - Map

ONE
TWO
THREE
FOUR
FIVE
ONE

Example - flatMap

I
am
Mahesh.
I
love
Java
8
Streams.

Example - distinct

One
Two
Three
Four
Five

Example - limit

One
Two