Java 8 Streams and its operations

Streams are sequences of objects from a source, which support aggregate operations. These were introduced in Java 8.

With Java 8, Collection interface has two methods to generate a Stream.

• stream() − Returns a sequential stream considering collection as its source.

• parallelStream() − Returns a parallel Stream considering collection as its source.

Some operations on streams are −

sorted − The sorted method is use to sort the stream lexicographically or in ascending order

List id = Arrays.asList("Objects","Classes","Interfaces");
List output = id.stream().sorted().collect(Collectors.toList());

map − The map method maps the elements in the collection to other objects according to the Predicate passed as a parameter

List list1= Arrays.asList(1,3,5,7);
List finalList = list1.stream().map(a -> a * a * a).collect(Collectors.toList());

filter − The filter method is used to select elements according to the Predicate parameter passed

List id = Arrays.asList(“Classes","Methods","Members");
List output = id.stream().filter(s -> s.startsWith("M")).collect(Collectors.toList());

The three operations above are intermediate operations. Now let us have a look at the terminal operations

collect − The collect method returns the outcome of the intermediate operations

List id = Arrays.asList(“Classes","Methods","Members");
List output = id.stream().filter(s -> s.startsWith("M")).collect(Collectors.toList());

forEach − This method iterates through every element in the stream

List list1 = Arrays.asList(1,3,5,7);
List finalList = list1.stream().map(a ->a * a * a).forEach(b->System.out.println(b));

Let us see an example which illustrates the use of Stream −

Example

 Live Demo

import java.util.*;
import java.util.stream.*;
public class Example {
   public static void main(String args[]) {
      List<Integer> list1 = Arrays.asList(11,22,44,21); //creating an integer list
      List<String> id = Arrays.asList("Objects","Classes","Methods");
      //creating a String list
      // map method
      List<Integer> answer = list1.stream().map(x -> x*x).collect(Collectors.toList());
      System.out.println(answer);
      // filter method
      List<String> output = id.stream().filter(x->x.startsWith("M")).collect(Collectors.toList());
      System.out.println(output);
   }
}

Output

[121, 484, 1936, 441]
[Methods]