Java had introduced streams in Java 8 to help us fasten the development by providing ways to perform operations on data streams (collections) without writing bulky code.

In short, stream can be thought of stream or pipeline of data, on which you need to perform some operations. There can be two types of operations, intermediate and terminal. Intermediate operations are the ones which transforms the data in the stream, but the output is still a stream, for example filter or map operations. On the other hand terminal operations are ones which are applied collectively on the stream and output is something other than stream for example sum or reduce methods.

Here is an example usage to make it more clear. Or Download code from git repo

import java.util.ArrayList;
import java.util.List;
import java.util.Optional;
import java.util.stream.Collectors;
import java.util.stream.Stream;

public class Java8Streams {
	
	public static void main(String s[]) {
		System.out.println("Welcome to streams!");
		
		//There are multiple ways to create streams
		Stream stream = Stream.of(new Integer[]{1,2,3,4}); 
		Stream intStream = Stream.of(1,2,3,4);


		// Using streams to find sum of a integer list
		List intlist= new ArrayList();
		intlist.add(2);
		intlist.add(7);
		intlist.add(12);
		intlist.add(17);
		Stream streamnew = intlist.stream(); 
		
		// No more looping through the array
		Integer sum = intlist.stream().mapToInt(i->i).sum();
		System.out.println("Sum is :"+sum);
		
		// Lets check if the array contains even number, without looping
		boolean anyEven= intlist.stream().anyMatch(i->i%2==0);
		System.out.println(anyEven);
		
		// Check if all elements are even in the list
		boolean allEven = intlist.stream().allMatch(i->i%2==0);
		System.out.println(allEven);
		
		// Find first even number from the list
		Optional firsteven= intlist.stream().filter(i->i%2==0).findFirst();
		System.out.println(firsteven);
		
		// Now lets sum all even numbers
		int sumeven = intlist.stream().filter(i->i%2==0).mapToInt(i->i).sum();
		System.out.println(sumeven);
		
		// Lets use forEach to do operations on each element, say add a constant number to even numbers and print
		intlist.stream().forEach(i->{
			if(i%2==0) {
				i = i+100;
				System.out.print(i+",");
			}
		});
		System.out.println();
		//Better way to achieve the above
		intlist.stream().filter(i->i%2==0).forEach(i->{
			i = i+100;
			System.out.print(i+",");
		});
		
		System.out.println();
		// Simliar to filter, we gave map function
		List newlist = intlist.stream().map(i->{
			return i+100;
		}).collect(Collectors.toList());
		System.out.println(newlist);
		
		// Use reduce to perform some custom operation
		Optional output = intlist.stream().map(i->{
			return i+10;
		}).reduce((i,j)->{
			return i*j;
		});
		System.out.println(output);
		
	}

}