Java Stream Revisited

What is a Stream ?

A stream is a sequence of objects on which various methods can be applied to get the desired results.

Stream is not a data structure.

How to work with Stream ?

Step 1: Create a Stream.
Step 2: Perform intermediate operation on the initial stream to transform it to another stream and do further intermediate operation on it.
Step 3: Perform terminal operation on the final stream.

How to create a Stream ?

Below are the some of the ways to create a stream:

Stream stream1=Stream.of(1,2,3,4);
Stream stream2=Stream.of(array);
Stream stream3=Stream.of(list);
Stream stream4=list.stream();//sequential stream will be created.
Stream stream5=list.parallelStream();//parallel stream will be created.
Stream stream6=Arrays.asList(1,2,3,4).stream();
Stream stream7=Stream.generate( () -> return "abc";);
Stream stream8=Stream.iterate("abc", (i) -> i);                       

What are Intermediate Operations ?

Intermediate operations return another Stream which allows you to call multiple operations in the form of a query.
Stream intermediate operations do not get executed until a terminal operation is invoked.
All Intermediate operations are lazy, so they’re not executed until a result of a processing is actually needed.
Traversal of the Stream does not begin until the terminal operation of the pipeline is executed.

Here is the list of all Stream intermediate operations:

filter()
map()
flatMap()
distinct()
sorted()
peek()
limit()
skip()

What are Terminal Operations ?

These are the operations that are at the end of the statement. Below are some of the methods:

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

List number = Arrays.asList(2,3,4,5,3);
Set square = number.stream().map(x -> x*x).collect(Collectors.toSet()); 
List square = number.stream().map(x-> x*x).collect(Collectors.toList()); 

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

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

3. reduce:  The reduce method is used to reduce the elements of a stream to a single value.
The reduce method takes a BinaryOperator as a parameter.

List number = Arrays.asList(2,3,4,5);
//In below example, ans variable is assigned 0 as the initial value and i is added to it:
int even = number.stream().filter(x -> x%2==0).reduce(0,(ans,i)-> ans+i); 
//In below example, doesn’t make much sense of this statement. Before printing it you are collecting it as a Set, but saving it to a Set:
number.stream().map(x->x*x).collect(Collectors.toSet()).forEach(s -> System.out.println(s)); 

How to iterate a stream ?

stream1.forEach(s -> System.out.println(s));

Is there any other way to iterate a stream ?

stream1.forEach(new MyConsumer());

import java.util.function.Consumer;
class MyConsumer implements Consumer<Integer>{
      public void  accept(Integer i){
           System.out.println(i);
      }
}

How does parallel stream work interanally, does it uses threads ?

How to convert a Stream to Collection object ?

List list=(List)stream.collect(Collectors.toList());

How to convert a Stream to Array ?

Integer[] intArray = (Integer[])stream.toArray(s -> new Integer[s]);

Lets see some of the programs using Streams. These are very important for an interview perspective:

How to print 10 random numbers ?

new Random().ints().limit(10).forEach(s -> System.out.println(s));

How to print 10 even numbers staring from 1 ?

Stream.iterate(1, c->c+1)  
		.filter(s -> s%2==0)  
		.limit(10)  
		.forEach(k -> System.out.println(k));

How to iterate over a list of integers and find out sum of all the integers greater than 10 ?

list.stream().filter(i -> i > 10).mapToInt(i -> i).sum(); //list is an ArrayList object.

How to print only the salary from all the Employee objects in a ArrayList object ?

List<Employee> empList=new ArrayList<>(); //Make sure to use generics for the empList object, otherwise the below line will give CTE. Why ? 
//add code here to add Employee objects to empList object.

empList.stream().filter(s2->s2.getSalary() > 0).forEach(s -> System.out.println(s.salary));

//Another way below:
empList.stream().filter(s2->s2.getAge() > 0).map(s -> s.age).forEach(s3 -> System.out.println(s3));