Stream In Java

• 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 –

• A stream is not a data structure instead it takes input from the Collections, Arrays or I/O channels.
• Streams don’t change the original data structure, they only provide the result as per the pipelined methods.
• Each intermediate operation is lazily executed and returns a stream as a result, hence various intermediate
• operations can be pipelined. Terminal operations mark the end of the stream and return the result.

Different Operations On Streams-
Intermediate Operations:

• map: The map method is used to returns a stream consisting of the results of applying the given function to the elements of this stream.
• List number = Arrays.asList(2,3,4,5);
• List square = number.stream().map(x->x*x).collect(Collectors.toList());
• filter: The 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());
• sorted: The 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:

• 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());
• 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));
• 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);
• int even = number.stream().filter(x->x%2==0).reduce(0,(ans,i)-> ans+i);

Here ans variable is assigned 0 as the initial value and i is added to it .

Program to demonstrate the use of Stream

//a simple program to demonstrate the use of stream in java
import java.util.*;
import java.util.stream.*;

class Demo
{
public static void main(String args[])
{

// create a list of integers
List<Integer> number = Arrays.asList(2,3,4,5);

// demonstration of map method
List<Integer> square = number.stream().map(x -> x*x).
collect(Collectors.toList());
System.out.println(square);

// create a list of String
List<String> names =
Arrays.asList("Reflection","Collection","Stream");

// demonstration of filter method
List<String> result = names.stream().filter(s->s.startsWith("S")).
collect(Collectors.toList());
System.out.println(result);

// demonstration of sorted method
List<String> show =
names.stream().sorted().collect(Collectors.toList());
System.out.println(show);

// create a list of integers
List<Integer> numbers = Arrays.asList(2,3,4,5,2);

// collect method returns a set
Set<Integer> squareSet =
numbers.stream().map(x->x*x).collect(Collectors.toSet());
System.out.println(squareSet);

// demonstration of forEach method
number.stream().map(x->x*x).forEach(y->System.out.println(y));

// demonstration of reduce method
int even =
number.stream().filter(x->x%2==0).reduce(0,(ans,i)-> ans+i);

System.out.println(even);
}
}
Result :

[4, 9, 16, 25]
[Stream]
[Collection, Reflection, Stream]
[16, 4, 9, 25]
4
9
16
25
6

Important Points/Observations:

• A stream consists of source followed by zero or more intermediate methods combined together (pipelined) and a terminal method to process the objects obtained from the source as per the methods described.
• Stream is used to compute elements as per the pipelined methods without altering the original value of the object.