Java -Stream API Introduction with Example

Java 8 introduced the concept of stream, Stream represents a sequence of objects from a source, which supports aggregate operations. 

Following are the characteristics of a Stream −

1. It takes input from the Collections, Arrays, or I/O channels.
2. It never stores the elements.
3. Stream supports intermediate operations like filter, map, limit, reduce, find, match, and so on.
4. Streams only provide the result as per the pipelined methods.
5. Stream operations do the iterations internally over the source elements provided.

Different Operations On Streams-

 forEach

The stream has provided a new method ‘forEach’ to iterate each element of the stream

 

List<Integer> obj = new ArrayList<>();

obj.add(7);

obj.add(12);

obj.add(3);

obj.add(5);

obj.forEach((o) -> System.out.println("Item : " + o));

map

The map method is used to map the items in the collection to other objects according to the Function passed as the argument.

List<String> obj = Arrays.asList("h", "bh", "ca", "d");

List<String> collectUpperCaseLetters = obj.stream().

map(String::toUpperCase).collect(Collectors.toList());

System.out.println(collectUpperCaseLetters);

filter

The ‘filter’ method is used to eliminate elements based on criteria.

 

List<String> names = Arrays.asList("Sibin", "Rahul", "Jose");

List<String> filteredNames = names.stream().filter(s -> s.startsWith("S")).

collect(Collectors.toList());

System.out.println(filteredNames); 

sorted

The sorted method is used to sort the stream.

 

List<String> names = Arrays.asList("Sibin", "Rahul", "Jose");

List<String> result = names.stream().sorted().collect(Collectors.toList());

System.out.println(result);

reduce

The reduce method is used to reduce the elements of a stream to a single value.

int[] array = {2,4,5,7,2};

Arrays.stream(array).reduce((x,y) -> x+y).ifPresent(s -> System.out.println(s));

Program to reveal the use of Stream

import java.util.ArrayList;

import java.util.Arrays;

import java.util.List;

import java.util.stream.Collectors;

public class StreamTest {

public static void main(String[] args) {

// demonstration of forEach method

List<Integer> obj = new ArrayList<>();

obj.add(7);

obj.add(12);

obj.add(3);

obj.add(5);

obj.forEach((o) -> System.out.println(o));

/* End */

// demonstration of map method

List<String> alplalist = Arrays.asList("h", "bh", "ca", "d");

List<String> collectUpperCaseLetters = alplalist.stream().

map(String::toUpperCase).collect(Collectors.toList());

System.out.println(collectUpperCaseLetters);

/* End */

// demonstration of filter method

List<String> names = Arrays.asList("Sibin", "Rahul", "Jose");

List<String> filteredNames = names.stream().

filter(s -> s.startsWith("S")).collect(Collectors.toList());

System.out.println(filteredNames);

/* End */

// demonstration of sorted method

List<String> namelist = Arrays.asList("Sibin", "Rahul", "Jose");

List<String> result = namelist.stream().sorted().

collect(Collectors.toList());

System.out.println(result);

/* End */

// demonstration of reduce method

int[] array = { 2, 4, 5, 7, 2 };

Arrays.stream(array).reduce((x, y) -> x + y).

ifPresent(s -> System.out.println(s));

/* End */

}

}

Output: 

7

12

3

5

[H, BH, CA, D]

[Sibin]

[Jose, Rahul, Sibin]

20