Functional Programming in Java 8+ using the Stream API with Example

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API is an acronym for Application Programming Interface, which is software and the java streams work on a data source. Consider a stream like a flow of water in a small canal. Let’s take a real-life example. Each time a user uses an application that is popular these days like WhatsApp in order to communicate via delivering text messages or calls to other users. Both users are using an API.

Java streams work on three operations which as mentioned below

Data Source
Intermediate operation
Terminal operation

Methods: Streams can be created in three ways

Using an object of any class from the collection framework
Using an array of the reference data type
Using the interface defined in the ‘java.util.stream’ package

Method 1: Data Source

The data source can be widely varied such as an array, List, etc

Syntax:

ArrayList<Integer> numbers = new ArrayList<>();
Integer[] numbers = {1,2,3};

Example 1: Using an object as a data source

Java

// Java Program showcasing data source
// using an object as a data source
 
// Importing input output classes
import java.io.*;
 
// Class
class GFG {
 
    // Main driver method
    public static void main(String[] args)
    {
        // Data Source
 
        // Creating an arrayList object
        // Declaring object of String type
        ArrayList<String> gfgNames = new ArrayList<>();
 
        // Custom input elements to above object
        gfgNames.add("Dean");
        gfgNames.add("castee");
        gfgNames.add("robert");
 
        // Creating object of Stream where Stream is created
        // using arrayList and object as data source
        Stream<String> streamOfNames = gfgNames.stream();
 
        // Print and display element
        System.out.print(streamOfNames);
    }
}

Example 2: Using an array as a data source

// Data Source
Integer[] numbers = {1,2,3,4,5};

// Stream using an array
Stream<Integer> streamOfNumbers = Arrays.stream(numbers);

// using predefined Instream interface
integerStream = IntStream.range(1,100); // a stream from 1 to 99;

Java

// Java Program showcasing data source
// using an array as a data source
 
// Importing java input output class
import java.io.*;
 
// Importing all classes from
// java.util package
import java.util.*;
 
// Importing class for additional operations,
// additionls and pipelines
import java.util.stream.IntStream;
 
// Class
class GFG {
 
    // Main driver method
    public static void main(String[] args)
    {
        // Creating a predefined stream using range method
        // Custom inputs for range as parameters
        var stream = IntStream.range(1, 100);
 
        // Stream is created
        var max = stream.filter(number -> number % 4 == 0)
                      .count();
 
        // Print and display the maximum number
        // from the stream
        System.out.println(max);
    }
}

Method 2: Intermediate Operation

Intermediate operations are some methods that one can apply on a stream.

Note: It can be of any number

filter()

Example:

Java

import java.io.*;
 
class GFG {
    public static void main(String[] args)
    {
        // Data Source
        Integer[] numbers = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 0 }
 
        // Stream
        Stream<Integer> streamOfNumbers
            = Arrays.stream(numbers);
 
        // filter all the even numbers
        Stream<Integer> evenNumbersStream
            = streamOfNumbers.filter(
                number -> number % 2 == 0)
    }
}

Method 3: Terminal operation

Terminal operation methods that we can apply on a stream that will cause a stream to be “closed”.

Concept:

Some terminal operations can be used to iterate on the elements of the stream.

min(),max(),count()

forEach(),noneMatch()

Example 1: Explaining stream API

Java

// Importing input output classes
import java.io.*;
// Importing all classes from
// java.util package
import java.util.*;
 
// Class
class GFG {
 
    // Main driver method
    public static void main(String[] args)
    {
        // Data source
        // Custom integer inputs
        Integer[] numbers
            = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 0 };
 
        // Stream
        var streamOfNumbers = Arrays.stream(numbers);
 
        // Using filter method
        var evenNumbersStream
            = streamOfNumbers
                  .filter(number -> number % 3 == 0)
                  .count();
 
        // Print and display
        System.out.println(evenNumbersStream);
    }
}

Output

Example 2:

Java

// Importing all classes from java.util package
import java.io.*;
import java.util.*;
 
// Class
class GFG {
 
  // Main driver method
    public static void main(String[] args)
    {
      // Creating an ArrayList of Integer type
        ArrayList<Integer> list = new ArrayList<>();
       
      // Adding elements to above object of Arraylist
      // Custom inputs
        list.add(20);
        list.add(4);
        list.add(76);
        list.add(21);
        list.add(3);
        list.add(80);
 
        var stream = list.stream();
 
        var numbers
            = stream.filter(number -> number % 2 == 0)
                  .filter(number -> number > 20);
       
      // Print all the elements of the stream on the console
        numbers.forEach(System.out::println);
    }
}