Streams in Java

A stream is an Object, type of data

Stream and file streams are different
You can use Stream once, cannot be using again after consumption


import java.util.Arrays ;
import java.util.List;
import java.util.function.Consumer;
import java.util.stream.Stream;
public class Demo{

public static void main(String[] args)
List<Integer> nums = Arrays.asList(4,5,7,8,9);
Stream<Integer> data =nums.stream() ;
Stream<Integer> sortedData= data.sorted();
sortedData.forEach(n->System.out.println(n));
}

other example can be seen in

import java.util.Arrays ;
import java.util.List;
import java.util.function.Consumer;
import java.util.stream.Stream;
public class Demo{

public static void main(String[] args)
List<Integer> nums = Arrays.asList(4,5,7,8,9);
Stream<Integer> data =nums.stream() ;
Stream<Integer> sortedData= data.sorted();
sortedData.forEach(n->System.out.println(n));
}

More Examples and Use cases

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import java.util.stream.Collector;
import java.util.stream.Collectors;
import java.util.stream.Stream;

public class Streams{
    public static void main(String[] args) {
         System.out.println("this is streams");
        final User[] arrayOfUsers = {
            new User(1, "Jeff Bezos", 100000.0), 
            new User(2, "Bill Gates", 200000.0), 
            new User(3, "Mark Zuckerberg", 300000.0)
        };


        //for each 
        List<Integer> nums = Arrays.asList(4,5,7,8,9);
        Stream<Integer> dataStream1 =nums.stream() ;
        Stream<Integer> sortedData= dataStream1.sorted();
        sortedData.forEach(n->System.out.print("| "+n+" | "));
        System.out.println("printing each value");
        Stream<Integer> dataStream2= nums.stream();
        dataStream2.forEach(n->System.out.print(n));

        //filter - criteria based result
         List<User> userList= Arrays.asList(arrayOfUsers);
         Stream<User> userListStream=userList.stream(); 
         List<User> userFilteredListStream=userListStream.filter(string-> !string.getName().isEmpty())
        .collect(Collectors.toList()); 
         System.out.println(userFilteredListStream);

         List<Integer> intList1= Arrays.asList(2,4,3,8,9,10);
         System.out.println("even and double");
         intList1.stream()
         .filter(n -> n%2==0)
         .map(n-> n*2)
         .forEach(n-> System.out.print(n+" "));


        //Count
        Long userFilteredCountListStream=userList.stream().filter(string-> string.getName().length()>13).count(); 
        System.out.println(userFilteredCountListStream);

        //Map - map each element corresponding to result
        List<Integer> numbers = Arrays.asList(3, 2, 2, 3, 7, 3, 5);
        List<Integer> squaredNumbers=numbers.stream().map(num->num*num).distinct().collect(Collectors.toList());
        System.out.print("squaredNumbers" +squaredNumbers+"\n");



        //limit- limit the number of list
        List<Integer> list1= Arrays.asList(1,2,3,4,5,6,7, 8,9,10,11,12,13);
        System.out.println("limiting result");
        list1.stream().limit(10).forEach(System.out::print); 
    }
}

Collectors

Collectors are used to combine the result of processing on the elements of a stream. Collectors can be used to return a list or a string.like in the example we have used, I am putting new usernames so that you can run and test it

         List<User> userList= Arrays.asList("Elon Musk", "Bill Gates", "Mark");
         Stream<User> userListStream=userList.stream(); 
         List<User> userFilteredListStream=userListStream.filter(string-> !string.getName().isEmpty())
        .collect(Collectors.toList()); 
         System.out.println(userFilteredListStream);

Mathematical and Statistical operations


List numbers = Arrays.asList(3, 2, 2, 3, 7, 3, 5);
IntSummaryStatistics stats = integers.stream().mapToInt((x) -> x).summaryStatistics();
System.out.println("Highest number in List : " + stats.getMax());
System.out.println("Lowest number in List : " + stats.getMin());
System.out.println("Sum of all numbers : " + stats.getSum());
System.out.println("Average of all numbers : " + stats.getAverage());

Map Filter Reduce

meaning of Predicate

Predicate is the object that is accepted by the filter, it can be illustrated by example


List<lnteger> nums = Arrays.asList(
4,5,7,3,2);
Predicate<lnteger> p = new Predicate<lnteger>() {
    public boolean test(Integer n) {
        return n%2==0;
    }
};
//can be written as 
//predicate p =n->n%2=0;



int result = nums.stream( )
        . fitter(p)
        .map(n -> n*2)
        . reduce( 0, (c,e) -> c+e);

System.out.println(result);