minte9
LearnRemember




R Q

Streams

With Java 8 Streams we can make bulk operations. 
 
/**
 * With Java 8 Streams we can make bulk operations ...
 * sequentiallly or paralel.
 * 
 * The source for streams can be arrays, files, regex, etc.
 * We don't have to create a collection in order to work with streams.
 */

package com.minte9.collections.streams;
import java.util.Arrays;
import java.util.List;
import java.util.stream.Stream;

public class Streams {
    public static void main(String[] args) {
        
        List<String> myList = Arrays.asList("aa", "ba", "bc");
        myList
            .stream()
            .filter(s -> s.startsWith("b"))
            .map(String::toUpperCase)
            .sorted()
            .forEach(System.out::println);  
                // BA BC

        Stream.of("aa", "ba", "bc") // no collection
            .filter(s -> s.startsWith("b"))
            .map(String::toUpperCase)
            .sorted()
            .forEach(System.out::println); 
                // BA BC    

        Arrays.stream(new int[] {1, 2, 3}) // primitives
            .map(n -> n * 2)
            .average()
            .ifPresent(System.out::println); 
                // 4.0
    }
}

Declarative

Using declarative approach we can create code that is reusable and testable. 
 
/**
 * The Problem: 
 * 
 * Given a list of integers, 
 * find the square of all the even numbers
 * and return them in a new list.
 */

package com.minte9.collections.streams;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import java.util.function.Function;
import java.util.function.Predicate;
import java.util.stream.Collectors;

public class Declarative {

    static List<Integer> mySquareList = new ArrayList<>();
    static List<Integer> mylist = Arrays.asList(
        1, 2, 3, 4, 5, 6, 7, 8, 9, 10
    );
    
    public static void main(String[] args) {

        // Imperative programming
        for(Integer n : mylist) {
            if (n % 2 == 0) {
                mySquareList.add(n * n);
            }
        }
        System.out.println(mySquareList); // [4, 16, 36, 64, 100]


        // Declarative - no lambdas
        mySquareList = mylist.stream()
            .filter(
                new Predicate<Integer>() {
                    @Override public boolean test(Integer n) {
                        return n % 2 == 0;
                    }
                })
            .map(
                new Function<Integer, Integer>() {
                    @Override public Integer apply(Integer n) {
                        return n * n;
                    }
                }
            )
            .collect(Collectors.toList());
        System.out.println(mySquareList); // [4, 16, 36, 64, 100]


        // Declarative - lambdas
        mySquareList = mylist.stream()
            .filter(x -> x % 2 == 0)
            .map(y -> y * y)
            .collect(Collectors.toList());
        System.out.println(mySquareList); // [4, 16, 36, 64, 100]
    }
}

Anonimous

Lambdas removed the need of writing anonymous functions. 
 
/**
 * Streams are collection of objects on which you can apply ...
 * various methods while they are in the pipeline of execution.
 * 
 * Lambdas are language constructs that remove the need of ...
 * writing anonymous functions.
 */

package com.minte9.collections.streams;

import java.util.Arrays;
import java.util.List;
import java.util.Set;
import java.util.stream.Collectors;

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

        /**
         * Filter:
         * ----------------
         * Only elements that meet the condition ...
         * will be passed to the next function in the pipeline
         */
        List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5);
        numbers.stream()
            .filter(x -> x % 2 == 0)
            .forEach(System.out::println); 
                // 2 4
            
        /** 
         * Map:
         * ---------------
         * Used to transform an element into other element
         */
        List<String> letters = Arrays.asList("a", "b", "c");
        letters.stream()
            .map(x -> x.toUpperCase())
            .forEach(System.out::println); 
                // A B C

        /**
         * ForEach:
         * ---------------
         * Runs for every element in the stream
         */
        List<Integer> myNumbers = Arrays.asList(1, 2, 3, 4, 5);
        myNumbers.stream()
            .filter(x -> x % 2 == 0)
            .map(x -> x * x)
            .forEach(System.out::println); 
                // 4 16

        /**
         * Collect:
         * ----------------
         * Collects every element in the stream to another collection
         */
        List<String> myList = Arrays.asList("London", "Paris", "London");
        Set<String> mySet = myList.stream()
            .filter(x -> x.length() % 2 == 0)
            .collect(Collectors.toSet()); // no duplicates (Set)
        System.out.println(mySet); 
            // [London]
        
    }
}

➥ Questions    
➟
Collections, Json