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The All-in-One Guide to Java Programming

For Each and Map/Reduce

Learn about the `forEach` and map/reduce method in Java 8.

We'll cover the following...

For each

The most basic thing we can do with a stream is loop through it using the forEach method.

For example, to print out all of the files in the current directory, we could do the following:

Java
import java.nio.file.Path;

import java.nio.file.Paths;

import java.nio.file.Files;

import java.io.File;

import java.io.IOException;



public class Program{



    public static void main(String[] args)throws IOException{ 

        Files.list(Paths.get("."))

            .forEach(System.out::println);

    }



}

For the most part, this replaces the for loop. It is more concise and more object-oriented since we are delegating the implementation of the actual loop.

Map, filter, and reduce

Lambda expressions and default methods allow us to implement map/filter/reduce in Java 8. Actually, it is already implemented for us in the standard library.

Let’s see how these methods can be implemented:

Java
import java.util.*;
class Mapexample {
  public static void main(String args[]) {
    Map < String, Integer > map = new HashMap < String, Integer > ();
    map.put("Hello", 23);
    map.put("Gary", 123);
    map.put("Larry", 145);
    
    //Elements can traverse in any order  
    for (Map.Entry m: map.entrySet()) {
      System.out.println(m.getKey() + " " + m.getValue());
    }
  }
}

The above program creates a map with ...