The new Java main()

public static void main(String[] args) is (or was) the infamous entry point in all Java programs

class MyApp1 {
	public static void main(String[] args) {
		System.out.println("Hello World!");
	}
}

# Using Java 21

$ javac MyApp1.java
$ java MyApp1
Hello World!

Now, since Java 21 the Java Team added the instance main methods feature that allows Java Programs to be launched with simple instance methods named main that no need to be static, nor public (but they cannot be private for obvious reasons), nor receive parameters:

class MyApp2 {
	void main() {
		System.out.println("Hello World!");
	}
}

# Using Java 21

$ javac MyApp2.java
$ java --enable-preview MyApp2
Hello World!

then also they added another feature called unnamed classes that add the ability to declare classes in an implicit manner. Lets say we create a file called MyApp3.java and then in that file we write:

void main() {
	System.out.println("Hello World!");
}

this main method is an instance method in a implicit class named MyApp3 (the class its taking its name from the file name).

# Using Java 21

$ javac --enable-preview --release 21 MyApp3.java
$ java --enable-preview MyApp3
Hello World!

and soon in Java 23 every implicit class will automatically import static methods from package java.io.IO.* which will allow to write the main method much simpler.

void main() {
	println("Hello World!");
}

but again this feature is not yet released not even as a preview.

A Sip of Java: Maps from collections of objects using Collectors (groupingBy and partitioningBy)

 We can use streams and collectors to get Maps from a collection of objects. These Maps could be the result of our collection grouped or partitioned in specific ways.

Let's say we have a collection of Employees, where an Employee has attributes like: name, years of experience, tenure (in the company), role name, experience level.

	record Employee(String name, int yoe, int tenure, String role, ExperienceLevel experienceLevel) {
	}
	enum ExperienceLevel {
	ENTRY, JR, MID, SR, STAFF
	}
	List<Employee> employeeList = List.of(
	new Employee("Jhon", 4, 4, "DevOps Engineer", ExperienceLevel.MID),
	new Employee("Emma", 2, 2, "Fullstack Engineer", ExperienceLevel.JR),
	new Employee("Lucas", 7, 10, "Frontend Engineer", ExperienceLevel.SR),
	new Employee("Oliver", 17, 20, "Frontend Engineer", ExperienceLevel.STAFF),
	new Employee("Ethan", 5, 7, "Frontend Engineer", ExperienceLevel.MID)
	);

view raw a_sip_of_java:_maps_from_collections_of_objects_using_collectors__code01.java hosted with ❤ by GitHub

Now let's say we want to group our employees by the experience level:

	Map<ExperienceLevel, List<Employee>> map =
	employeeList.stream().collect(Collectors.groupingBy(Employee::experienceLevel));
	// if we print map as json:
	{
	"JR" : [ {
	"name" : "Emma",
	"yoe" : 2,
	"tenure" : 2,
	"role" : "Fullstack Engineer",
	"experienceLevel" : "JR"
	} ],
	"STAFF" : [ {
	"name" : "Oliver",
	"yoe" : 17,
	"tenure" : 20,
	"role" : "Frontend Engineer",
	"experienceLevel" : "STAFF"
	} ],
	"SR" : [ {
	"name" : "Lucas",
	"yoe" : 7,
	"tenure" : 10,
	"role" : "Frontend Engineer",
	"experienceLevel" : "SR"
	} ],
	"MID" : [ {
	"name" : "Jhon",
	"yoe" : 4,
	"tenure" : 4,
	"role" : "DevOps Engineer",
	"experienceLevel" : "MID"
	}, {
	"name" : "Ethan",
	"yoe" : 5,
	"tenure" : 7,
	"role" : "Frontend Engineer",
	"experienceLevel" : "MID"
	} ]
	}

view raw a_sip_of_java:_maps_from_collections_of_objects_using_collectors__code02.java hosted with ❤ by GitHub

What if we dont need to know the employees with some experience level but the total number of employees with that experience level?:

	Map<ExperienceLevel, Long> countMap =
	employeeList.stream().collect(Collectors.groupingBy(
	Employee::experienceLevel, // groups by experience level
	Collectors.counting())); // downstream collector that counts
	// if we print countMap as json:
	{
	"STAFF" : 1,
	"MID" : 2,
	"JR" : 1,
	"SR" : 1
	}

view raw a_sip_of_java:_maps_from_collections_of_objects_using_collectors__code03.java hosted with ❤ by GitHub

In this case we are using the version of the method Collectors.groupingBy that uses another Collector as downstream collector.

Ok, what if then, we have to partition the collection based on some condition that could be true or false? In that case we use Collectors.partitioningBy:

	Map<Boolean, List<Employee>> map =
	employeeList.stream().collect(Collectors.partitioningBy(e -> e.tenure() > 2));
	// if we pring map as json:
	{
	"false" : [ {
	"name" : "Emma",
	"yoe" : 2,
	"tenure" : 2,
	"role" : "Fullstack Engineer",
	"experienceLevel" : "JR"
	} ],
	"true" : [ {
	"name" : "Jhon",
	"yoe" : 4,
	"tenure" : 4,
	"role" : "DevOps Engineer",
	"experienceLevel" : "MID"
	}, {
	"name" : "Lucas",
	"yoe" : 7,
	"tenure" : 10,
	"role" : "Frontend Engineer",
	"experienceLevel" : "SR"
	}, {
	"name" : "Oliver",
	"yoe" : 17,
	"tenure" : 20,
	"role" : "Frontend Engineer",
	"experienceLevel" : "STAFF"
	}, {
	"name" : "Ethan",
	"yoe" : 5,
	"tenure" : 7,
	"role" : "Frontend Engineer",
	"experienceLevel" : "MID"
	} ]
	}

view raw a_sip_of_java:_maps_from_collections_of_objects_using_collectors__code04.java hosted with ❤ by GitHub

Collectors.partitioningBy method also has a version that receives another Collector as downstream collector where we can execute other interesting logic, like grouping the result.

	Map<Boolean, Map<ExperienceLevel, List<Employee>>> map =
	employeeList.stream().collect(Collectors.partitioningBy(
	e -> e.tenure() > 2,
	Collectors.groupingBy(Employee::experienceLevel)));
	// if we pring map as json:
	"false" : {
	"JR" : [ {
	"name" : "Emma",
	"yoe" : 2,
	"tenure" : 2,
	"role" : "Fullstack Engineer",
	"experienceLevel" : "JR"
	} ]
	},
	"true" : {
	"SR" : [ {
	"name" : "Lucas",
	"yoe" : 7,
	"tenure" : 10,
	"role" : "Frontend Engineer",
	"experienceLevel" : "SR"
	} ],
	"STAFF" : [ {
	"name" : "Oliver",
	"yoe" : 17,
	"tenure" : 20,
	"role" : "Frontend Engineer",
	"experienceLevel" : "STAFF"
	} ],
	"MID" : [ {
	"name" : "Jhon",
	"yoe" : 4,
	"tenure" : 4,
	"role" : "DevOps Engineer",
	"experienceLevel" : "MID"
	}, {
	"name" : "Ethan",
	"yoe" : 5,
	"tenure" : 7,
	"role" : "Frontend Engineer",
	"experienceLevel" : "MID"
	} ]
	}
	}

view raw a_sip_of_java:_maps_from_collections_of_objects_using_collectors__code05.java hosted with ❤ by GitHub

A Sip of Java: Encapsulate collection attributes

Encapsulation is the ability we give our classes (and objects created from them) to hide the implementation details of their inner workings and control the consistency of the internal state and invariants.

It happens all the time that we expose some attribute in our class to the outside world in the form of a collection like the following:

	// Our domain class Contact
	public class Contact {
	private List<Email> emails;
	// ... some code
	public List<Email> getEmails() {
	return this.emails;
	}
	}

view raw a-sip-of-java__encapsulate_collection_attributes_code01.java hosted with ❤ by GitHub

This is actually pretty common and I think a bad practice, we tend to create private fields in our classes and as a reflex create setters and getters immediately without thinking about the consequences of exposing the state that way.

If we simply return the reference to the attribute of the object there is a lot of danger there because now others have a copy of the reference to that attribute that is supposed to be private and under the control of the owner object.

Imagine if some code that gets that list of emails from our class now tries to modify the content of that list by adding or removing some objects from it. How can we control that? what if there is some kind of limit on the number of emails a Contact can have? How can we ensure only valid emails are added?

What if, instead, we write the following:

	// Our domain class Contact
	public class Contact {
	private List<Emails> emails;
	// ... some code
	public List<Email> getEmails() {
	// lets return a unmodifiable version of the list
	return Collections.unmodifiableList(this.emails);
	}
	}

view raw a-sip-of-java__encapsulate_collection_attributes_code02.java hosted with ❤ by GitHub

Now users of our class will get a list of emails that cannot be modified. New Email objects cannot be added or removed (but still the objects in the list can be changed if the public methods in the Email class allow that 😉). You could also create a copy of the list of emails by instantiating a new ArrayList sending the list of emails as an argument to the constructor new ArrayList<>(this.emails) but I still think it is better to fail when trying to modify the list.

Probably it is even better to just expose public methods with just the exact functionality that we want to allow like the following:

	// Our domain class Contact
	public class Contact {
	private List<Emails> emails;
	// ...
	// private, just for internal use
	private List<Email> getEmails() {
	return this.emails;
	}
	public void addEmail(Email email) {
	// here is some code validating and controlling how email is added
	this.emails.add(email);
	}
	public void removeEmail(Email email) {
	this.emails.remove(this.emails.indexOf(email));
	}
	// allow for-each
	public void forEachEmail(Consumer<Email> consumer) {
	this.emails.stream().forEach(consumer);
	}
	// or maybe return iterators
	public Iterator<Email> getEmailsIterator() {
	return this.emails.iterator();
	}
	}

view raw a-sip-of-java__encapsulate_collection_attributes_code03.java hosted with ❤ by GitHub

With the last implementation, we are hiding more and then have even more control. For example, let's say a Contact can have thousands of Emails (I know, it is unlikely, but let's say that is possible), then we can even change internally the way we store Emails from a List to a Map and then be able to have better performance for some operations, like removing an Email.

Do you have suggestions on how to improve the information hiding here? please comment!

A Sip of Java: Stream.flatMap()

 Let's say you have a map where you store a list of values for each key, something like:

And now you want a single list with all values in the Map. Then you need to use flatMap:

Map.values() returns a collection of the values for each key in the map, since each value is a list of Strings, it will return a collection of list of strings.

But you want a single list with all strings right?

Ok, then use Stream.flatMap() method to get a stream for each list in the collection of values and then merge all lists into a single list.

Simply put, convert a Stream of Stream of values into a single list of values.

Elixir Development with Visual Studio Code

Hi! the actual post is here: https://medium.com/@abadon.gutierrez/elixir-development-with-visual-studio-code-16b923e82653