#java
Сохранение данных в постоянной памяти, используя Preferences.

Если вам нужно сохранять конфигурацию, информацию о пользователе, о программе на всех платформах одинаково и вы не хотите беспокоиться о разных путях, или писать отдельные методы для работы с файлами и переживать об их безопасности, то самый лучший и самый простой вариант это Preferences.

Если очень просто то это обычное API, позволяющее записывать значения по принципу ключ/значения. И основная особенность того, что на всех платформах оно будет адаптироваться. Фактическое хранение данных зависит от платформы.

Вот пример кода (Читайте комментарии):

import java.util.prefs.Preferences;

class PreferenceTest {
    //Created node for storage
    private final Preferences prefs = Preferences.userRoot().node(this.getClass().getName());

    public void setPreference() {
        //Keys
        final String key1 = "key1";
        final String key2 = "key2";
        final String key3 = "key3";

        //Here we define variables with default values
        System.out.println(prefs.getBoolean(key1, true));
        System.out.println(prefs.get(key2, "Hello World"));
        System.out.println(prefs.getInt(key3, 50));

        prefs.putBoolean(key1, false);
        prefs.put(key2, "Bye bye");
        prefs.putInt(key3, 0);

        //You should write key and default value for getting present value
        System.out.println(prefs.getInt(key3, 50));

        prefs.remove(key1);
    }

    public static void main(String[] args) {
        PreferenceTest test = new PreferenceTest();
        test.setPreference();
    }
}

Запустите программу дважды. Значение «key1» должно быть записано со значением по умолчанию (true) в командную строку, так как значение preference было удалено в конце метода. Значение «key2» и «key3» должно было измениться после первого вызова.

Useful link - https://spec-zone.ru/RU/Java/Docs/7/api/java/util/prefs/Preferences.html

#info
👋🏻 Мы решили делать посты на английском.

На это есть несколько причин:
1) количество охватываемой аудитории
2) хоть какая, но прокачка языка
3) легкость поиска контента

Всем тем, кто отпишется - спасибо что оставались с нами, несмотря на то, что мы часто забиваем на это дело ввиду работы и учебы в универе.

Если кому-то было интересно и/или полезно - не зря мы их писали 🐸

#java
Difference between Concurrency and Parallelism

Concurrency:
By Concurrency, we mean executing multiple tasks on the same core. In simpler terms, it relates to processing more than one task at the same time. It is a state in which multiple tasks start, run, and complete in overlapping time periods. An application capable of executing multiple tasks virtually at the same time is called a Concurrent application.

In case the computer only has one CPU or one Core the application may not make progress on more than one task at the exact same time. Instead, divide the time and the Core/CPU among various tasks. Concurrency is useful in decreasing the response time of the system.

Parallelism:
Parallelism is the ability of an application to split up its processes or tasks into smaller subtasks that are processed simultaneously or in parallel. It is the mechanism in which multiple processes execute independently of each other where each process acquires a separate core. This utilizes all the cores of the CPU or

#java
Sealed Classes

Java 15 introduces Sealed Classes, a preview language feature, that allows classes/interfaces to restrict which other classes/interfaces may extend or implement them. Here is an example:

In the example above, Vehicle is a sealed class, which specifies three permitted subclasses; Car, Truck and Motorcycle.

Sealing serves two main purposes:

It restricts which classes or interfaces can be a subtype of a class or interface and thus preserves the integrity of your API.
It allows the compiler to list all the permitted subtypes of a sealed type (exhaustiveness analysis), which will (in a future Java release) enable switching over type patterns in a sealed type (and other features). For example, given the following switch statement, the compiler will detect that there is a case statement for every permitted subclass of Vehicle (so no default clause is needed) and it will also give an error if any of them are missing

#java
What is the structure of Java Heap?

The JVM has a heap of runtime data. Memory for all class instances and arrays is allocated to this heap, which is created at the JVM start-up. Heap memory for objects is reclaimed by the garbage collector, which is an automatic memory management system.

Both living objects, which are accessible by the application and won’t be part of garbage collection, and dead objects, which will never be accessible by the application but haven’t yet been collected by the garbage collector, make up the heap memory space until the dead objects eventually end up in the garbage collector.

#java StreamAPI
We have a source - our collection. We don't copy all the elements at once, but stretch it, loading the elements in stages, doing some operations.

It's like a file stream, only instead of reading byte by byte, we get references to objects[1] (if we don't have a primitive stream - IntStream, etc.)

Therefore, when the stream is open (we are working with the address space of our data structure), we cannot change it.

[1]By the way, this is a stream of object references, not a stream of bytes. It means we can modify objects of the collection by reference(setters, getters, and so on).

That is, if we had a collection with objects of the Person type, we could do something like this:
personList.stream().forEach(x->x.setName("New name"));

Hi there. If there are any suggestions for improving the channel - write.

One of them, did you have a desire to improve the post, or even write your own?

If so, how do you like the idea of making a bot in which you can write your post, or some additions to one of the existing ones.

#jvm
Metaspace?

Metaspace is a special partition of memory in the Java virtual machine. It is located in the Runtime Data Area and is not part of the heap.

It is used to manage memory for metadata class.

Metadata of a class is a describtion of class fields, methods stored in a special object. Every class has own (meta) Class object.

Metadata class are allocated when classes are loaded. Their lifetime is usually scoped to that of the loading classloader - when a loader gets collected, all class metadata it accumulated are released in bulk. The memory manager does not need to track individual allocations for the purpose of freeing them.

Notes:
- Metaspace by default auto increases its size
- Max size can be set using XX: MetaspaceSize
- Comparatively effective Garbage collection. Deallocate class data concurrently and not during GC pause.
- In versions before Java 8 instead of Metaspace was PermGen Space

The best but not ideal map of JVM

Obviously not the prettiest blue line😁

#news #java

Oracle doesn't want Java EE any more

Oracle wants someone else to lead enterprise Java, though it says it will stay involved. Apache and Eclipse are likely candidates to take over Java EE.

Oracle wants to end its leadership in the development of enterprise Java and is looking for an open source foundation to take on the role.

The company said today that the upcoming Java EE (Enterprise Edition) 8 presents an opportunity to rethink how the platform is developed. Although development is done via open source with community participation, the current Oracle-led process is not seen agile, flexible, or open enough. ”We believe that moving Java EE technologies to an open source foundation may be the right next step, to adopt more agile processes, implement more flexible licensing and change the governance process,” Oracle said in a statement.

Why in functional programming languages recursion is faster compare to cycle?

Hi folks. I haven't written anything in a long time but now I have interesting topic to share.

You probably know that in Java recursion is much slower compare to loops. And It makes sense, because in recursion each time is created some stack frame. Each stack frame require some memory allocation and so on. So when we use recursion all time time during invocation your size of stack frames increasing. Same stuff in C++/C and in C#.

You can check it by writing Fibonacci numbers i.e.

But recently I figure out that in Haskell recursion works faster. And in mostly functional programming languages. Bot of course id depends on algorithm. The main reason is - Compiler is better in optimizing pure functions. He can make optimization more eagerly and aggressive.

"The key concept here is purity: a pure function is a function with no side effects and no state. Functional programming languages generally embrace purity for many reasons, such as reasoning about code and avoiding non-obvious dependencies. Some languages, most notably Haskell, even go so far as to allow only pure code; any side effects a program may have (such as performing I/O) are moved to a non-pure runtime, keeping the language itself pure."

Hi, we often work with JVM tuning.

Correct resource allocation, best performance is very important to each program.

Here is useful command to see all JVM options for tuning:

java -XX:+UnlockDiagnosticVMOptions -XX:+PrintFlagsFinal -version

Why is better always to use custom initial capacity for ArrayList?

So, ArrayList have two main parameters.

First one is capacity which defined by default as 10. Capacity is used to create empty array thus help to optimize list by static creation

And size — is size of actual elements which list contains.

When size of list = capacity — list recreate himself as array with newCapacity.

newCapacity = newCapacity + (oldCapacity >> 1)

Which means each time when your list will be full java will create new array.

That's why for better performance is good to use expected initial capacity.