Extends function in java

Functional Interfaces in Java

Java has forever remained an Object-Oriented Programming language. By object-oriented programming language, we can declare that everything present in the Java programming language rotates throughout the Objects, except for some of the primitive data types and primitive methods for integrity and simplicity. There are no solely functions present in a programming language called Java. Functions in the Java programming language are part of a class, and if someone wants to use them, they have to use the class or object of the class to call any function.

Java Functional Interfaces

A functional interface is an interface that contains only one abstract method. They can have only one functionality to exhibit. From Java 8 onwards, lambda expressions can be used to represent the instance of a functional interface. A functional interface can have any number of default methods. Runnable, ActionListener, and Comparable are some of the examples of functional interfaces.

Functional Interface is additionally recognized as Single Abstract Method Interfaces. In short, they are also known as SAM interfaces. Functional interfaces in Java are the new feature that provides users with the approach of fundamental programming.

Functional interfaces are included in Java SE 8 with Lambda expressions and Method references in order to make code more readable, clean, and straightforward. Functional interfaces are interfaces that ensure that they include precisely only one abstract method. Functional interfaces are used and executed by representing the interface with an annotation called @FunctionalInterface. As described earlier, functional interfaces can contain only one abstract method. However, they can include any quantity of default and static methods.

In Functional interfaces, there is no need to use the abstract keyword as it is optional to use the abstract keyword because, by default, the method defined inside the interface is abstract only. We can also call Lambda expressions as the instance of functional interface.

Java Functional Interfaces Example

Before Java 8, we had to create anonymous inner class objects or implement these interfaces.

Источник

Методы расширения в Java

В таких языках программирования, как C#, Kotlin, Groovy, Scala есть возможность расширять класс путем добавления нового функционала, при этом не требуется наследование или изменение самого изначального класса. Это реализовано с помощью специальных выражений, называемых расширения. Java, в отличие от этих языков, не имеет такой возможности из коробки и даже не планирует в ближайших релизах. Благодаря Lombok это стало возможным. Методы расширения были реализованы в Lombok еще 8 лет назад (с поддержкой Eclipse), но для многих все упиралось в поддержку плагином в IDEA (код компилировался, но IDE его не распознавала как валидный). Lombok плагин теперь предустановлен в IDEA 2021.1 EAP, и теперь он поддерживает методы расширения lombok (спасибо Anna Kozlova, Tagir Valeev, NekoCaffeine и Michail Plushnikov).
Рассмотрим пример классического статического импорта:

import static org.apache.commons.lang3.StringUtils.capitalize; public class ExtensionMethods < public static void main(String[] args) < String str = "test"; String capitalized = capitalize(str); // "Test" System.out.println(capitalized); >>

при переходе на метод расширения код станет выглядеть так:

import lombok.experimental.ExtensionMethod; import org.apache.commons.lang3.StringUtils; @ExtensionMethod(StringUtils.class) public class ExtensionMethods < public static void main(String[] args) < String str = "test"; String capitalized = str.capitalize(); // "Test" System.out.println(capitalized); >>

Заворачивания аргументов в скобки заменяются на цепочки вызовов, т.е. код вида call3(call2(call1(arg))) превратится в

Во многих ситуациях это может облегчить чтение кода, особенно когда цепочки длинные, здесь есть некая аналогия со Stream Api или преобразования значения java.util.Optional .
Фактически это просто синтаксический сахар. Код при компиляции будет заменен на вызов статического метода. Первый аргумент статического метода и станет объектом » this «.

null-значения

В отличие от обычных instance-методов, методы расширения могут работать и с null-значениями, т.е. подобный вызов вполне допустим:

import org.apache.commons.lang3.StringUtils; @ExtensionMethod(StringUtils.class) public class MethodExtensions < public static void main(String[] args) throws Exception < String nullStr = null; // "isEmpty=true" System.out.println("isEmpty esche-primery">Еще примеры 
Можно добавить в проект на JDK 8 метод, который появится только в JDK 11:
 
@UtilityClass public class CollectionExtensions < public static T[] toArray(Collection list, IntFunction generator) < return list.stream().toArray(generator); >> @ExtensionMethod(CollectionExtensions.class) public class MethodExtensions < public static void main(String[] args) throws Exception < Listlist = Arrays.asList(1, 2, 3); // toArray(IntFunction) добавлен только в Java 11 Integer[] array = list.toArray(Integer[]::new); // "[1, 2, 3]" System.out.println(Arrays.toString(array)); > >
 
Или добавить более лаконичный вызов Stream.collect(toList()):
 
@UtilityClass public class StreamExtensions < public static List toList(Stream stream) < return stream.collect(Collectors.toList()); >> @ExtensionMethod(StreamExtensions.class) public class MethodExtensions < public static void main(String[] args) throws Exception < Listlist = Arrays.asList(3, 1, 2); List sorted = list.stream() .sorted() .toList(); // "[1, 2, 3]" System.out.println(sorted); > >
 
Настройка проекта
 
 
Установите последнюю версию IDEA EAP, важно: EAP версии не стабильны, зато бесплатны. Плагин доступен и в Ultimate, и в Community Edition.
 
Добавьте зависимость lombok: maven
 
 
 org.projectlombok lombok 1.18.16 provided 
 
compileOnly 'org.projectlombok:lombok:1.18.16' annotationProcessor 'org.projectlombok:lombok:1.18.16' testCompileOnly 'org.projectlombok:lombok:1.18.16' testAnnotationProcessor 'org.projectlombok:lombok:1.18.16'
 
 
Убедитесь, что включена опция проекта Build, Execution, Deployment -> Compiler -> Annotations processor -> Enable annotation processing
 
Добавьте аннотацию @ExtensionMethod на класс (откуда будет вызов), перечисляя все утилитные классы, из которых необходимо импортировать вызовы.
 
 
Источник
 
Java extend a function in java
 
Trying to extend these all known classes/interfaces will cause artefacts like rewrapping old classes with new classes and so on. Solution 1: (This answer is based upon the comments, which say that the question is about extension functions) Java extension functions Java does not allow extension functions, but there are alternatives you can use instead Alternatives Wrapping You can have a class which contains the original object, and passes through the existing ones you want to use, while implementing the new ones Extension You can extend the original object type, but then the instances must be of your new type, rather than the original one You only need to include the new methods, the rest pass to the super type Other languages Some other JVM languages, like Kotlin, do support extension functions, but
 
Java extends class as function argument
 
Not new to java, but this question bothers me. I think I do not have a solid foundation.
 
Let's say have classes A , B , C and B extends A and C extends A . My question is, how can I define a method f() so that it can take one of List , List and List as argument?
 
Use an upper-bounded wildcard:
 
See Oracle's tutorial for more information.
 
Note that this restricts you to only be able to get things out of the list in the method body; you can't call consumer methods on the list:
 
A item = list.get(0); // OK. list.add(new A()); // Not OK! list might be a List. 
 
Extends vs Implements in Java, Extends: In Java, the extends keyword is used to indicate that the class which is being defined is derived from the base class using inheritance. So basically, extends keyword is used to extend the functionality of the parent class to the subclass. In Java, multiple inheritances are not allowed due to ambiguity.
 
How to convert a function into a method that extends an existing Java class
 
I have the following simple function that returns the position of a String in a string array ( String[] ).
 
private int getIndexOf(String needle, String[] haystack) < int indexOfNeedle = -1; for (int index=0;index> > 
 
I want to convert this to be a method of the ArrayList class so that I can call it with:
 
String[] myArray = ; int indexOfGreen = myArray.getIndexOf("green"); 
 
(I know there are existing libraries I can include that include indexOf but reinventing this is a lesson for me as well.)
 
I believe the approach will involve extends ArrayList and that is it probably the class that I need to extend so my function above will need to be modified to fit into that extension class.
 
Rather than using haystack in the method version I would also expect to be able to search inside this referring to the array object that the method was called from.
 
I'm also thinking that I may need to replace my String[] arrays with this new extended class of array (or can I use an override to include my new method in the excusing String Array class?).
 
As I say, this is to help me learn how to do this in general in Java so the exercise itself may seem a bit pointless and the point of the question isn't really to learn how to find the position of an element in an array - that's just by way of an example.
 
 (This answer is based upon the comments, which say that the question is about extension functions) 
 
Java extension functions
 
Java does not allow extension functions, but there are alternatives you can use instead
 
Alternatives
 
 
Wrapping 
 
You can have a class which contains the original object, and passes through the existing ones you want to use, while implementing the new ones
 
 
 
You can extend the original object type, but then the instances must be of your new type, rather than the original one
 
You only need to include the new methods, the rest pass to the super type
 
 
Other languages
 
Some other JVM languages, like Kotlin, do support extension functions, but Java itself does not
 
  Not a good idea.  
 
The best practice is to have a more general type (interface) in the declaration.
 
void f(List list) < . >List list = new ArrayList<>(); 
 
This allows other implementations, and being used as parameter is more flexible/powerfull.
 
Trying to extend these all known classes/interfaces will cause artefacts like rewrapping old classes with new classes and so on.
 
Instead use static functions in a utility class. Like the Collections and Arrays classes. With a binarySearch and other goodies.
 
 By the way, such functionality as mentioned, probably exists, and one would like to build upon existing general functionality, instead of reinventing the wheel. 
 
int index = Arrays.asList(haystack).indexOf(needle); 
 
Extend Two Classes in Java, Java allows extending class to any class, but it has a limit. It means a class can extend only a single class at a time. Extending more than one class will lead to code execution failure. When a class extends a class, then …
 
Java - method argument for any instance extending a given object
 
Since I don't know how to phrase my question so that I get a helping result just by searching via Google I decided to ask here. I'm just searching for a way to tell a method that it should take every Object extending a certain class. Here are the things I tried so far:
 
public void method( Object obj ); public void method( Component c ); 
 
The problem with the second one is that i have to cast every instance to Component again before method() accepts it and the first one just didn't work for me. Can anyone give me a quick solution?
 
 
The problem with the second one is that i have to cast every instance to Component again before method() accepts it
 
 
No, you don't have to cast anything, that's how polymorphism works.
 
class A < >class B extends A < >class C extends B < >public class Main < private void method(final A a) < >public static void main(final String[] args) < final Main main = new Main(); main.method(new B()); main.method(new C()); >> 
 
If you're looking to define a method that takes subtypes of Component, use public void method(T t) . This will work for any object, i.e. Component can be replaced with any other class that has subclasses.
 
The second form public void method( Component c ); is correct, If you need to typecast then you are trying to pass as parameter a variable which type is not Component or a subclass of Component. I don't know why.
 
Declaring variables with type Object is not usually the right thing to do, you should define them with the correct type when you know it.
 
Typecast is the way to tell the compiler you know the type of the object pointed by a reference when that reference does not have that type for any reason.
 
Can We Extend final Method in Java?, Final method in java can be extended but alongside the main concept to be taken into consideration is that extend means that you can extend that particular class or not which is having final method, …
 
Источник