Add currying pattern

currying/README.md

@@ -0,0 +1,128 @@
+---
+layout: pattern title: Currying folder: currying permalink: /patterns/currying/ categories: Functional language: en
+tags:
+
+- Decoupling
+
+---
+
+## Intent
+
+Currying pattern transforms an arbitrary arity into a sequence of unary functions.
+
+## Explanation
+
+Real-world example
+
+> We have a Staff class for storing employees' information, e.g. firstName, lastName,
+> gender, email, etc. We want to adapt currying pattern to simplify the parameters handling,
+> we don't need to provide all parameters at the same time. We can provide them one by one
+> when we have that data.
+
+In plain words
+
+> It helps you to avoid passing the same variable again and again.
+> It helps to create a higher order function. It extremely helpful in event handling.
+> Little snippets of code can be written and reused with ease.
+
+Wikipedia says
+
+> Currying provides a way for working with functions that take multiple arguments,
+> and using them in frameworks where functions might take only one argument.
+> For example, some analytical techniques can only be applied to functions with a
+> single argument. Practical functions frequently take more arguments than this.
+> Frege showed that it was sufficient to provide solutions for the single argument
+> case, as it was possible to transform a function with multiple arguments into a
+> chain of single-argument functions instead. This transformation is the process
+> now known as currying.
+
+**Programmatic Example**
+
+First, we have the `Staff` class:
+
+```java
+
+@AllArgsConstructor
+@Data
+public class Staff {
+    private String firstName;
+    private String lastName;
+    private Gender gender;
+    private String email;
+    private LocalDate dateOfBirth;
+}
+```
+
+By using @AllArgsConstructor from Lombok library, it will generate a constructor with all attributes. All paramaters
+need to provide at the same time.
+
+Next, we will try to use Curry pattern to transforms an arbitrary arity into a sequence of unary functions.
+
+```java
+static Function<String,
+        Function<String,
+        Function<Gender,
+        Function<String,
+        Function<LocalDate, Staff>>>>>CREATOR=
+        firstName->lastName
+        ->gender->email
+        ->dateOfBirth
+        ->new Staff(firstName,lastName,gender,
+        email,dateOfBirth);
+```
+
+With using this way, we can create Staff object by providing parameters one by one:
+
+```java
+Staff.CREATOR
+        .apply(firstName)
+        .apply(lastName)
+        .apply(gender)
+        .apply(email)
+        .apply(dateOfBirth);
+```
+
+We can also use Functional Interface to implement currying function.
+
+```java
+static AddFirstName builder(){
+        return firstName->lastName
+        ->gender->email
+        ->dateOfBirth
+        ->new Staff(firstName,lastName,gender,email,dateOfBirth);
+        }
+
+interface AddFirstName {
+    AddLastName withReturnFirstName(String firstName);
+}
+
+interface AddLastName {
+    AddGender withReturnLastName(String lastName);
+}
+
+interface AddGender {
+    AddEmail withReturnGender(Gender gender);
+}
+
+interface AddEmail {
+    AddDateOfBirth withReturnEmail(String email);
+}
+
+interface AddDateOfBirth {
+    Staff withReturnDateOfBirth(LocalDate dateOfBirth);
+}
+```
+
+## Applicability
+
+Use the currying pattern in any of the following situations
+
+* when you want to break a function with many arguments into many functions with single argument
+
+## Tutorials
+
+* [Currying in Java](https://www.baeldung.com/java-currying)
+
+## Credits
+
+* [Baeldung](https://www.baeldung.com/)

currying/pom.xml

@@ -0,0 +1,62 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!--
+
+    The MIT License
+    Copyright (c) 2014-2016 Ilkka Seppälä
+
+    Permission is hereby granted, free of charge, to any person obtaining a copy
+    of this software and associated documentation files (the "Software"), to deal
+    in the Software without restriction, including without limitation the rights
+    to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+    copies of the Software, and to permit persons to whom the Software is
+    furnished to do so, subject to the following conditions:
+
+    The above copyright notice and this permission notice shall be included in
+    all copies or substantial portions of the Software.
+
+    THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+    IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+    FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+    AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+    LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+    OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+    THE SOFTWARE.
+
+-->
+<project xmlns="http://maven.apache.org/POM/4.0.0"
+         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
+    <parent>
+        <artifactId>java-design-patterns</artifactId>
+        <groupId>com.iluwatar</groupId>
+        <version>1.25.0-SNAPSHOT</version>
+    </parent>
+    <modelVersion>4.0.0</modelVersion>
+    <artifactId>currying</artifactId>
+    <dependencies>
+        <dependency>
+            <groupId>junit</groupId>
+            <artifactId>junit</artifactId>
+            <scope>test</scope>
+        </dependency>
+    </dependencies>
+    <build>
+        <plugins>
+            <plugin>
+                <groupId>org.apache.maven.plugins</groupId>
+                <artifactId>maven-assembly-plugin</artifactId>
+                <executions>
+                    <execution>
+                        <configuration>
+                            <archive>
+                                <manifest>
+                                    <mainClass>com.iluwatar.dao.App</mainClass>
+                                </manifest>
+                            </archive>
+                        </configuration>
+                    </execution>
+                </executions>
+            </plugin>
+        </plugins>
+    </build>
+</project>

currying/src/main/java/com/iluwatar/currying/Staff.java

@@ -0,0 +1,97 @@
+package com.iluwatar.currying;
+
+import java.time.LocalDate;
+import java.util.function.Function;
+import lombok.AllArgsConstructor;
+import lombok.Data;
+
+/**
+ * Staff Object for demonstrating how to use currying pattern.
+ */
+@AllArgsConstructor
+@Data
+public class Staff {
+  private String firstName;
+  private String lastName;
+  private Gender gender;
+  private String email;
+  private LocalDate dateOfBirth;
+
+  /**
+   * Use {@link Function} for currying.
+   */
+  static Function<String,
+            Function<String,
+                    Function<Gender,
+                            Function<String,
+                                    Function<LocalDate, Staff>>>>> CREATOR =
+                                        firstName -> lastName
+                                            -> gender -> email
+                                                -> dateOfBirth
+                                                    -> new Staff(firstName, lastName, gender,
+                                                         email, dateOfBirth);
+
+  /**
+   * Use functional interfaces for currying.
+   */
+  static AddFirstName builder() {
+    return firstName -> lastName
+        -> gender -> email
+            -> dateOfBirth
+                -> new Staff(firstName, lastName, gender, email, dateOfBirth);
+  }
+
+  interface AddFirstName {
+    AddLastName withReturnFirstName(String firstName);
+  }
+
+  interface AddLastName {
+    AddGender withReturnLastName(String lastName);
+  }
+
+  interface AddGender {
+    AddEmail withReturnGender(Gender gender);
+  }
+
+  interface AddEmail {
+    AddDateOfBirth withReturnEmail(String email);
+  }
+
+  interface AddDateOfBirth {
+    Staff withReturnDateOfBirth(LocalDate dateOfBirth);
+  }
+
+  enum Gender {
+    Male, Female
+  }
+
+  /**
+   * Main method for maven-assembly-plugin.
+   */
+  public static void main(String[] args) {
+    final String firstName = "Janus";
+    final String lastName = "Lin";
+    final Staff.Gender gender = Staff.Gender.Male;
+    final String email = "[email protected]";
+    final LocalDate dateOfBirth = LocalDate.now();
+
+    Staff staff1 = Staff.CREATOR
+            .apply(firstName)
+            .apply(lastName)
+            .apply(gender)
+            .apply(email)
+            .apply(dateOfBirth);
+
+    System.out.println(String.format("Staff created with basic currying: %s", staff1));
+
+    Staff staff2 = Staff.builder()
+            .withReturnFirstName(firstName)
+            .withReturnLastName(lastName)
+            .withReturnGender(gender)
+            .withReturnEmail(email)
+            .withReturnDateOfBirth(dateOfBirth);
+
+    System.out.println(
+            String.format("Staff created with currying and functional interfaces: %s", staff2));
+  }
+}

currying/src/test/java/com/iluwatar/currying/StaffTest.java

@@ -0,0 +1,58 @@
+package com.iluwatar.currying;
+
+import org.junit.Assert;
+import org.junit.Test;
+
+import java.time.LocalDate;
+
+import static org.junit.Assert.*;
+
+public class StaffTest {
+    private final String firstName = "Janus";
+    private final String lastName = "Lin";
+    private final Staff.Gender gender = Staff.Gender.Male;
+    private final String email = "[email protected]";
+    private final LocalDate dateOfBirth = LocalDate.now();
+
+    private final Staff expectedResult = new Staff(firstName, lastName, gender, email, dateOfBirth);
+
+    @Test
+    public void createStaffWithBasicCurrying() {
+        Staff actualResult = Staff.CREATOR
+                .apply(firstName)
+                .apply(lastName)
+                .apply(gender)
+                .apply(email)
+                .apply(dateOfBirth);
+        assertEquals(expectedResult, actualResult);
+    }
+
+    @Test
+    public void createStaffWithFunctionalInterface() {
+        Staff actualResult = Staff.builder()
+                .withReturnFirstName(firstName)
+                .withReturnLastName(lastName)
+                .withReturnGender(gender)
+                .withReturnEmail(email)
+                .withReturnDateOfBirth(dateOfBirth);
+        assertEquals(expectedResult, actualResult);
+    }
+
+    @Test
+    public void mainTest() {
+        Staff.main(new String[]{});
+    }
+
+    @Test
+    public void hashTest() {
+        expectedResult.hashCode();
+    }
+
+    @Test
+    public void equalTest() {
+        assertTrue(expectedResult.equals(expectedResult));
+        assertFalse(expectedResult.equals(new Integer(1)));
+        Staff o2 = new Staff(firstName, lastName, gender, email, dateOfBirth);
+        assertTrue(expectedResult.equals(o2));
+    }
+}