[JAVA] I wrote a design pattern in kotlin Adapter edition

To learn the concept of Interface and the reusability of objects, which are important in object orientation ["Introduction to design patterns learned in Java language"](https://www.amazon.co.jp/%E5%A2%97% E8% A3% 9C% E6% 94% B9% E8% A8% 82% E7% 89% 88Java% E8% A8% 80% E8% AA% 9E% E3% 81% A7% E5% AD% A6% E3% 81% B6% E3% 83% 87% E3% 82% B6% E3% 82% A4% E3% 83% B3% E3% 83% 91% E3% 82% BF% E3% 83% BC% E3% 83% B3% E5% 85% A5% E9% 96% 80-% E7% B5% 90% E5% 9F% 8E-% E6% B5% A9 / dp / 4797327030 / ref = sr_1_1? __mk_ja_JP =% E3% 82% AB % E3% 82% BF% E3% 82% AB% E3% 83% 8A & keywords = java% E8% A8% 80% E8% AA% 9E% E3% 81% A7% E5% AD% A6% E3% 81% B6 % E3% 83% 87% E3% 82% B6% E3% 82% A4% E3% 83% B3% E3% 83% 91% E3% 82% BF% E3% 83% BC% E3% 83% B3% E5 I learned about% 85% A5% E9% 96% 80 & qid = 1559563427 & s = gateway & sr = 8-1) and decided to write in Java and then in kotlin. This time I will write about Adapter.

What is Adapter

It is a design pattern that bridges the gap between what is already provided and what is needed, and is also called the Wrapper pattern. A pattern of roles that wraps the original class and meets the required specifications.

Two types of Adapter pattern implementation

1. Pattern with inheritance in class
2. Pattern with delegation on instance Two types are introduced, but this time I would like to implement a pattern using inheritance in the first class.

Pattern with inheritance in class

Era class

This class is assumed to be the originally provided class. When inheriting in kotlin, class is treated as java final, so it is necessary to open and inherit, so specify open before class definition reference: https://www.atmarkit.co.jp/ait/articles/1804/24/news008.html

Era.java

class Era{
	private int year;
	public Era(int year) {
		this.year = year;
	}
	
	public int yearToReiwa() {
		return year - 2018;
	}
	
	public int yearToHeisei() {
		return year - 1988;
	}
}

Era.kt

open class Era(private val year: Int){
    fun yearToReiwa(): Int = this.year - 2018
    fun yearToHeisei(): Int = this.year - 1988
}

Print interface

This is an interface that meets the requirements required as a specification. In kolin, use Unit when specifying void in Java

Print.java

interface Print{
	public abstract void printAsReiwa();
	public abstract void printAsHeisei();
}

Print.kt

interface Print{
    fun printAsReiwa(): Unit
    fun printAsHeisei(): Unit
}

PrintEra class

This class will be an Adapter and will wrap the Era class to meet the required requirements. The point is that the required specifications can be implemented without changing the implementation of the Era class. When inheriting, specify like ": Era", and when calling the inherited constructor, you can call it with ": Era (year)"

PrintEra.java

class PrintEra extends Era implements Print{
	public PrintEra(int year) {
		super(year);
	}
	public void printAsHeisei() {
		System.out.println("Heisei" + yearToHeisei() + "Year");
	}
	public void printAsReiwa() {
		if(yearToReiwa() == 1) System.out.println("First year of Reiwa");
		else System.out.println("Reiwa" + yearToReiwa() + "Year");	
	}
}

PrintEra.kt

class PrintEra(private val year: Int) :Era(year), Print{
    override fun printAsHeisei(): Unit = println("Heisei" + yearToHeisei() + "Year")
    override fun printAsReiwa() {
        if (yearToReiwa() == 1) println("First year of Reiwa") else println("Reiwa" + yearToReiwa() + "Year")
    }
}

Main class

This is the Main class that actually operates the above class.

AdapterSample.java

public class AdapterSample {
	public static void main(String[] args) {
		Print p = new PrintEra(2019);
		p.printAsHeisei();
		p.printAsReiwa();
	}
}

AdapterSample.kt

fun main(args: Array<String>){
    val p: Print = PrintEra(2019)
    p.printAsHeisei()
    p.printAsReiwa()
}

Execution result

2019
First year of Reiwa

Class diagram

Impressions

• I learned that it can be implemented without breaking the original method because it can be implemented according to the new requirements without changing the originally prepared class.
• Also, since the implementation of the Adapter class that implements the method of the original Era class is completely invisible from the main class, reusability will also increase.
• With kotlin, you can call the superclass constructor in the definition for inheritance, so the number of lines of code is small

reference

It was very easy to read and understand by referring to the following.

Introduction to Kotlin for Java Programmers