[JAVA] [Gang of Four] Design pattern learning --Singleton

Singleton --One card

** Table of Contents ** The last of the creational patterns is a singleton. I think this is a pattern that everyone is familiar with. I feel that the first design pattern I learned was a singleton.

Classes that provide DB connections and classes that get assets are generally implemented in this pattern. Guarantees the uniqueness of an instance of that class in your program.

Purpose

It guarantees that there is only one instance for a class and provides a global way to access it.

Component

· A class that creates only one Singleton instance

Implementation

If you code this pattern in kotlin, it will be in a simple form as shown below, so I'm not sure, so I will code it in java.

SingletonObject.kt

package singleton

object SingletonObject {
    var str = ""

    fun print() {
        println("Output result:$str")
    }
}

Create images and music files from multiple screens via singleton objects.

A class that creates only one Singleton instance

Singleton object

JavaSingletonObject.java

package singleton;

public class JavaSingletonObject {
    /**
     *Defined as static and make the instance of own class unique
     */
    public static JavaSingletonObject instance = new JavaSingletonObject();
    private String address = "/assets/temp";
    enum Assets {
        IMAGE1("image_1"),
        IMAGE2("image_2"),
        AUDIO1("audio_1"),
        AUDIO2("audio_2");

        private final String value;

        Assets(final String value) {
            this.value = value;
        }
    }

    /**
     *Define the constructor privately so that no new instance can be created.
     */
    private JavaSingletonObject(){
        System.out.println("Connection:" + address);
    }

    public String getAssets(Assets target) {
        System.out.println("Material acquisition with singleton object:" + target.value);
        return target.value;
    }
}

Screen 1 class

DisplayOne.kt

package singleton

class DisplayOne {
    fun draw() {
        println("In User1${JavaSingletonObject.instance.getAssets(JavaSingletonObject.Assets.IMAGE1)}Draw!!")
    }
}

Screen 2 class

DisplayTwo.kt

package singleton

class DisplayTwo {
    fun start() {
        println("In User2${JavaSingletonObject.instance.getAssets(JavaSingletonObject.Assets.AUDIO1)}Play!!")
    }
}

User

Screen management class

Manager.kt

package singleton

class Manager {
    init {
        DisplayOne().draw()
        DisplayTwo().start()
    }
}

This completes the implementation of the singleton pattern. This process is supposed to be executed only once because the connection to assets is established in the constructor of the `` `JavaSingletonObject``` class.

Also, since the constructor is defined as private, it is not possible to create a new instance of the class, and uniqueness is guaranteed.

Output result

[output]
Connection:/assets/temp
Material acquisition with singleton object: image_1
Image in User1_Draw 1!!
Material acquisition with singleton object: audio_1
Audio with User2_Play 1!!