Private Constructor Singleton Object

To stop multiple creations of Singleton objects using a public constructor, The constructor will be private, so that it cannot be called outside of the class. In the below code, the object creation will only happen when the getAppleInstance method will be called.

package org.wesome.dsalgo.design.pattern.singleton;

public class Apple {
    static Apple appleInstance;

    private Apple() {
        System.out.println("Apple private default constructor");
    }

   public static Apple getAppleInstance() {
        System.out.println("Apple getAppleInstance");
        if (Objects.isNull(appleInstance)) {
            appleInstance = new Apple();
        }
        return appleInstance;
    }
}

package org.wesome.dsalgo.design.pattern.singleton;

public class SingletonClass {
    public static void main(String[] args) {
        System.out.println(Apple.getAppleInstance());
        /*  private constructor is not accessible outside the Apple class, hence below line will throw compile time exception    */
        Apple apple = new Apple();
        System.out.println("apple = " + apple);
    }
}

plugins {
    id 'java'
    id "io.freefair.lombok" version "6.4.1"
}

group = 'org.wesome'
version = '0.0.1-SNAPSHOT'
sourceCompatibility = JavaVersion.VERSION_1_8

configurations {
    compileOnly {
        extendsFrom annotationProcessor
    }
}

repositories {
    mavenCentral()
}

dependencies {
    testImplementation 'org.junit.jupiter:junit-jupiter:5.6.2'
}

test {
    useJUnitPlatform()
}